implemented shared with me + verification

crates/pdfs-cli/src/main.rs

@@ -13,7 +13,7 @@ use pdfs_core::auth;
 use pdfs_core::cache::ContentCache;
 use pdfs_core::config::AppDirs;
 use pdfs_core::control::{
-    ErrorKind, RefreshScope, Request as CtlRequest, Response as CtlResponse,
+    ErrorKind, MountKind, RefreshScope, Request as CtlRequest, Response as CtlResponse,
     RestoreItem as CtlRestoreItem, ShareEntryKind, SyncPhase, pending_summary,
 };
 use pdfs_core::db::Db;
@@ -28,7 +28,7 @@ struct Cli {
     /// Emit machine-readable JSON instead of formatted text.
     ///
     /// Applies to the query commands (`status`, `ls`, `pins`, `sync list`,
-    /// `devices list`, `transfers`, `activity`, `cache inspect`). Commands that
+    /// `devices list`, `locations`, `transfers`, `activity`, `cache inspect`). Commands that
     /// perform an action keep their human output — a script that needs to know
     /// whether one succeeded has the exit code.
     #[arg(long, global = true)]
@@ -105,6 +105,8 @@ enum Command {
     Logout,
     /// Show account login state, and mount status if a daemon is running.
     Status,
+    /// List every local Proton Drive location.
+    Locations,
     /// Mount Proton Drive at the given (or default) path. Blocks until unmounted.
     Mount {
         /// Mountpoint; defaults to ~/ProtonDrive.
@@ -507,6 +509,7 @@ fn main() -> Result<()> {
         Command::Login { username } => cmd_login(username),
         Command::Logout => cmd_logout(),
         Command::Status => cmd_status(),
+        Command::Locations => cmd_locations(),
         Command::Mount { mountpoint } => cmd_mount(mountpoint),
         Command::Daemon { mountpoint } => cmd_daemon(mountpoint),
         Command::Pin { path } => cmd_pin(path),
@@ -1201,6 +1204,45 @@ fn cmd_status() -> Result<()> {
     Ok(())
 }
 
+fn cmd_locations() -> Result<()> {
+    let response = control_request(CtlRequest::ListLocations)?;
+    if emit_json(&response)? {
+        return Ok(());
+    }
+    match response {
+        CtlResponse::Locations { items } if items.is_empty() => {
+            println!("No Proton Drive locations.");
+        }
+        CtlResponse::Locations { items } => {
+            for location in items {
+                let kind = match &location.kind {
+                    MountKind::MyFiles => "My Files".to_string(),
+                    MountKind::Device { sync_folder_id } => {
+                        format!("Device folder {sync_folder_id}")
+                    }
+                    MountKind::Shared { .. } => "Shared folder".to_string(),
+                };
+                let mode = match location.pending_mode {
+                    Some(pending) => format!("{} -> {pending}", location.mode),
+                    None => location.mode.to_string(),
+                };
+                let mounted = if location.mounted {
+                    "mounted"
+                } else {
+                    "not mounted"
+                };
+                println!(
+                    "{kind}: {} ({mode}, {}, {}, {mounted})",
+                    location.local_path, location.access, location.state
+                );
+            }
+        }
+        CtlResponse::Error { message, kind } => bail!("{}", cli_error(kind, &message)),
+        other => bail!("unexpected response: {other:?}"),
+    }
+    Ok(())
+}
+
 fn cmd_mount(mountpoint: Option<PathBuf>) -> Result<()> {
     mount_once(mountpoint)?;
     Ok(())

crates/pdfs-core/src/control.rs

@@ -14,6 +14,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::cache::Pin;
 use crate::error::Result;
+pub use crate::mounts::{MountAccess, MountKind, MountMode, MountSpec};
 
 /// Cap on the *write* half of a round-trip. A crashed daemon can leave its
 /// listening socket in the kernel (e.g. as a `<defunct>` zombie): `connect`
@@ -194,6 +195,11 @@ pub enum Request {
     /// it invalidates, it does not fetch.
     Refresh { scope: RefreshScope },
 
+    // ---- locations --------------------------------------------------------
+    /// List every local Proton Drive location. Replies with
+    /// [`Response::Locations`].
+    ListLocations,
+
     // ---- devices ----------------------------------------------------------
     /// List the account's registered devices. Replies with [`Response::Devices`].
     ListDevices,
@@ -1127,6 +1133,8 @@ pub enum Response {
     },
     /// The account's devices (reply to [`Request::ListDevices`]).
     Devices { items: Vec<DeviceInfo> },
+    /// Every local Proton Drive location (reply to [`Request::ListLocations`]).
+    Locations { items: Vec<MountSpec> },
     /// This device's synced folders (reply to [`Request::ListSyncFolders`]).
     SyncFolders { items: Vec<SyncFolderInfo> },
     /// Folders offered for restore (reply to [`Request::ListRestorableFolders`]).
@@ -1574,6 +1582,7 @@ mod tests {
     #[test]
     fn sharing_requests_roundtrip() {
         let reqs = [
+            Request::ListLocations,
             Request::ListDevices,
             Request::RenameDevice {
                 uid: "dev-1".into(),
@@ -1698,6 +1707,107 @@ mod tests {
         }
     }
 
+    #[test]
+    fn locations_response_roundtrips_typed_mounts() {
+        let response = Response::Locations {
+            items: vec![
+                MountSpec {
+                    id: 1,
+                    kind: MountKind::MyFiles,
+                    local_path: "/home/me/ProtonDrive".into(),
+                    root_uid: "vol~root".into(),
+                    root_share_id: "share-main".into(),
+                    mode: MountMode::OnDemand,
+                    access: MountAccess::Rw,
+                    state: "idle".into(),
+                    last_sync: 0,
+                    pending_mode: None,
+                    mounted: true,
+                    progress: None,
+                },
+                MountSpec {
+                    id: 2,
+                    kind: MountKind::Device { sync_folder_id: 7 },
+                    local_path: "/home/me/Work".into(),
+                    root_uid: "device-vol~work".into(),
+                    root_share_id: "device-share".into(),
+                    mode: MountMode::Mirror,
+                    access: MountAccess::Rw,
+                    state: "syncing".into(),
+                    last_sync: 42,
+                    pending_mode: Some(MountMode::OnDemand),
+                    mounted: false,
+                    progress: None,
+                },
+                MountSpec {
+                    id: 3,
+                    kind: MountKind::Shared {
+                        share_root_uid: "shared-vol~root".into(),
+                    },
+                    local_path: "/home/me/Shared".into(),
+                    root_uid: "shared-vol~root".into(),
+                    root_share_id: "shared-share".into(),
+                    mode: MountMode::Unknown,
+                    access: MountAccess::Ro,
+                    state: "error".into(),
+                    last_sync: 0,
+                    pending_mode: Some(MountMode::Unknown),
+                    mounted: false,
+                    progress: Some(SyncProgress {
+                        phase: SyncPhase::Applying,
+                        done: 2,
+                        total: 5,
+                        current: "report.pdf".into(),
+                    }),
+                },
+            ],
+        };
+        let wire = serde_json::to_string(&response).unwrap();
+        assert!(!wire.contains('\n'), "wire form must be a single line");
+        assert!(wire.contains(r#""mode":"ondemand""#));
+        assert!(wire.contains(r#""kind":"shared""#));
+        assert!(wire.contains(r#""access":"ro""#));
+        assert!(wire.contains(r#""mode":"unknown""#));
+        let back: Response = serde_json::from_str(&wire).unwrap();
+        match back {
+            Response::Locations { items } => {
+                assert_eq!(items.len(), 3);
+                assert!(matches!(
+                    items[1].kind,
+                    MountKind::Device { sync_folder_id: 7 }
+                ));
+                assert_eq!(items[1].pending_mode, Some(MountMode::OnDemand));
+                assert!(matches!(
+                    &items[2].kind,
+                    MountKind::Shared { share_root_uid }
+                        if share_root_uid == "shared-vol~root"
+                ));
+                assert_eq!(items[2].access, MountAccess::Ro);
+                assert_eq!(items[2].mode, MountMode::Unknown);
+                assert_eq!(items[2].pending_mode, Some(MountMode::Unknown));
+                let progress = items[2].progress.as_ref().unwrap();
+                assert_eq!(progress.phase, SyncPhase::Applying);
+                assert_eq!((progress.done, progress.total), (2, 5));
+            }
+            other => panic!("unexpected response: {other:?}"),
+        }
+    }
+
+    #[test]
+    fn legacy_daemons_reject_list_locations_cleanly() {
+        #[allow(dead_code)]
+        #[derive(serde::Deserialize)]
+        enum LegacyLocationRequest {
+            ListSyncFolders,
+        }
+
+        let wire = serde_json::to_string(&Request::ListLocations).unwrap();
+        assert!(
+            serde_json::from_str::<LegacyLocationRequest>(&wire).is_err(),
+            "an old daemon must reject ListLocations instead of interpreting another request"
+        );
+    }
+
     #[test]
     fn path_share_requests_keep_their_existing_wire_shape() {
         let cases = [

crates/pdfs-core/src/db/maintenance.rs

@@ -77,6 +77,7 @@ const REPORTED_TABLES: &[&str] = &[
     "trash",
     "device",
     "sync_folder",
+    "mount",
     "sync_entry",
     "pending_op",
     "activity",

crates/pdfs-core/src/db/migrations.rs

@@ -9,7 +9,7 @@ use super::Db;
 use crate::Result;
 
 /// Current schema version. Bump on every forward migration added below.
-pub(super) const SCHEMA_VERSION: i64 = 17;
+pub(super) const SCHEMA_VERSION: i64 = 18;
 
 impl Db {
     pub(super) fn migrate(&self) -> Result<()> {
@@ -92,6 +92,9 @@ impl Db {
         if current < 17 {
             tx.execute_batch(MIGRATION_V17)?;
         }
+        if current < 18 {
+            tx.execute_batch(MIGRATION_V18)?;
+        }
         tx.execute(
             "INSERT INTO sync_state (key, value) VALUES ('schema_version', ?1)
              ON CONFLICT(key) DO UPDATE SET value = excluded.value",
@@ -425,3 +428,47 @@ CREATE TABLE share_access (
            CHECK (access IN ('owner', 'editor', 'viewer', 'unknown'))
 );
 ";
+
+/// Schema v18: unified location presentation. Device rows carry only their
+/// `sync_folder` identity; reads join all device state from that authoritative
+/// table. The trigger projects future folders, while the foreign key removes a
+/// presentation row with its sync folder.
+const MIGRATION_V18: &str = "
+CREATE TABLE mount (
+  id              INTEGER PRIMARY KEY,
+  kind            TEXT NOT NULL
+                  CHECK (kind IN ('myfiles', 'device', 'shared')),
+  sync_folder_id  INTEGER REFERENCES sync_folder(id) ON DELETE CASCADE,
+  share_root_uid  TEXT,
+  local_path      TEXT NOT NULL DEFAULT '',
+  root_uid        TEXT NOT NULL DEFAULT '',
+  root_share_id   TEXT NOT NULL DEFAULT '',
+  mode            TEXT NOT NULL DEFAULT 'unknown',
+  access          TEXT NOT NULL DEFAULT 'rw'
+                  CHECK (access IN ('rw', 'ro')),
+  CHECK (
+    (kind = 'myfiles' AND sync_folder_id IS NULL AND share_root_uid IS NULL)
+    OR
+    (kind = 'device' AND sync_folder_id IS NOT NULL AND share_root_uid IS NULL)
+    OR
+    (kind = 'shared' AND sync_folder_id IS NULL AND share_root_uid IS NOT NULL)
+  )
+);
+
+CREATE UNIQUE INDEX mount_myfiles
+  ON mount(kind) WHERE kind = 'myfiles';
+CREATE UNIQUE INDEX mount_device
+  ON mount(sync_folder_id) WHERE kind = 'device';
+CREATE UNIQUE INDEX mount_shared
+  ON mount(share_root_uid) WHERE kind = 'shared';
+
+INSERT INTO mount (kind, sync_folder_id)
+SELECT 'device', id FROM sync_folder;
+
+CREATE TRIGGER mount_sync_folder_insert
+AFTER INSERT ON sync_folder
+BEGIN
+  INSERT OR IGNORE INTO mount (kind, sync_folder_id)
+  VALUES ('device', NEW.id);
+END;
+";

crates/pdfs-core/src/db/mod.rs

@@ -27,6 +27,7 @@ mod devices;
 mod local;
 mod maintenance;
 mod migrations;
+mod mounts;
 mod nodes;
 mod ops;
 mod photos;
@@ -110,6 +111,10 @@ impl Db {
     #[cfg(test)]
     pub fn open_in_memory() -> Result<Self> {
         let conn = Connection::open_in_memory()?;
+        // Keep relational behavior identical to `open`: V18 relies on the
+        // `mount.sync_folder_id` cascade, and SQLite disables foreign keys per
+        // connection unless explicitly enabled.
+        conn.pragma_update(None, "foreign_keys", "ON")?;
         let db = Self {
             conn: Mutex::new(conn),
         };

crates/pdfs-core/src/db/mounts.rs

@@ -0,0 +1,178 @@
+//! Unified mount/location presentation rows.
+//!
+//! A device row stores only its `sync_folder` identity. Its path, remote root,
+//! mode, state, pending mode, and last-sync timestamp are selected from
+//! `sync_folder`, which remains the authoritative sync-engine table.
+
+use rusqlite::params;
+
+use super::Db;
+use crate::mounts::{MountAccess, MountKind, MountMode, MountSpec};
+use crate::{Error, Result};
+
+impl Db {
+    /// Project the configured primary mount into the presentation table.
+    ///
+    /// `root_share_id` is optional because an offline startup may only have the
+    /// cached root node. A previously learned share id is retained only when it
+    /// belongs to the same root uid.
+    pub fn mount_upsert_my_files(
+        &self,
+        local_path: &str,
+        root_uid: &str,
+        root_share_id: Option<&str>,
+    ) -> Result<i64> {
+        let mut conn = self.conn.lock();
+        let tx = conn.transaction()?;
+        let updated = tx.execute(
+            "UPDATE mount
+                SET local_path = ?1,
+                    root_share_id = CASE
+                        WHEN root_uid = ?2
+                          THEN COALESCE(NULLIF(?3, ''), root_share_id)
+                        ELSE COALESCE(?3, '')
+                    END,
+                    root_uid = ?2,
+                    mode = 'ondemand',
+                    access = 'rw'
+              WHERE kind = 'myfiles'",
+            params![local_path, root_uid, root_share_id],
+        )?;
+        let id = if updated == 0 {
+            tx.execute(
+                "INSERT INTO mount
+                    (kind, local_path, root_uid, root_share_id, mode, access)
+                 VALUES ('myfiles', ?1, ?2, COALESCE(?3, ''), 'ondemand', 'rw')",
+                params![local_path, root_uid, root_share_id],
+            )?;
+            tx.last_insert_rowid()
+        } else {
+            tx.query_row("SELECT id FROM mount WHERE kind = 'myfiles'", [], |row| {
+                row.get(0)
+            })?
+        };
+        tx.commit()?;
+        Ok(id)
+    }
+
+    /// Store a share id learned asynchronously, but only if the projection still
+    /// names the root it was resolved for.
+    pub fn mount_repair_my_files_share_id(
+        &self,
+        root_uid: &str,
+        root_share_id: &str,
+    ) -> Result<bool> {
+        let conn = self.conn.lock();
+        Ok(conn.execute(
+            "UPDATE mount
+                SET root_share_id = ?2
+              WHERE kind = 'myfiles'
+                AND root_uid = ?1",
+            params![root_uid, root_share_id],
+        )? > 0)
+    }
+
+    /// Every local location, with device fields joined from `sync_folder`.
+    pub fn mount_list(&self) -> Result<Vec<MountSpec>> {
+        let conn = self.conn.lock();
+        let mut stmt = conn.prepare(
+            "SELECT
+                 m.id,
+                 m.kind,
+                 m.sync_folder_id,
+                 m.share_root_uid,
+                 CASE WHEN m.kind = 'device' THEN sf.local_path ELSE m.local_path END,
+                 CASE WHEN m.kind = 'device' THEN sf.remote_uid ELSE m.root_uid END,
+                 CASE WHEN m.kind = 'device' THEN sf.remote_share_id ELSE m.root_share_id END,
+                 CASE WHEN m.kind = 'device' THEN sf.mode ELSE m.mode END,
+                 m.access,
+                 CASE WHEN m.kind = 'device' THEN sf.state ELSE 'idle' END,
+                 CASE WHEN m.kind = 'device' THEN sf.last_sync ELSE 0 END,
+                 CASE WHEN m.kind = 'device' THEN sf.pending_mode ELSE NULL END
+               FROM mount AS m
+               LEFT JOIN sync_folder AS sf ON sf.id = m.sync_folder_id
+              ORDER BY CASE m.kind
+                         WHEN 'myfiles' THEN 0
+                         WHEN 'device' THEN 1
+                         ELSE 2
+                       END,
+                       m.id",
+        )?;
+        let rows = stmt.query_map([], |row| {
+            Ok((
+                row.get::<_, i64>(0)?,
+                row.get::<_, String>(1)?,
+                row.get::<_, Option<i64>>(2)?,
+                row.get::<_, Option<String>>(3)?,
+                row.get::<_, String>(4)?,
+                row.get::<_, String>(5)?,
+                row.get::<_, String>(6)?,
+                row.get::<_, String>(7)?,
+                row.get::<_, String>(8)?,
+                row.get::<_, String>(9)?,
+                row.get::<_, i64>(10)?,
+                row.get::<_, Option<String>>(11)?,
+            ))
+        })?;
+
+        let mut mounts = Vec::new();
+        for row in rows {
+            let (
+                id,
+                kind,
+                sync_folder_id,
+                share_root_uid,
+                local_path,
+                root_uid,
+                root_share_id,
+                mode,
+                access,
+                state,
+                last_sync,
+                pending_mode,
+            ) = row?;
+            let kind = match kind.as_str() {
+                "myfiles" => MountKind::MyFiles,
+                "device" => MountKind::Device {
+                    sync_folder_id: sync_folder_id.ok_or_else(|| {
+                        Error::Other(format!("mount row {id} has no sync_folder_id"))
+                    })?,
+                },
+                "shared" => MountKind::Shared {
+                    share_root_uid: share_root_uid.ok_or_else(|| {
+                        Error::Other(format!("mount row {id} has no share_root_uid"))
+                    })?,
+                },
+                other => {
+                    return Err(Error::Other(format!(
+                        "mount row {id} has unknown kind {other:?}"
+                    )));
+                }
+            };
+            let access = match access.as_str() {
+                "rw" => MountAccess::Rw,
+                "ro" => MountAccess::Ro,
+                other => {
+                    return Err(Error::Other(format!(
+                        "mount row {id} has unknown access {other:?}"
+                    )));
+                }
+            };
+            mounts.push(MountSpec {
+                id,
+                kind,
+                local_path,
+                root_uid,
+                root_share_id,
+                mode: MountMode::from(mode.as_str()),
+                access,
+                state,
+                last_sync,
+                pending_mode: pending_mode.as_deref().map(MountMode::from),
+                mounted: false,
+                progress: None,
+            });
+        }
+        Ok(mounts)
+    }
+}

crates/pdfs-core/src/db/sync.rs

@@ -115,14 +115,17 @@ impl Db {
         Ok(n > 0)
     }
 
-    /// Update a synced folder's mode (`mirror`/`ondemand`). The folder has
-    /// reached the mode it was asked for, so any queued request is satisfied and
-    /// cleared in the same write — a `pending_mode` outliving the switch it asked
-    /// for would have the engine try to apply it again on the next pass.
+    /// Update a synced folder's mode (`mirror`/`ondemand`). A queued request for
+    /// that mode is satisfied and cleared in the same write. An opposite pending
+    /// mode arrived after this transition started, so preserve it as the newer
+    /// last-request intent.
     pub fn sync_folder_set_mode(&self, id: i64, mode: &str) -> Result<()> {
         let conn = self.conn.lock();
         conn.execute(
-            "UPDATE sync_folder SET mode = ?2, pending_mode = NULL WHERE id = ?1",
+            "UPDATE sync_folder
+                SET mode = ?2,
+                    pending_mode = CASE WHEN pending_mode = ?2 THEN NULL ELSE pending_mode END
+              WHERE id = ?1",
             params![id, mode],
         )?;
         Ok(())
@@ -139,6 +142,18 @@ impl Db {
         Ok(())
     }
 
+    /// Withdraw a queued mode only if it is still the intent the caller
+    /// inspected. A newer request must survive an older settlement attempt.
+    pub fn sync_folder_clear_pending_mode_if(&self, id: i64, expected: &str) -> Result<bool> {
+        let conn = self.conn.lock();
+        let changed = conn.execute(
+            "UPDATE sync_folder SET pending_mode = NULL
+             WHERE id = ?1 AND pending_mode = ?2",
+            params![id, expected],
+        )?;
+        Ok(changed > 0)
+    }
+
     /// Update a synced folder's state and stamp `last_sync` to now.
     pub fn sync_folder_set_state(&self, id: i64, state: &str, last_sync: i64) -> Result<()> {
         let conn = self.conn.lock();

crates/pdfs-core/src/db/tests.rs

@@ -1323,15 +1323,17 @@ fn migration_v17_adds_share_access_to_a_v16_fixture() {
             .as_nanos()
     ));
     {
-        // V17 only adds `share_access`, so removing it and restoring the version
-        // stamp produces the exact schema a released V16 database had.
+        // Remove the later tables too, so restoring the version stamp produces
+        // the exact schema a released V16 database had.
         let db = Db::open(&path).unwrap();
         db.set_state_str("fixture_value", "survives").unwrap();
     }
     {
         let conn = rusqlite::Connection::open(&path).unwrap();
         conn.execute_batch(
-            "DROP TABLE share_access;
+            "DROP TRIGGER mount_sync_folder_insert;
+             DROP TABLE mount;
+             DROP TABLE share_access;
              UPDATE sync_state SET value = '16' WHERE key = 'schema_version';",
         )
         .unwrap();
@@ -1347,13 +1349,211 @@ fn migration_v17_adds_share_access_to_a_v16_fixture() {
     assert_eq!(db.share_access(&root).unwrap(), Some(crate::Access::Viewer));
     assert_eq!(
         db.state_str("schema_version").unwrap().as_deref(),
-        Some("17")
+        Some("18")
     );
 
     drop(db);
     let _ = std::fs::remove_file(path);
 }
 
+#[test]
+fn migration_v18_projects_v17_sync_folders_and_cascades_deletes() {
+    let path = std::env::temp_dir().join(format!(
+        "pdfs-db-v17-fixture-{}-{}.db",
+        std::process::id(),
+        std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap()
+            .as_nanos()
+    ));
+    {
+        // Pin the released V17 shape that V18 reads instead of deriving a
+        // fixture from today's head schema and dropping newer objects.
+        let conn = rusqlite::Connection::open(&path).unwrap();
+        conn.execute_batch(
+            "CREATE TABLE sync_state (key TEXT PRIMARY KEY, value TEXT);
+             INSERT INTO sync_state VALUES ('schema_version', '17');
+             INSERT INTO sync_state VALUES ('fixture_value', 'survives');
+             CREATE TABLE sync_folder (
+               id              INTEGER PRIMARY KEY,
+               local_path      TEXT NOT NULL UNIQUE,
+               remote_uid      TEXT NOT NULL,
+               remote_share_id TEXT NOT NULL,
+               mode            TEXT NOT NULL DEFAULT 'mirror',
+               state           TEXT NOT NULL DEFAULT 'idle',
+               last_sync       INTEGER NOT NULL DEFAULT 0,
+               pending_mode    TEXT
+             );
+             CREATE TABLE sync_entry (
+               folder_id   INTEGER NOT NULL,
+               rel_path    TEXT NOT NULL,
+               remote_uid  TEXT,
+               local_mtime INTEGER NOT NULL DEFAULT 0,
+               local_size  INTEGER NOT NULL DEFAULT 0,
+               remote_hash TEXT,
+               remote_rev  TEXT,
+               PRIMARY KEY (folder_id, rel_path)
+             );
+             INSERT INTO sync_folder
+               (id, local_path, remote_uid, remote_share_id, mode, state, last_sync)
+             VALUES
+               (41, '/home/me/Existing', 'vol~existing', 'share-existing',
+                'ondemand', 'syncing', 123);
+             CREATE TABLE share_access (
+               root_uid TEXT PRIMARY KEY,
+               access   TEXT NOT NULL
+                        CHECK (access IN ('owner', 'editor', 'viewer', 'unknown'))
+             );",
+        )
+        .unwrap();
+    }
+
+    let db = Db::open(&path).unwrap();
+    let existing = 41;
+    let locations = db.mount_list().unwrap();
+    assert_eq!(locations.len(), 1);
+    let projected = &locations[0];
+    assert!(matches!(
+        projected.kind,
+        crate::mounts::MountKind::Device {
+            sync_folder_id
+        } if sync_folder_id == existing
+    ));
+    assert_eq!(projected.local_path, "/home/me/Existing");
+    assert_eq!(projected.mode, crate::mounts::MountMode::OnDemand);
+    assert_eq!(projected.state, "syncing");
+    assert_eq!(projected.last_sync, 123);
+    assert_eq!(
+        db.state_str("fixture_value").unwrap().as_deref(),
+        Some("survives")
+    );
+
+    let added = db
+        .sync_folder_add("/home/me/New", "vol~new", "share-new")
+        .unwrap();
+    assert_eq!(db.mount_list().unwrap().len(), 2);
+    assert!(db.sync_folder_remove(added).unwrap());
+    assert_eq!(
+        db.mount_list().unwrap().len(),
+        1,
+        "the V18 foreign key must remove the device projection"
+    );
+    assert_eq!(
+        db.state_str("schema_version").unwrap().as_deref(),
+        Some("18")
+    );
+
+    drop(db);
+    let _ = std::fs::remove_file(path);
+}
+
+#[test]
+fn my_files_projection_retains_share_id_only_for_the_same_root() {
+    let db = Db::open_in_memory().unwrap();
+    let id = db
+        .mount_upsert_my_files("/home/me/ProtonDrive", "vol~root", Some("share-main"))
+        .unwrap();
+    let same_id = db
+        .mount_upsert_my_files("/mnt/proton", "vol~root", None)
+        .unwrap();
+    assert_eq!(same_id, id);
+    assert_eq!(db.mount_list().unwrap()[0].root_share_id, "share-main");
+
+    db.mount_upsert_my_files("/mnt/proton", "vol~root-new", None)
+        .unwrap();
+
+    let locations = db.mount_list().unwrap();
+    assert_eq!(locations.len(), 1);
+    assert!(matches!(
+        locations[0].kind,
+        crate::mounts::MountKind::MyFiles
+    ));
+    assert_eq!(locations[0].local_path, "/mnt/proton");
+    assert_eq!(locations[0].root_uid, "vol~root-new");
+    assert_eq!(
+        locations[0].root_share_id, "",
+        "a share id must never be paired with a different root uid"
+    );
+}
+
+#[test]
+fn my_files_share_id_repair_is_conditional_on_root_uid() {
+    let db = Db::open_in_memory().unwrap();
+    db.mount_upsert_my_files("/mnt/proton", "vol~root", None)
+        .unwrap();
+
+    assert!(
+        !db.mount_repair_my_files_share_id("vol~stale", "share-stale")
+            .unwrap()
+    );
+    assert_eq!(db.mount_list().unwrap()[0].root_share_id, "");
+
+    assert!(
+        db.mount_repair_my_files_share_id("vol~root", "share-current")
+            .unwrap()
+    );
+    assert_eq!(db.mount_list().unwrap()[0].root_share_id, "share-current");
+}
+
+#[test]
+fn mount_constraints_reject_malformed_kinds_and_missing_sync_folders() {
+    let db = Db::open_in_memory().unwrap();
+    let conn = db.conn.lock();
+
+    assert!(
+        conn.execute(
+            "INSERT INTO mount (kind, local_path, root_uid)
+             VALUES ('device', '/tmp/device', 'vol~device')",
+            [],
+        )
+        .is_err(),
+        "a device projection without sync_folder_id must fail its CHECK"
+    );
+    assert!(
+        conn.execute(
+            "INSERT INTO mount (kind, sync_folder_id)
+             VALUES ('device', 999999)",
+            [],
+        )
+        .is_err(),
+        "a device projection cannot reference a missing sync folder"
+    );
+    assert!(
+        conn.execute(
+            "INSERT INTO mount (kind, share_root_uid, local_path, root_uid)
+             VALUES ('myfiles', 'vol~shared', '/tmp/root', 'vol~root')",
+            [],
+        )
+        .is_err(),
+        "kind-specific columns must not be mixed"
+    );
+}
+
+#[test]
+fn deleting_a_device_projection_does_not_delete_its_sync_folder() {
+    let db = Db::open_in_memory().unwrap();
+    let folder_id = db
+        .sync_folder_add("/home/me/Keep", "vol~keep", "share-keep")
+        .unwrap();
+    {
+        let conn = db.conn.lock();
+        assert_eq!(
+            conn.execute(
+                "DELETE FROM mount WHERE kind = 'device' AND sync_folder_id = ?1",
+                [folder_id],
+            )
+            .unwrap(),
+            1
+        );
+    }
+
+    assert!(db.sync_folder_get(folder_id).unwrap().is_some());
+    assert!(
+        db.mount_list().unwrap().is_empty(),
+        "the foreign key direction must not recreate or delete sync authority"
+    );
+}
+
 #[test]
 fn refuses_a_schema_newer_than_the_running_build() {
     let path = std::env::temp_dir().join(format!(
@@ -1953,6 +2153,56 @@ fn pending_mode_is_queued_until_the_mode_is_reached() {
     assert_eq!(folder.pending_mode, None);
 }
 
+#[test]
+fn mode_commit_clears_only_the_intent_it_satisfies() {
+    let db = Db::open_in_memory().unwrap();
+    let id = db
+        .sync_folder_add("/tmp/pdfs-mode-intent", "vol~folder", "share")
+        .unwrap();
+
+    db.sync_folder_set_pending_mode(id, Some("ondemand"))
+        .unwrap();
+    db.sync_folder_set_mode(id, "ondemand").unwrap();
+    assert_eq!(db.sync_folder_get(id).unwrap().unwrap().pending_mode, None);
+
+    db.sync_folder_set_pending_mode(id, Some("mirror")).unwrap();
+    db.sync_folder_set_mode(id, "ondemand").unwrap();
+    assert_eq!(
+        db.sync_folder_get(id)
+            .unwrap()
+            .unwrap()
+            .pending_mode
+            .as_deref(),
+        Some("mirror")
+    );
+}
+
+#[test]
+fn clearing_a_stale_pending_mode_preserves_the_newer_intent() {
+    let db = Db::open_in_memory().unwrap();
+    let id = db
+        .sync_folder_add("/tmp/pdfs-pending", "vol~folder", "share")
+        .unwrap();
+    db.sync_folder_set_pending_mode(id, Some("ondemand"))
+        .unwrap();
+    db.sync_folder_set_pending_mode(id, Some("mirror")).unwrap();
+
+    assert!(
+        !db.sync_folder_clear_pending_mode_if(id, "ondemand")
+            .unwrap()
+    );
+    assert_eq!(
+        db.sync_folder_get(id)
+            .unwrap()
+            .unwrap()
+            .pending_mode
+            .as_deref(),
+        Some("mirror")
+    );
+    assert!(db.sync_folder_clear_pending_mode_if(id, "mirror").unwrap());
+    assert_eq!(db.sync_folder_get(id).unwrap().unwrap().pending_mode, None);
+}
+
 #[test]
 fn mirror_uid_lookup_covers_the_root_and_synced_descendants_only() {
     let db = Db::open_in_memory().unwrap();

crates/pdfs-core/src/lib.rs

@@ -10,6 +10,7 @@ pub mod control;
 pub mod db;
 pub mod error;
 pub mod localindex;
+pub mod mounts;
 pub mod profile;
 pub mod search;
 pub mod service;

crates/pdfs-core/src/mounts.rs

@@ -0,0 +1,146 @@
+//! Unified presentation model for every local Proton Drive location.
+//!
+//! Device sync state remains owned by `sync_folder`; these types are the stable
+//! shape exposed to front-ends after that state is joined with the `mount`
+//! presentation table.
+
+use std::fmt;
+
+use serde::{Deserialize, Serialize};
+
+use crate::control::SyncProgress;
+
+/// How a location makes remote content available locally.
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[serde(rename_all = "lowercase")]
+pub enum MountMode {
+    /// A full local copy reconciled in both directions.
+    Mirror,
+    /// A FUSE session that fetches content as it is accessed.
+    OnDemand,
+    /// A mode written by a newer client.
+    #[default]
+    #[serde(other)]
+    Unknown,
+}
+
+impl MountMode {
+    pub const fn as_str(self) -> &'static str {
+        match self {
+            Self::Mirror => "mirror",
+            Self::OnDemand => "ondemand",
+            Self::Unknown => "unknown",
+        }
+    }
+}
+
+impl From<&str> for MountMode {
+    fn from(value: &str) -> Self {
+        match value {
+            "mirror" => Self::Mirror,
+            "ondemand" => Self::OnDemand,
+            _ => Self::Unknown,
+        }
+    }
+}
+
+impl fmt::Display for MountMode {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+/// Whether the location itself was declared writable.
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq)]
+#[serde(rename_all = "lowercase")]
+pub enum MountAccess {
+    Rw,
+    Ro,
+}
+
+impl MountAccess {
+    pub const fn as_str(self) -> &'static str {
+        match self {
+            Self::Rw => "rw",
+            Self::Ro => "ro",
+        }
+    }
+}
+
+impl fmt::Display for MountAccess {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+/// What remote root a local location presents.
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
+#[serde(tag = "kind", rename_all = "lowercase")]
+pub enum MountKind {
+    /// The account's primary My Files mount.
+    MyFiles,
+    /// A folder owned by this machine's Proton Drive device.
+    Device { sync_folder_id: i64 },
+    /// A future standalone mount rooted at a node shared with this account.
+    Shared { share_root_uid: String },
+}
+
+/// One local Proton Drive location as exposed over the control protocol.
+#[derive(Serialize, Deserialize, Debug, Clone)]
+pub struct MountSpec {
+    /// Presentation-row id. Device actions use the `sync_folder_id` carried by
+    /// [`MountKind::Device`], not this id.
+    pub id: i64,
+    pub kind: MountKind,
+    pub local_path: String,
+    pub root_uid: String,
+    pub root_share_id: String,
+    pub mode: MountMode,
+    pub access: MountAccess,
+    /// `idle` | `syncing` | `error` | `conflict` for device locations.
+    pub state: String,
+    /// Last successful device sync, epoch seconds; `0` when not applicable.
+    pub last_sync: i64,
+    /// A device mode switch accepted but not yet applied.
+    #[serde(default)]
+    pub pending_mode: Option<MountMode>,
+    /// Whether a FUSE session currently owns `local_path`.
+    #[serde(default)]
+    pub mounted: bool,
+    /// Live device sync progress, absent when no pass is running.
+    #[serde(default)]
+    pub progress: Option<SyncProgress>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn mount_mode_keeps_legacy_wire_strings() {
+        assert_eq!(
+            serde_json::to_string(&MountMode::Mirror).unwrap(),
+            r#""mirror""#
+        );
+        assert_eq!(
+            serde_json::to_string(&MountMode::OnDemand).unwrap(),
+            r#""ondemand""#
+        );
+        assert_eq!(
+            serde_json::from_str::<MountMode>(r#""mirror""#).unwrap(),
+            MountMode::Mirror
+        );
+        assert_eq!(
+            serde_json::from_str::<MountMode>(r#""ondemand""#).unwrap(),
+            MountMode::OnDemand
+        );
+    }
+
+    #[test]
+    fn mount_mode_tolerates_unknown_values() {
+        assert_eq!(
+            serde_json::from_str::<MountMode>(r#""from-the-future""#).unwrap(),
+            MountMode::Unknown
+        );
+    }
+}

crates/pdfs-core/src/profile.rs

@@ -17,6 +17,8 @@
 
 use serde::{Deserialize, Serialize};
 
+use crate::mounts::MountMode;
+
 /// File name of the profile inside the device root folder.
 pub const PROFILE_FILE_NAME: &str = "profile.json";
 
@@ -39,8 +41,9 @@ pub struct ProfileFolder {
     /// Absolute local path it was synced to. A *suggestion* on restore: another
     /// machine may have a different username, home, or mount layout.
     pub local_path: String,
-    /// `mirror` or `ondemand`.
-    pub mode: String,
+    /// `mirror` or `ondemand`. Unknown values written by a newer client do not
+    /// invalidate the rest of the profile.
+    pub mode: MountMode,
 }
 
 /// One pinned node (P5 pins), as `pins` stores it.
@@ -114,7 +117,7 @@ mod tests {
             folders: vec![ProfileFolder {
                 remote_uid: "vol~link".to_string(),
                 local_path: "/home/nils/Documents".to_string(),
-                mode: "mirror".to_string(),
+                mode: MountMode::Mirror,
             }],
             pins: vec![ProfilePin {
                 uid: "vol~pin".to_string(),
@@ -133,6 +136,52 @@ mod tests {
         assert_eq!(Profile::parse(&p.to_bytes().unwrap()).unwrap(), p);
     }
 
+    #[test]
+    fn profile_keeps_legacy_mode_bytes_and_parses_old_documents() {
+        let bytes = sample().to_bytes().unwrap();
+        let json = String::from_utf8(bytes).unwrap();
+        assert!(json.contains(r#""mode": "mirror""#));
+
+        let old = br#"{
+            "version": 1,
+            "device_uid": "dev-1",
+            "hostname": "old-client",
+            "saved_at": 1,
+            "folders": [
+                {
+                    "remote_uid": "vol~mirror",
+                    "local_path": "/home/me/Mirror",
+                    "mode": "mirror"
+                },
+                {
+                    "remote_uid": "vol~ondemand",
+                    "local_path": "/home/me/Cloud",
+                    "mode": "ondemand"
+                }
+            ]
+        }"#;
+        let profile = Profile::parse(old).unwrap();
+        assert_eq!(profile.folders[0].mode, MountMode::Mirror);
+        assert_eq!(profile.folders[1].mode, MountMode::OnDemand);
+    }
+
+    #[test]
+    fn unknown_folder_mode_does_not_reject_profile() {
+        let json = br#"{
+            "version": 1,
+            "device_uid": "dev-1",
+            "hostname": "future-client",
+            "saved_at": 1,
+            "folders": [{
+                "remote_uid": "vol~future",
+                "local_path": "/home/me/Future",
+                "mode": "streaming"
+            }]
+        }"#;
+        let profile = Profile::parse(json).unwrap();
+        assert_eq!(profile.folders[0].mode, MountMode::Unknown);
+    }
+
     /// A field added by a later client must not break this one's restore: the
     /// common subset is still usable, which is the whole reason `version` is
     /// reserved for breaking changes only.

crates/pdfs-fuse/src/control.rs

@@ -561,6 +561,10 @@ fn handle_control_conn(core: &Core, username: &str, mountpoint: &Path, stream: U
             Ok(items) => CtlResponse::Devices { items },
             Err(e) => CtlResponse::error(e),
         },
+        Ok(CtlRequest::ListLocations) => match core.list_locations() {
+            Ok(items) => CtlResponse::Locations { items },
+            Err(e) => CtlResponse::error(e),
+        },
         Ok(CtlRequest::RenameDevice { uid, name }) => match core.rename_device(&uid, &name) {
             Ok(()) => CtlResponse::Ok {
                 message: format!("renamed device to {name}"),

crates/pdfs-fuse/src/devices.rs

@@ -11,12 +11,13 @@ use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
-use fuser::{Config, MountOption, Session};
 use parking_lot::Mutex;
 use pdfs_core::control::{
-    ActivityKind, DeviceInfo, JobItem, SyncFolderInfo, SyncPhase, SyncProgress,
+    ActivityKind, DeviceInfo, JobItem, MountKind, MountSpec, SyncFolderInfo, SyncPhase,
+    SyncProgress,
 };
 use pdfs_core::db::{StoredDevice, StoredSyncFolder};
+use pdfs_core::mounts::MountMode;
 use pdfs_core::{CoreError, CoreResult};
 use proton_drive_rs::proton_sdk::ids::{DeviceUid, NodeUid};
 use proton_drive_rs::{DeviceType, Node};
@@ -25,11 +26,69 @@ use tracing::{info, warn};
 
 use super::sync::{self, base_name};
 use super::{
-    BackgroundSession, Core, ProtonFs, State, SwitchBlocked, clear_stale_mount, device_type_str,
-    dir_is_empty, evict_dir_contents, fuse_connection_id, is_stale_mount, now_secs, parse_uid,
-    sync_folder_info, this_hostname, umount_session_unblocked,
+    Core, SecondaryInsertRejection, SecondaryMount, State, SwitchBlocked, clear_stale_mount,
+    device_type_str, dir_is_empty, evict_dir_contents, fuse_connection_id, is_stale_mount,
+    now_secs, parse_uid, spawn_session, sync_folder_info, this_hostname,
 };
 
+/// A mode that this daemon is allowed to apply.
+///
+/// Persisted and control-protocol values first pass through [`MountMode`] so
+/// forward-compatible unknown values never reach either destructive transition.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(crate) enum ApplicableMountMode {
+    Mirror,
+    OnDemand,
+}
+
+impl ApplicableMountMode {
+    fn parse(value: &str) -> Result<Self, String> {
+        match MountMode::from(value) {
+            MountMode::Mirror => Ok(Self::Mirror),
+            MountMode::OnDemand => Ok(Self::OnDemand),
+            MountMode::Unknown => Err(format!("unknown mode {value:?} (want mirror|ondemand)")),
+        }
+    }
+
+    const fn as_str(self) -> &'static str {
+        match self {
+            Self::Mirror => "mirror",
+            Self::OnDemand => "ondemand",
+        }
+    }
+}
+
+fn select_mode_transition(
+    current: &str,
+    target: &str,
+) -> Result<Option<ApplicableMountMode>, String> {
+    let current = ApplicableMountMode::parse(current)
+        .map_err(|error| format!("unsupported current mode: {error}"))?;
+    let target = ApplicableMountMode::parse(target)
+        .map_err(|error| format!("unsupported target mode: {error}"))?;
+    Ok((current != target).then_some(target))
+}
+
+fn restore_snapshot_matches(snapshot: &StoredSyncFolder, current: &StoredSyncFolder) -> bool {
+    current.id == snapshot.id
+        && current.local_path == snapshot.local_path
+        && current.remote_uid == snapshot.remote_uid
+        && current.remote_share_id == snapshot.remote_share_id
+        && current.mode == "ondemand"
+}
+
+fn pending_intent_matches(pending: Option<&str>, expected: Option<&str>) -> bool {
+    expected.is_none_or(|expected| pending == Some(expected))
+}
+
+fn commit_after_teardown<T, E>(
+    teardown: Result<(), E>,
+    commit: impl FnOnce() -> T,
+) -> Result<T, E> {
+    teardown?;
+    Ok(commit())
+}
+
 /// The device uid this machine has been adopted into, from `config.json`.
 ///
 /// Read fresh each time rather than cached on [`Core`]: adoption rewrites the
@@ -57,6 +116,22 @@ pub(crate) fn set_adopted_device_uid(uid: Option<&str>) -> CoreResult<()> {
 impl Core {
     // ---- devices ----------------------------------------------------------
 
+    /// Unified local-location listing for the control protocol.
+    pub(crate) fn list_locations(&self) -> CoreResult<Vec<MountSpec>> {
+        let progress = self.sync_progress.lock();
+        let mut locations = self
+            .db
+            .mount_list()
+            .map_err(|e| CoreError::internal(format!("db: {e:?}")))?;
+        for location in &mut locations {
+            location.mounted = self.states.is_mounted_at(Path::new(&location.local_path));
+            if let MountKind::Device { sync_folder_id } = &location.kind {
+                location.progress = progress.get(sync_folder_id).cloned();
+            }
+        }
+        Ok(locations)
+    }
+
     /// List the account's registered devices, flagging the one *this* machine
     /// syncs to so a front-end can treat it as more than another computer in the
     /// list — deleting it takes this machine's synced folders down with it.
@@ -439,6 +514,8 @@ impl Core {
     /// Remove a synced folder from the sync set. `delete_remote` also deletes its
     /// folder under the device root; otherwise the cloud copy is left in place.
     pub(crate) fn remove_sync_folder(&self, id: i64, delete_remote: bool) -> CoreResult<()> {
+        let lock = self.sync_lock(id);
+        let _guard = lock.lock();
         let folder = self
             .db
             .sync_folder_get(id)
@@ -449,19 +526,16 @@ impl Core {
         // folder the daemon no longer tracks, and nothing would ever unmount it.
         // Tear it down first — including before trashing the remote tree it
         // serves, which would otherwise leave it answering for deleted nodes.
-        if let Some((session, conn)) = self.mounts.lock().remove(&id) {
-            if let Err(e) = umount_session_unblocked(session, conn) {
-                warn!(id, error = %e, "unmount on-demand folder failed");
-            } else {
-                info!(id, path = %folder.local_path, "unmounted on-demand folder");
-            }
-        }
-        if delete_remote
-            && let Some(uid) = parse_uid(&folder.remote_uid)
-            && let Err(e) = self.rt.block_on(self.client.trash_nodes(&[uid]))
+        if let Some(mount) = self.mounts.lock().remove(&id)
+            && let Err(error) = commit_after_teardown(mount.teardown(), || ())
         {
-            warn!(id, error = %e, "delete remote device folder failed");
+            let _ = self.db.sync_folder_set_state(id, "error", now_secs());
+            return Err(CoreError::internal(format!(
+                "cannot remove synced folder: unmount {} failed: {error}",
+                folder.local_path
+            )));
         }
+        info!(id, path = %folder.local_path, "unmounted on-demand folder");
         if !self
             .db
             .sync_folder_remove(id)
@@ -469,8 +543,15 @@ impl Core {
         {
             return Err(CoreError::invalid(format!("no synced folder with id {id}")));
         }
+        drop(_guard);
         // Stop watching the folder we just dropped.
         let _ = self.sync_tx.send(sync::SyncMsg::Rewatch);
+        if delete_remote
+            && let Some(uid) = parse_uid(&folder.remote_uid)
+            && let Err(e) = self.rt.block_on(self.client.trash_nodes(&[uid]))
+        {
+            warn!(id, error = %e, "delete remote device folder failed");
+        }
         Ok(())
     }
 
@@ -507,13 +588,9 @@ impl Core {
         // session is the only one that knows these inodes, so it must be the one
         // notified about them. Filled in by `spawn_ondemand_mount`.
         fork.notifier = Arc::new(OnceLock::new());
+        fork.session_live = Arc::new(std::sync::atomic::AtomicBool::new(false));
         // Size upgrades are keyed by inode, which is per-fork too.
         fork.size_upgrades = Arc::new(Mutex::new(HashMap::new()));
-        // The daemon's single drain thread belongs to the primary mount, but it
-        // drains ops for nodes that live *here*. Publishing this inode space is
-        // what lets it find them; without it a queued create's node keeps its
-        // `local~` uid after the upload lands and reads it as empty (B74).
-        fork.register_state();
         fork
     }
 
@@ -529,49 +606,84 @@ impl Core {
     /// running — which, seeing a queued `ondemand`, also stops pulling down files
     /// it is about to evict ([`Core::push_pass`]).
     pub(crate) fn request_sync_folder_mode(&self, id: i64, mode: &str) -> CoreResult<String> {
-        if mode != "mirror" && mode != "ondemand" {
-            return Err(CoreError::invalid(format!(
-                "unknown mode {mode:?} (want mirror|ondemand)"
-            )));
-        }
+        // Preserve the existing control behavior: reject an unsupported request
+        // before looking up the folder, then validate the persisted source too.
+        let target_mode = ApplicableMountMode::parse(mode).map_err(CoreError::invalid)?;
         let folder = self
             .db
             .sync_folder_get(id)
             .map_err(|e| CoreError::internal(format!("db: {e:?}")))?
             .ok_or_else(|| CoreError::not_found(format!("no synced folder with id {id}")))?;
-        if folder.mode == mode {
-            // Already there. A queued switch the other way is the user changing
-            // their mind back before it landed, so withdraw it.
-            if folder.pending_mode.is_some() {
-                self.db
-                    .sync_folder_set_pending_mode(id, None)
-                    .map_err(|e| CoreError::internal(format!("db: {e:?}")))?;
-                let _ = self.sync_tx.send(sync::SyncMsg::Rewatch);
-                return Ok(format!("staying {mode}"));
+        let current_mode = ApplicableMountMode::parse(&folder.mode)
+            .map_err(|error| CoreError::invalid(format!("unsupported current mode: {error}")))?;
+        if current_mode == target_mode {
+            let mode_str = target_mode.as_str();
+            let lock = self.sync_lock(id);
+            match lock.try_lock() {
+                Some(_guard) => {
+                    let current = self
+                        .db
+                        .sync_folder_get(id)
+                        .map_err(|e| CoreError::internal(format!("db: {e:?}")))?
+                        .ok_or_else(|| {
+                            CoreError::not_found(format!("no synced folder with id {id}"))
+                        })?;
+                    let locked_mode =
+                        ApplicableMountMode::parse(&current.mode).map_err(|error| {
+                            CoreError::invalid(format!("unsupported current mode: {error}"))
+                        })?;
+                    if locked_mode == target_mode {
+                        self.db
+                            .sync_folder_set_pending_mode(id, None)
+                            .map_err(|e| CoreError::internal(format!("db: {e:?}")))?;
+                        let _ = self.sync_tx.send(sync::SyncMsg::Rewatch);
+                        return Ok(if current.pending_mode.is_some() {
+                            format!("staying {mode_str}")
+                        } else {
+                            format!("already {mode_str}")
+                        });
+                    }
+                }
+                None => {
+                    // A transition or reconcile owns the folder lock. Record
+                    // this request instead of clearing the old target: if a
+                    // queued switch already passed its recheck, its mode write
+                    // preserves this opposite marker and the engine switches
+                    // back next.
+                    self.db
+                        .sync_folder_set_pending_mode(id, Some(mode_str))
+                        .map_err(|e| CoreError::internal(format!("db: {e:?}")))?;
+                    let _ = self.sync_tx.send(sync::SyncMsg::Reconcile(id));
+                    return Ok(format!("staying {mode_str}"));
+                }
             }
-            return Ok(format!("already {mode}"));
         }
+        let mode = target_mode;
+        let mode_str = target_mode.as_str();
 
-        match self.apply_sync_folder_mode(id, mode) {
+        match self.apply_sync_folder_mode(id, mode, None) {
             Ok(message) => Ok(message),
             // Not switchable this instant — remember the intent instead of making
             // the user retry, and kick a pass to clear whatever is in the way.
             Err(SwitchBlocked::NotNow) => {
                 self.db
-                    .sync_folder_set_pending_mode(id, Some(mode))
+                    .sync_folder_set_pending_mode(id, Some(mode_str))
                     .map_err(|e| CoreError::internal(format!("db: {e:?}")))?;
                 let _ = self.sync_tx.send(sync::SyncMsg::Reconcile(id));
                 Ok(match mode {
-                    "ondemand" => format!(
+                    ApplicableMountMode::OnDemand => format!(
                         "{} will go on-demand once its local changes are uploaded",
                         base_name(&folder.local_path)
                     ),
-                    _ => format!(
+                    ApplicableMountMode::Mirror => format!(
                         "{} will start mirroring once the current sync finishes",
                         base_name(&folder.local_path)
                     ),
                 })
             }
+            Err(SwitchBlocked::Superseded) => Err(CoreError::internal(
+                "direct mode request was unexpectedly superseded",
+            )),
             Err(SwitchBlocked::Failed(e)) => Err(CoreError::internal(e)),
         }
     }
@@ -585,27 +697,45 @@ impl Core {
         let Ok(Some(folder)) = self.db.sync_folder_get(id) else {
             return;
         };
-        let Some(mode) = folder.pending_mode.as_deref() else {
+        let Some(expected_mode) = folder.pending_mode.as_deref() else {
             return;
         };
-        if folder.mode == mode {
-            let _ = self.db.sync_folder_set_pending_mode(id, None);
-            return;
-        }
-        match self.apply_sync_folder_mode(id, mode) {
+        let mode = match select_mode_transition(&folder.mode, expected_mode) {
+            Ok(Some(mode)) => mode,
+            Ok(None) => {
+                let _ = self.db.sync_folder_clear_pending_mode_if(id, expected_mode);
+                return;
+            }
+            Err(error) => {
+                warn!(
+                    id,
+                    current_mode = %folder.mode,
+                    pending_mode = %expected_mode,
+                    %error,
+                    "withdrawing unsafe queued mode transition"
+                );
+                let _ = self.db.sync_folder_clear_pending_mode_if(id, expected_mode);
+                return;
+            }
+        };
+        let mode_str = mode.as_str();
+        match self.apply_sync_folder_mode(id, mode, Some(expected_mode)) {
             Ok(message) => {
-                info!(id, mode, "applied queued mode switch");
+                info!(id, mode = mode_str, "applied queued mode switch");
                 self.log_activity(ActivityKind::Sync, &message, "", true);
             }
             // Still blocked: the pass could not get everything up, so the local copy
             // is not safe to evict yet. Leave the request standing — the next pass
             // (poll, or the retry the engine schedules) tries again.
             Err(SwitchBlocked::NotNow) => {
-                info!(id, mode, "queued mode switch still waiting");
+                info!(id, mode = mode_str, "queued mode switch still waiting");
+            }
+            Err(SwitchBlocked::Superseded) => {
+                info!(id, mode = mode_str, "queued mode switch was replaced");
             }
             Err(SwitchBlocked::Failed(e)) => {
-                warn!(id, mode, error = %e, "queued mode switch failed; withdrawing");
-                let _ = self.db.sync_folder_set_pending_mode(id, None);
+                warn!(id, mode = mode_str, error = %e, "queued mode switch failed; withdrawing");
+                let _ = self.db.sync_folder_clear_pending_mode_if(id, expected_mode);
                 self.log_activity(
                     ActivityKind::Sync,
                     format!("couldn't switch {}", base_name(&folder.local_path)),
@@ -631,8 +761,10 @@ impl Core {
     pub(crate) fn apply_sync_folder_mode(
         &self,
         id: i64,
-        mode: &str,
+        mode: ApplicableMountMode,
+        expected_pending: Option<&str>,
     ) -> Result<String, SwitchBlocked> {
+        let mode_str = mode.as_str();
         // Hold the folder's lock for the whole switch so no reconcile pass can be
         // running over the tree we are about to evict (or start while we mount over
         // it). A pass in flight holds the lock for its full duration, so `try_lock`
@@ -650,13 +782,18 @@ impl Core {
             .sync_folder_get(id)
             .map_err(|e| SwitchBlocked::Failed(format!("db: {e:?}")))?
             .ok_or_else(|| SwitchBlocked::Failed(format!("no synced folder with id {id}")))?;
-        if folder.mode == mode {
-            return Ok(format!("already {mode}"));
+        if !pending_intent_matches(folder.pending_mode.as_deref(), expected_pending) {
+            return Err(SwitchBlocked::Superseded);
+        }
+        let current_mode = ApplicableMountMode::parse(&folder.mode)
+            .map_err(|e| SwitchBlocked::Failed(format!("unsupported current mode: {e}")))?;
+        if current_mode == mode {
+            return Ok(format!("already {mode_str}"));
         }
         let local = PathBuf::from(&folder.local_path);
 
         match mode {
-            "ondemand" => {
+            ApplicableMountMode::OnDemand => {
                 // Only flip a folder that is fully in sync — a failed reconcile means
                 // local edits could still be un-uploaded, and we are about to delete
                 // the local copy. Not an error: a pass makes this true, and the queued
@@ -703,25 +840,28 @@ impl Core {
                 if let Err(e) = evict_dir_contents(&local) {
                     warn!(id, path = %local.display(), error = %e, "evict local dir contents failed");
                 }
-                let session = self.spawn_ondemand_mount(&local, root).map_err(|e| {
+                let mount = self.spawn_ondemand_mount(&local, root).map_err(|e| {
                     SwitchBlocked::Failed(format!(
                         "mount ondemand failed after local cache eviction; switch back to mirror to restore it: {e}"
                     ))
                 })?;
-                self.mounts
-                    .lock()
-                    .insert(id, (session, fuse_connection_id(&local)));
+                self.publish_secondary_mount(id, mount)
+                    .map_err(SwitchBlocked::Failed)?;
                 let _ = self.sync_tx.send(sync::SyncMsg::Rewatch);
                 self.db.sync_folder_set_state(id, "idle", now_secs()).ok();
                 info!(id, path = %local.display(), "mounted sync folder on-demand");
                 Ok(format!("{} is now on-demand", local.display()))
             }
-            _ => {
+            ApplicableMountMode::Mirror => {
                 // ondemand→mirror: tear down the secondary mount first.
-                if let Some((session, conn)) = self.mounts.lock().remove(&id)
-                    && let Err(e) = umount_session_unblocked(session, conn)
+                if let Some(mount) = self.mounts.lock().remove(&id)
+                    && let Err(error) = commit_after_teardown(mount.teardown(), || ())
                 {
-                    warn!(id, error = %e, "unmount on-demand folder failed");
+                    let _ = self.db.sync_folder_set_state(id, "error", now_secs());
+                    return Err(SwitchBlocked::Failed(format!(
+                        "unmount {} failed; folder remains on-demand: {error}",
+                        local.display()
+                    )));
                 }
                 // The evicted local tree makes the old baseline lie ("everything
                 // deleted locally"); clear it so the reconcile is a pure download.
@@ -749,23 +889,52 @@ impl Core {
         &self,
         local: &Path,
         root: Node,
-    ) -> CoreResult<BackgroundSession> {
-        clear_stale_mount(local);
-        let mut config = Config::default();
-        config.mount_options = vec![
-            MountOption::FSName("protondrive".to_string()),
-            MountOption::Subtype("protondrive".to_string()),
-            MountOption::DefaultPermissions,
-        ];
+    ) -> CoreResult<SecondaryMount> {
         let core = self.fork_state();
-        let fs = ProtonFs::new(core.clone(), root);
-        let session = Session::new(fs, local, &config)
-            .and_then(|s| s.spawn())
+        let session_live = core.session_live.clone();
+        // The fork owns this state/path registration. `spawn_session` only
+        // marks it live after FUSE has spawned successfully.
+        core.register_state(local);
+        let session = spawn_session(&core, local, root)
             .map_err(|e| CoreError::internal(format!("mount {}: {e}", local.display())))?;
-        // Hand this fork its own session's notification channel, so a background
-        // size upgrade can drop the kernel's cached attrs for these inodes.
-        let _ = core.notifier.set(session.notifier());
-        Ok(session)
+        Ok(SecondaryMount::new(
+            session,
+            fuse_connection_id(local),
+            session_live,
+        ))
+    }
+
+    /// Publish one newly spawned secondary or explicitly tear it down when
+    /// shutdown has closed publication or another session already owns the id.
+    fn publish_secondary_mount(&self, id: i64, mount: SecondaryMount) -> Result<(), String> {
+        let rejected = self.mounts.lock().insert(id, mount);
+        let Err((reason, mount)) = rejected else {
+            return Ok(());
+        };
+        let reason = match reason {
+            SecondaryInsertRejection::Closed => "daemon is shutting down",
+            SecondaryInsertRejection::Duplicate => "folder already has a live mount",
+        };
+        match mount.teardown() {
+            Ok(()) => Err(reason.to_string()),
+            Err(error) => Err(format!("{reason}; rejected mount teardown failed: {error}")),
+        }
+    }
+
+    fn mark_restore_error_if_current(&self, snapshot: &StoredSyncFolder) {
+        let lock = self.sync_lock(snapshot.id);
+        let _guard = lock.lock();
+        if self
+            .db
+            .sync_folder_get(snapshot.id)
+            .ok()
+            .flatten()
+            .is_some_and(|current| restore_snapshot_matches(snapshot, &current))
+        {
+            let _ = self
+                .db
+                .sync_folder_set_state(snapshot.id, "error", now_secs());
+        }
     }
 
     /// Re-establish FUSE mounts for folders left in `ondemand` mode across a daemon
@@ -786,14 +955,57 @@ impl Core {
             if folder.mode != "ondemand" {
                 continue;
             }
-            let local = PathBuf::from(&folder.local_path);
-            // A daemon killed before it could unmount (systemd's stop timeout,
-            // say) leaves this path attached to a dead FUSE connection: it still
-            // exists, but every stat on it fails with ENOTCONN — so `is_dir()`
-            // reads it as *missing* and the folder gets parked in `error` on
-            // every subsequent start, with no way back but a manual
-            // `fusermount -u`. Detach the corpse first; the directory
-            // underneath then restores normally.
+            let Some(root_uid) = parse_uid(&folder.remote_uid) else {
+                warn!(id = folder.id, "restore on-demand: bad remote uid");
+                self.mark_restore_error_if_current(&folder);
+                continue;
+            };
+            let root = match self
+                .rt
+                .block_on(self.client.enumerate_nodes(std::slice::from_ref(&root_uid)))
+            {
+                Ok(v) => match v.into_iter().next() {
+                    Some(n) => n,
+                    None => {
+                        warn!(id = folder.id, "restore on-demand: remote folder gone");
+                        self.mark_restore_error_if_current(&folder);
+                        continue;
+                    }
+                },
+                Err(e) => {
+                    warn!(id = folder.id, error = %e, "restore on-demand: fetch remote failed");
+                    self.mark_restore_error_if_current(&folder);
+                    continue;
+                }
+            };
+            let lock = self.sync_lock(folder.id);
+            let _guard = lock.lock();
+            let current = match self.db.sync_folder_get(folder.id) {
+                Ok(Some(current)) if restore_snapshot_matches(&folder, &current) => current,
+                Ok(_) => {
+                    info!(
+                        id = folder.id,
+                        "restore on-demand: folder changed while fetching"
+                    );
+                    continue;
+                }
+                Err(error) => {
+                    warn!(
+                        id = folder.id,
+                        ?error,
+                        "restore on-demand: cannot re-read folder"
+                    );
+                    continue;
+                }
+            };
+            if !self.mounts.lock().is_accepting() {
+                info!(id = folder.id, "restore on-demand: shutdown in progress");
+                continue;
+            }
+            let local = PathBuf::from(&current.local_path);
+            // A daemon killed before it could unmount can leave a dead FUSE
+            // connection over the path. Clear it only after the row identity
+            // and mode have been revalidated under the folder lock.
             if is_stale_mount(&local) {
                 warn!(id = folder.id, path = %local.display(), "restore on-demand: clearing stale mount");
                 clear_stale_mount(&local);
@@ -805,11 +1017,6 @@ impl Core {
                     .sync_folder_set_state(folder.id, "error", now_secs());
                 continue;
             }
-            // An `ondemand` folder's local dir is empty by construction — the switch
-            // evicts it. Finding files there means the row is lying (a switch that
-            // died between persisting the mode and evicting), and mounting over them
-            // would hide real local data behind the remote tree. Leave the files
-            // alone and let the user resolve it.
             match dir_is_empty(&local) {
                 Ok(true) => {}
                 Ok(false) => {
@@ -831,40 +1038,16 @@ impl Core {
                     continue;
                 }
             }
-            let Some(root_uid) = parse_uid(&folder.remote_uid) else {
-                warn!(id = folder.id, "restore on-demand: bad remote uid");
-                continue;
-            };
-            let root = match self
-                .rt
-                .block_on(self.client.enumerate_nodes(std::slice::from_ref(&root_uid)))
-            {
-                Ok(v) => match v.into_iter().next() {
-                    Some(n) => n,
-                    None => {
-                        warn!(id = folder.id, "restore on-demand: remote folder gone");
-                        let _ = self
-                            .db
-                            .sync_folder_set_state(folder.id, "error", now_secs());
-                        continue;
+            match self.spawn_ondemand_mount(&local, root) {
+                Ok(mount) => match self.publish_secondary_mount(folder.id, mount) {
+                    Ok(()) => {
+                        let _ = self.db.sync_folder_set_state(folder.id, "idle", now_secs());
+                        info!(id = folder.id, path = %local.display(), "remounted on-demand folder");
+                    }
+                    Err(error) => {
+                        warn!(id = folder.id, %error, "restore on-demand: mount rejected");
                     }
                 },
-                Err(e) => {
-                    warn!(id = folder.id, error = %e, "restore on-demand: fetch remote failed");
-                    let _ = self
-                        .db
-                        .sync_folder_set_state(folder.id, "error", now_secs());
-                    continue;
-                }
-            };
-            match self.spawn_ondemand_mount(&local, root) {
-                Ok(session) => {
-                    self.mounts
-                        .lock()
-                        .insert(folder.id, (session, fuse_connection_id(&local)));
-                    let _ = self.db.sync_folder_set_state(folder.id, "idle", now_secs());
-                    info!(id = folder.id, path = %local.display(), "remounted on-demand folder");
-                }
                 Err(e) => {
                     warn!(id = folder.id, error = %e, "restore on-demand: mount failed");
                     let _ = self
@@ -875,3 +1058,118 @@ impl Core {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use pdfs_core::db::StoredSyncFolder;
+
+    use super::{
+        ApplicableMountMode, commit_after_teardown, pending_intent_matches,
+        restore_snapshot_matches, select_mode_transition,
+    };
+
+    #[test]
+    fn unknown_mode_cannot_select_a_destructive_transition() {
+        let mut unmounts = 0;
+        let mut baseline_clears = 0;
+        let mut mode_writes = 0;
+        if let Ok(mode) = ApplicableMountMode::parse("streaming") {
+            match mode {
+                ApplicableMountMode::Mirror => {
+                    unmounts += 1;
+                    baseline_clears += 1;
+                    mode_writes += 1;
+                }
+                ApplicableMountMode::OnDemand => {
+                    mode_writes += 1;
+                }
+            }
+        }
+        assert_eq!((unmounts, baseline_clears, mode_writes), (0, 0, 0));
+        assert_eq!(
+            ApplicableMountMode::parse("streaming"),
+            Err("unknown mode \"streaming\" (want mirror|ondemand)".to_string())
+        );
+        assert_eq!(
+            ApplicableMountMode::parse("mirror"),
+            Ok(ApplicableMountMode::Mirror)
+        );
+        assert_eq!(
+            ApplicableMountMode::parse("ondemand"),
+            Ok(ApplicableMountMode::OnDemand)
+        );
+    }
+
+    #[test]
+    fn unknown_source_mode_with_pending_mirror_cannot_select_a_transition() {
+        let mut destructive_helper_reached = false;
+        let selected = select_mode_transition("streaming", "mirror");
+        if let Ok(Some(_)) = selected {
+            destructive_helper_reached = true;
+        }
+
+        assert!(!destructive_helper_reached);
+        assert_eq!(
+            selected,
+            Err(
+                "unsupported current mode: unknown mode \"streaming\" (want mirror|ondemand)"
+                    .to_string()
+            )
+        );
+    }
+
+    fn folder(local_path: &str, remote_uid: &str, mode: &str) -> StoredSyncFolder {
+        StoredSyncFolder {
+            id: 7,
+            local_path: local_path.to_string(),
+            remote_uid: remote_uid.to_string(),
+            remote_share_id: "share".to_string(),
+            mode: mode.to_string(),
+            pending_mode: None,
+            state: "idle".to_string(),
+            last_sync: 0,
+        }
+    }
+
+    #[test]
+    fn stale_restore_identity_or_mode_is_rejected() {
+        let snapshot = folder("/mnt/folder", "volume~node", "ondemand");
+        assert!(restore_snapshot_matches(
+            &snapshot,
+            &folder("/mnt/folder", "volume~node", "ondemand")
+        ));
+        assert!(!restore_snapshot_matches(
+            &snapshot,
+            &folder("/mnt/replaced", "volume~node", "ondemand")
+        ));
+        assert!(!restore_snapshot_matches(
+            &snapshot,
+            &folder("/mnt/folder", "volume~other", "ondemand")
+        ));
+        assert!(!restore_snapshot_matches(
+            &snapshot,
+            &folder("/mnt/folder", "volume~node", "mirror")
+        ));
+    }
+
+    #[test]
+    fn teardown_failure_prevents_follow_up_state_changes() {
+        let mut commits = 0;
+        let result = commit_after_teardown::<_, &str>(Err("busy"), || {
+            commits += 1;
+        });
+        assert_eq!(result, Err("busy"));
+        assert_eq!(commits, 0);
+
+        commit_after_teardown::<_, &str>(Ok(()), || commits += 1).unwrap();
+        assert_eq!(commits, 1);
+    }
+
+    #[test]
+    fn canceled_or_replaced_queued_intent_cannot_transition() {
+        assert!(pending_intent_matches(Some("ondemand"), Some("ondemand")));
+        assert!(!pending_intent_matches(None, Some("ondemand")));
+        assert!(!pending_intent_matches(Some("mirror"), Some("ondemand")));
+        assert!(pending_intent_matches(Some("mirror"), None));
+    }
+}

crates/pdfs-fuse/src/lib.rs

@@ -98,7 +98,9 @@ mod workers;
 use background::{run_event_sync, run_local_index};
 pub(crate) use mount::is_stale_mount;
 pub use mount::{MountOptions, MountOutcome, mount};
-use mount::{SecondaryMount, clear_stale_mount, fuse_connection_id, umount_session_unblocked};
+use mount::{
+    SecondaryInsertRejection, SecondaryMount, clear_stale_mount, fuse_connection_id, spawn_session,
+};
 use reads::{ReaderSlot, STREAM_BYPASS_MIN, StreamRing};
 use state::{Entry, Intervals, PendingRevision, State, WriteHandle};
 use tracing::{debug, error, info, warn};
@@ -400,6 +402,10 @@ struct Core {
     /// would name inodes that session has never heard of. [`Core::fork_state`]
     /// gives each fork an empty cell of its own.
     notifier: Arc<OnceLock<Notifier>>,
+    /// True only after this inode space's FUSE session spawned successfully.
+    /// Registration happens earlier for the primary drain, so state residency
+    /// alone cannot answer whether the location is mounted.
+    session_live: Arc<AtomicBool>,
     /// In-flight upload/download progress, served to `GetQueueStatus`. Shared
     /// across the FUSE session and the control-socket task.
     transfers: Arc<TransferRegistry>,
@@ -417,10 +423,10 @@ struct Core {
     /// id (devices.md Phase 3). Each is a `ProtonFs` rooted at the folder's remote
     /// node, mounted over its local path, sharing this Core's client/cache/db but
     /// with its own inode space (`fork_state`). Held so we can unmount on toggle
-    /// back to `mirror` and on daemon shutdown. The `u32` is the FUSE connection
-    /// id (see [`fuse_connection_id`]), captured at mount time so teardown can
-    /// abort a mid-transfer connection instead of blocking on it.
-    mounts: Arc<Mutex<HashMap<i64, SecondaryMount>>>,
+    /// back to `mirror` and on daemon shutdown. Each entry retains the FUSE
+    /// connection id and the fork's exact liveness flag so teardown first makes
+    /// that inode space unroutable.
+    mounts: Arc<Mutex<mount::SecondaryMountRegistry<SecondaryMount>>>,
     /// Per-sync-folder locks, held for a whole reconcile pass and for a whole
     /// mode switch. A `mirror→ondemand` flip evicts the local tree and mounts
     /// FUSE over it, so it must never overlap a pass that is walking and
@@ -523,15 +529,23 @@ fn take_self_change(
 /// the kernel to drop inode 42 is only correct on the session that minted it.
 /// Background work that invalidates as well as mutates needs the matching pair,
 /// which is what [`Core::for_each_mount`] hands it.
-type LiveMount = (Arc<Mutex<State>>, Arc<OnceLock<Notifier>>);
+type LiveMount = (
+    PathBuf,
+    Arc<Mutex<State>>,
+    Arc<OnceLock<Notifier>>,
+    Arc<AtomicBool>,
+);
 
 struct MountedState {
+    /// Absolute local root of this inode space.
+    mountpoint: PathBuf,
     /// `Weak`, so an unmounted fork's state is dropped rather than pinned here;
     /// a dead entry is reaped on the next registry walk.
     state: std::sync::Weak<Mutex<State>>,
     /// This mount's kernel notification channel — the same cell as
     /// [`Core::notifier`], still empty until its session is spawned.
     notifier: Arc<OnceLock<Notifier>>,
+    session_live: Arc<AtomicBool>,
 }
 
 /// Every mounted inode space in the daemon, shared by the primary `Core` and
@@ -546,14 +560,28 @@ struct StateRegistry(Mutex<Vec<MountedState>>);
 impl StateRegistry {
     /// Publish an inode space and the channel to the session that owns it.
     ///
-    /// Registering the same `state` twice would make every walk visit it twice;
-    /// callers register exactly once, at mount time.
-    fn register(&self, state: &Arc<Mutex<State>>, notifier: Arc<OnceLock<Notifier>>) {
+    /// A state/path pair is registered exactly once by its owner. Session
+    /// construction only flips `session_live`; it never republishes the state.
+    fn register(
+        &self,
+        mountpoint: &Path,
+        state: &Arc<Mutex<State>>,
+        notifier: Arc<OnceLock<Notifier>>,
+        session_live: Arc<AtomicBool>,
+    ) {
         let mut states = self.0.lock();
         states.retain(|m| m.state.strong_count() > 0);
+        debug_assert!(
+            !states
+                .iter()
+                .any(|mounted| mounted.state.ptr_eq(&Arc::downgrade(state))),
+            "an inode state must be registered exactly once"
+        );
         states.push(MountedState {
+            mountpoint: mountpoint.to_path_buf(),
             state: Arc::downgrade(state),
             notifier,
+            session_live,
         });
     }
 
@@ -565,10 +593,39 @@ impl StateRegistry {
         states.retain(|m| m.state.strong_count() > 0);
         states
             .iter()
-            .filter_map(|m| Some((m.state.upgrade()?, m.notifier.clone())))
+            .filter_map(|m| {
+                Some((
+                    m.mountpoint.clone(),
+                    m.state.upgrade()?,
+                    m.notifier.clone(),
+                    m.session_live.clone(),
+                ))
+            })
             .collect()
     }
 
+    /// The live session whose local root most specifically covers `path`.
+    ///
+    /// On-demand roots can be nested below another location. Selecting by the
+    /// longest component prefix ensures the nested session wins rather than the
+    /// broader primary mount. Callers currently need the selected mount only;
+    /// a future path router can derive a relative suffix with
+    /// `path.strip_prefix(mountpoint)` without storing a second path value here.
+    fn covering(&self, path: &Path) -> Option<LiveMount> {
+        self.live()
+            .into_iter()
+            .filter(|(mountpoint, _, _, live)| {
+                live.load(Ordering::Acquire) && path.starts_with(mountpoint)
+            })
+            .max_by_key(|(mountpoint, _, _, _)| mountpoint.components().count())
+    }
+
+    fn is_mounted_at(&self, path: &Path) -> bool {
+        self.covering(path).is_some_and(|(mountpoint, _, _, live)| {
+            mountpoint == path && live.load(Ordering::Acquire)
+        })
+    }
+
     /// Whether any live inode space owns and currently exposes `uid`.
     ///
     /// Retained open inodes and revoked incoming shares remain interned after
@@ -578,7 +635,8 @@ impl StateRegistry {
     fn owns_visible_uid(&self, uid: &NodeUid) -> bool {
         self.live()
             .into_iter()
-            .any(|(state, _)| state.lock().owns_visible_uid(uid))
+            .filter(|(_, _, _, live)| live.load(Ordering::Acquire))
+            .any(|(_, state, _, _)| state.lock().owns_visible_uid(uid))
     }
 }
 
@@ -589,6 +647,9 @@ impl StateRegistry {
 enum SwitchBlocked {
     /// The folder is mid-pass, or not yet safe to switch. Try again after a pass.
     NotNow,
+    /// A queued target was canceled or replaced before the folder lock became
+    /// available. The newer intent owns the next transition.
+    Superseded,
     /// The switch was attempted and broke.
     Failed(String),
 }
@@ -596,8 +657,13 @@ enum SwitchBlocked {
 impl Core {
     /// Register an inode space so background work can reach it. Called once for
     /// the primary mount and once per on-demand fork.
-    fn register_state(&self) {
-        self.states.register(&self.state, self.notifier.clone());
+    fn register_state(&self, mountpoint: &Path) {
+        self.states.register(
+            mountpoint,
+            &self.state,
+            self.notifier.clone(),
+            self.session_live.clone(),
+        );
     }
 
     /// Run `apply` against every live inode space — this daemon's primary mount
@@ -610,7 +676,7 @@ impl Core {
     /// to all of them is correct because a uid is unique across mounts — at most
     /// one state has an entry for it, and the rest are no-ops.
     fn for_each_state(&self, mut apply: impl FnMut(&mut State)) {
-        for (state, notifier) in self.states.live() {
+        for (_, state, notifier, _) in self.states.live() {
             let changed = {
                 let mut state = state.lock();
                 apply(&mut state);
@@ -627,7 +693,7 @@ impl Core {
     /// `None` for a mount whose session has not been spawned yet — there is
     /// nothing to invalidate in a kernel that has never been told about it.
     fn for_each_mount(&self, mut apply: impl FnMut(&mut State, Option<&Notifier>)) {
-        for (state, notifier) in self.states.live() {
+        for (_, state, notifier, _) in self.states.live() {
             apply(&mut state.lock(), notifier.get());
         }
     }
@@ -660,7 +726,7 @@ impl Core {
                 live_access.push(access);
             }
         }
-        for (state, _) in self.states.live() {
+        for (_, state, _, _) in self.states.live() {
             if Arc::ptr_eq(&state, &self.state) {
                 continue;
             }
@@ -1152,6 +1218,7 @@ impl Core {
                     // exactly this.
                     self.wake_drain();
                     info!("back online");
+                    self.repair_primary_share_id(&root.uid);
                     return;
                 }
                 Err(e) => {
@@ -4627,6 +4694,10 @@ mod tests {
     use pdfs_core::cache::{Baseline, StagedWrite};
     use pdfs_core::db::{OP_REVISION, OP_TRASH, PendingOp};
 
+    fn session_flag(live: bool) -> std::sync::Arc<std::sync::atomic::AtomicBool> {
+        std::sync::Arc::new(std::sync::atomic::AtomicBool::new(live))
+    }
+
     /// The registry every mount publishes itself into is what lets the daemon's
     /// single drain thread reach a node living in an on-demand fork's inode
     /// space. A fork missing from it reads as an empty file for the life of the
@@ -4644,8 +4715,17 @@ mod tests {
         let primary = std::sync::Arc::new(parking_lot::Mutex::new(primary));
         let fork_a = std::sync::Arc::new(parking_lot::Mutex::new(fork_a));
         let fork_b = std::sync::Arc::new(parking_lot::Mutex::new(fork_b));
-        for state in [&primary, &fork_a, &fork_b] {
-            registry.register(state, std::sync::Arc::new(std::sync::OnceLock::new()));
+        for (mountpoint, state) in [
+            ("/mnt/primary", &primary),
+            ("/mnt/fork-a", &fork_a),
+            ("/mnt/fork-b", &fork_b),
+        ] {
+            registry.register(
+                std::path::Path::new(mountpoint),
+                state,
+                std::sync::Arc::new(std::sync::OnceLock::new()),
+                session_flag(false),
+            );
         }
         assert_eq!(registry.live().len(), 3, "every registered mount is walked");
 
@@ -4674,6 +4754,113 @@ mod tests {
         );
     }
 
+    #[test]
+    fn state_registry_covering_uses_longest_prefix_and_ignores_dead_forks() {
+        let registry = StateRegistry::default();
+        let (primary, _primary_dir) = rooted_state("primary-volume", "primary");
+        let (nested, _nested_dir) = rooted_state("nested-volume", "nested");
+        let (sibling, _sibling_dir) = rooted_state("sibling-volume", "sibling");
+        let primary_uid = primary.entries[&super::ROOT_INO].uid.clone();
+        let primary = std::sync::Arc::new(parking_lot::Mutex::new(primary));
+        let nested = std::sync::Arc::new(parking_lot::Mutex::new(nested));
+        let sibling = std::sync::Arc::new(parking_lot::Mutex::new(sibling));
+        let primary_path = std::path::Path::new("/home/me/ProtonDrive");
+        let nested_path = std::path::Path::new("/home/me/ProtonDrive/Device");
+        let sibling_path = std::path::Path::new("/home/me/Archive");
+        let primary_live = session_flag(false);
+        let nested_live = session_flag(false);
+        let sibling_live = session_flag(true);
+
+        registry.register(
+            primary_path,
+            &primary,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            primary_live.clone(),
+        );
+        registry.register(
+            nested_path,
+            &nested,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            nested_live.clone(),
+        );
+        registry.register(
+            sibling_path,
+            &sibling,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            sibling_live,
+        );
+        assert_eq!(registry.live().len(), 3);
+        assert!(!registry.is_mounted_at(primary_path));
+        assert!(!registry.is_mounted_at(nested_path));
+        assert!(
+            !registry.owns_visible_uid(&primary_uid),
+            "an unspawned session must not authorize resident uids"
+        );
+        assert!(
+            registry
+                .covering(&nested_path.join("folder/file.txt"))
+                .is_none(),
+            "a registered but unspawned session must not win path covering"
+        );
+        primary_live.store(true, std::sync::atomic::Ordering::Release);
+        nested_live.store(true, std::sync::atomic::Ordering::Release);
+        assert!(registry.owns_visible_uid(&primary_uid));
+
+        let (covering, covering_state, _, _) = registry
+            .covering(&nested_path.join("folder/file.txt"))
+            .unwrap();
+        assert_eq!(covering, nested_path);
+        assert!(std::sync::Arc::ptr_eq(&covering_state, &nested));
+        drop(covering_state);
+        let (covering, _, _, _) = registry.covering(nested_path).unwrap();
+        assert_eq!(covering, nested_path, "an exact mountpoint covers itself");
+        let (covering, _, _, _) = registry
+            .covering(std::path::Path::new("/home/me/ProtonDrive/Device/folder/"))
+            .unwrap();
+        assert_eq!(
+            covering, nested_path,
+            "a trailing separator does not change component-prefix selection"
+        );
+        let (covering, _, _, _) = registry
+            .covering(std::path::Path::new(
+                "/home/me/ProtonDrive/DeviceBackup/file",
+            ))
+            .unwrap();
+        assert_eq!(
+            covering, primary_path,
+            "/Device must not string-prefix-match /DeviceBackup"
+        );
+        let (covering, covering_state, _, _) =
+            registry.covering(&sibling_path.join("file")).unwrap();
+        assert_eq!(covering, sibling_path);
+        assert!(std::sync::Arc::ptr_eq(&covering_state, &sibling));
+        assert!(
+            registry
+                .covering(std::path::Path::new("/home/me/ProtonDriveBackup/file"))
+                .is_none(),
+            "/ProtonDrive must not string-prefix-match /ProtonDriveBackup"
+        );
+        assert!(
+            registry
+                .covering(std::path::Path::new("/tmp/outside"))
+                .is_none(),
+            "an outside path has no covering mount"
+        );
+        assert!(registry.is_mounted_at(primary_path));
+        assert!(registry.is_mounted_at(nested_path));
+        assert!(
+            !registry.is_mounted_at(&nested_path.join("folder")),
+            "a path covered by a session is not itself a mountpoint"
+        );
+
+        // This is also the failed-fork case: the caller registered the fork,
+        // session construction failed, and its last strong `Core`/state handle
+        // was dropped. It must not make ListLocations report mounted.
+        drop(nested);
+        assert!(!registry.is_mounted_at(nested_path));
+        assert_eq!(registry.live().len(), 2);
+    }
+
     #[test]
     fn state_registry_resolves_uids_from_any_resident_mount() {
         let registry = StateRegistry::default();
@@ -4687,8 +4874,18 @@ mod tests {
         fork.children.insert(super::ROOT_INO, vec![fork_ino]);
         let primary = std::sync::Arc::new(parking_lot::Mutex::new(primary));
         let fork = std::sync::Arc::new(parking_lot::Mutex::new(fork));
-        registry.register(&primary, std::sync::Arc::new(std::sync::OnceLock::new()));
-        registry.register(&fork, std::sync::Arc::new(std::sync::OnceLock::new()));
+        registry.register(
+            std::path::Path::new("/mnt/primary"),
+            &primary,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            session_flag(true),
+        );
+        registry.register(
+            std::path::Path::new("/mnt/device"),
+            &fork,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            session_flag(true),
+        );
 
         assert!(registry.owns_visible_uid(&primary_uid));
         assert!(registry.owns_visible_uid(&fork_uid));
@@ -4711,7 +4908,12 @@ mod tests {
         let child_ino = state.intern(super::ROOT_INO, child_node);
         state.children.insert(super::ROOT_INO, vec![child_ino]);
         let state = std::sync::Arc::new(parking_lot::Mutex::new(state));
-        registry.register(&state, std::sync::Arc::new(std::sync::OnceLock::new()));
+        registry.register(
+            std::path::Path::new("/mnt/device"),
+            &state,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            session_flag(true),
+        );
 
         assert!(registry.owns_visible_uid(&child_uid));
 
@@ -4769,7 +4971,12 @@ mod tests {
         );
 
         let state = std::sync::Arc::new(parking_lot::Mutex::new(state));
-        registry.register(&state, std::sync::Arc::new(std::sync::OnceLock::new()));
+        registry.register(
+            std::path::Path::new("/mnt/device"),
+            &state,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            session_flag(true),
+        );
         assert!(
             registry.owns_visible_uid(&child_uid),
             "the absent listing remains unknown until refresh, so a valid resident uid is allowed"
@@ -5015,7 +5222,12 @@ mod tests {
         state.children.insert(revoked_ino, vec![child_ino]);
 
         let state = std::sync::Arc::new(parking_lot::Mutex::new(state));
-        registry.register(&state, std::sync::Arc::new(std::sync::OnceLock::new()));
+        registry.register(
+            std::path::Path::new("/mnt/device"),
+            &state,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            session_flag(true),
+        );
         assert!(registry.owns_visible_uid(&open_uid));
         assert!(registry.owns_visible_uid(&child_uid));
 
@@ -5064,7 +5276,12 @@ mod tests {
         state.children.insert(cycle_b, vec![cycle_a]);
 
         let state = std::sync::Arc::new(parking_lot::Mutex::new(state));
-        registry.register(&state, std::sync::Arc::new(std::sync::OnceLock::new()));
+        registry.register(
+            std::path::Path::new("/mnt/device"),
+            &state,
+            std::sync::Arc::new(std::sync::OnceLock::new()),
+            session_flag(true),
+        );
 
         assert!(
             !registry.owns_visible_uid(&foreign_uid),

crates/pdfs-fuse/src/mount.rs

@@ -28,10 +28,101 @@ pub(crate) fn is_stale_mount(path: &Path) -> bool {
     )
 }
 
-/// A secondary (on-demand sync folder) FUSE session, paired with its FUSE
-/// connection id so teardown can abort a mid-transfer connection rather than
-/// block on it. See [`Core::mounts`] and [`umount_session_unblocked`].
-pub(super) type SecondaryMount = (BackgroundSession, Option<u32>);
+/// A secondary (on-demand sync folder) FUSE session and the exact liveness flag
+/// published with its forked inode state.
+///
+/// Keeping these together makes every teardown mark the fork unroutable before
+/// unmounting, including paths that can block on an in-flight FUSE request.
+pub(super) struct SecondaryMount {
+    session: BackgroundSession,
+    conn: Option<u32>,
+    session_live: Arc<AtomicBool>,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub(super) enum SecondaryInsertRejection {
+    Closed,
+    Duplicate,
+}
+
+/// Secondary sessions and the daemon-wide gate controlling whether newly
+/// spawned sessions may become visible. The gate and map share one lock so
+/// shutdown cannot race an insertion between checking and publishing.
+pub(super) struct SecondaryMountRegistry<T> {
+    accepting: bool,
+    entries: HashMap<i64, T>,
+}
+
+impl<T> Default for SecondaryMountRegistry<T> {
+    fn default() -> Self {
+        Self {
+            accepting: false,
+            entries: HashMap::new(),
+        }
+    }
+}
+
+impl<T> SecondaryMountRegistry<T> {
+    pub(super) fn open(&mut self) {
+        self.accepting = true;
+    }
+
+    pub(super) fn is_accepting(&self) -> bool {
+        self.accepting
+    }
+
+    pub(super) fn close(&mut self) {
+        self.accepting = false;
+    }
+
+    pub(super) fn insert(
+        &mut self,
+        id: i64,
+        mount: T,
+    ) -> Result<(), (SecondaryInsertRejection, T)> {
+        if !self.accepting {
+            return Err((SecondaryInsertRejection::Closed, mount));
+        }
+        if self.entries.contains_key(&id) {
+            return Err((SecondaryInsertRejection::Duplicate, mount));
+        }
+        self.entries.insert(id, mount);
+        Ok(())
+    }
+
+    pub(super) fn remove(&mut self, id: &i64) -> Option<T> {
+        self.entries.remove(id)
+    }
+
+    pub(super) fn contains_key(&self, id: &i64) -> bool {
+        self.entries.contains_key(id)
+    }
+
+    pub(super) fn close_and_drain(&mut self) -> Vec<(i64, T)> {
+        self.close();
+        self.entries.drain().collect()
+    }
+}
+
+impl SecondaryMount {
+    pub(super) fn new(
+        session: BackgroundSession,
+        conn: Option<u32>,
+        session_live: Arc<AtomicBool>,
+    ) -> Self {
+        Self {
+            session,
+            conn,
+            session_live,
+        }
+    }
+
+    pub(super) fn teardown(self) -> std::io::Result<()> {
+        teardown_session(&self.session_live, || {
+            umount_session_unblocked(self.session, self.conn)
+        })
+    }
+}
 
 /// The kernel's id for the FUSE connection backing the mount at `mountpoint` —
 /// the directory name under `/sys/fs/fuse/connections`, which is the minor
@@ -152,6 +243,71 @@ pub struct MountOptions {
     pub sweep_mode: SweepMode,
 }
 
+/// Spawn one FUSE session rooted at an arbitrary remote node.
+///
+/// Primary My Files and secondary on-demand device locations use this same
+/// construction path so mount options, stale-mount recovery, registry
+/// liveness, and notifier ownership cannot drift apart. Each caller owns its
+/// state/path registration and performs it exactly once before calling here.
+pub(super) fn spawn_session(
+    core: &Core,
+    mountpoint: &Path,
+    root: Node,
+) -> std::io::Result<BackgroundSession> {
+    let mut config = Config::default();
+    config.mount_options = vec![
+        MountOption::FSName("protondrive".to_string()),
+        MountOption::Subtype("protondrive".to_string()),
+        MountOption::DefaultPermissions,
+    ];
+
+    clear_stale_mount(mountpoint);
+    info!(mountpoint = %mountpoint.display(), "mounting Proton Drive location");
+    let fs = ProtonFs::new(core.clone(), root);
+    if core.primary {
+        // Only My Files rehydrates the daemon-wide node cache. A secondary
+        // inode space is rooted at one device subtree and must not import every
+        // unrelated persisted node.
+        core.hydrate();
+    }
+    let session = Session::new(fs, mountpoint, &config)?.spawn()?;
+    let _ = core.notifier.set(session.notifier());
+    core.session_live.store(true, Ordering::Release);
+    Ok(session)
+}
+
+/// Make a session unroutable before beginning any teardown that may block.
+fn teardown_session<T>(session_live: &AtomicBool, teardown: impl FnOnce() -> T) -> T {
+    session_live.store(false, Ordering::Release);
+    teardown()
+}
+
+impl Core {
+    /// Resolve the primary share id away from startup and publish it only if the
+    /// projected root has not changed while the request was in flight.
+    pub(crate) fn repair_primary_share_id(&self, root_uid: &NodeUid) {
+        match self.rt.block_on(self.client.context_share_id(root_uid)) {
+            Ok(share_id) => {
+                match self
+                    .db
+                    .mount_repair_my_files_share_id(&root_uid.to_string(), &share_id.to_string())
+                {
+                    Ok(true) => debug!(%root_uid, "repaired My Files share id"),
+                    Ok(false) => {
+                        debug!(%root_uid, "discarded stale My Files share-id result");
+                    }
+                    Err(error) => {
+                        warn!(%root_uid, error = %error, "persist My Files share id failed");
+                    }
+                }
+            }
+            Err(error) => {
+                debug!(%root_uid, error = %error, "resolve My Files share id failed");
+            }
+        }
+    }
+}
+
 /// Mount the filesystem at `mountpoint` and block until it is unmounted or the
 /// daemon is asked to stop.
 ///
@@ -179,6 +335,8 @@ pub fn mount(
     } = options;
     let (root, online) = fetch_or_recall_root(&client, &rt, &db)?;
     let scope = root.tree_event_scope_id();
+    db.mount_upsert_my_files(&mountpoint.to_string_lossy(), &root.uid.to_string(), None)
+        .map_err(|error| std::io::Error::other(format!("project My Files mount: {error}")))?;
     let share_access = db
         .all_share_access()
         .map_err(|error| std::io::Error::other(format!("load shared access: {error}")))?;
@@ -228,17 +386,18 @@ pub fn mount(
         no_thumbnail: Arc::new(Mutex::new(HashMap::new())),
         size_upgrades: Arc::new(Mutex::new(HashMap::new())),
         notifier: Arc::new(OnceLock::new()),
+        session_live: Arc::new(AtomicBool::new(false)),
         transfers: TransferRegistry::new(),
         indexing: Arc::new(AtomicBool::new(false)),
         sync_progress: Arc::new(Mutex::new(HashMap::new())),
         sync_tx,
-        mounts: Arc::new(Mutex::new(HashMap::new())),
+        mounts: Arc::new(Mutex::new(SecondaryMountRegistry::default())),
         sync_locks: Arc::new(Mutex::new(HashMap::new())),
         states: Arc::new(StateRegistry::default()),
     };
     // Before anything can queue work against it: the drain thread below reaches
     // every mount's inode space through this registry, not through `core.state`.
-    core.register_state();
+    core.register_state(mountpoint);
 
     // Writes queued by a previous run (or left behind by a crash) are still owed
     // an upload, and reads must be served from their staged blobs until they land.
@@ -282,16 +441,6 @@ pub fn mount(
             .spawn(move || core.run_online_probe())?;
     }
 
-    // Re-establish on-demand mounts left over from a previous run (devices.md
-    // Phase 4). On its own thread: each remount fetches a remote node, and we
-    // must not block the main mount below on the network.
-    {
-        let core = core.clone();
-        std::thread::Builder::new()
-            .name("pdfs-restore-ondemand".into())
-            .spawn(move || core.restore_ondemand_mounts())?;
-    }
-
     // Keep the launcher prompt's "This computer" index fresh. Its own thread:
     // the walk is I/O-heavy and must never sit in front of a FUSE callback.
     {
@@ -304,18 +453,6 @@ pub fn mount(
             .spawn(move || run_local_index(db, indexing, transfers, mountpoint))?;
     }
 
-    let mut config = Config::default();
-    config.mount_options = vec![
-        MountOption::FSName("protondrive".to_string()),
-        MountOption::Subtype("protondrive".to_string()),
-        MountOption::DefaultPermissions,
-    ];
-
-    // A crashed previous run can leave the kernel mount in place, which makes
-    // the fresh mount below fail with EBUSY. Lazily detach any leftover first.
-    clear_stale_mount(mountpoint);
-    info!(mountpoint = %mountpoint.display(), "mounting Proton Drive");
-
     // Bind the control socket before the FUSE session takes over the thread. A
     // stale socket file from a previous run would block the bind, so clear it.
     let _ = std::fs::remove_file(control_socket);
@@ -332,32 +469,70 @@ pub fn mount(
             "control socket permissions: {e}"
         )));
     }
-    {
-        let core = core.clone();
-        let username = username.clone();
-        let mountpoint = mountpoint.to_path_buf();
-        std::thread::Builder::new()
-            .name("pdfs-control".into())
-            .spawn(move || run_control_socket(core, username, mountpoint, listener))?;
-    }
-
-    let fs = ProtonFs::new(core.clone(), root);
-    // Warm the in-memory maps from the DB so a cold start serves previously
-    // seen metadata without re-hitting the API (plan.md P1).
-    core.hydrate();
-
-    // Build the session explicitly (not `mount2`) so we can grab a `Notifier`
-    // for the event task. `spawn` runs the session loop on its own background
-    // thread; we then wait here for either a stop signal or the mount ending.
-    let bg = Session::new(fs, mountpoint, &config)?.spawn()?;
+    // Do not accept control requests until `spawn_session` has transitioned the
+    // primary registration to live. The listener may queue a connection during
+    // this short window, but its handler cannot observe `mounted = false`.
+    let bg = match spawn_session(&core, mountpoint, root) {
+        Ok(session) => session,
+        Err(error) => {
+            let _ = std::fs::remove_file(control_socket);
+            return Err(error);
+        }
+    };
+    core.mounts.lock().open();
     // The connection id, captured now while the mount is live, so a stop signal
     // mid-transfer can abort it rather than block `join` (see `abort_fuse_connection`).
     let main_conn = fuse_connection_id(mountpoint);
-    let notifier = bg.notifier();
-    // Same channel, kept on the `Core` so background work (a size upgrade, say)
-    // can invalidate kernel-cached metadata without threading a handle through.
-    let _ = core.notifier.set(notifier.clone());
+    {
+        let control_core = core.clone();
+        let username = username.clone();
+        let mountpoint = mountpoint.to_path_buf();
+        if let Err(error) = std::thread::Builder::new()
+            .name("pdfs-control".into())
+            .spawn(move || run_control_socket(control_core, username, mountpoint, listener))
+        {
+            let secondaries = core.mounts.lock().close_and_drain();
+            for (id, mount) in secondaries {
+                if let Err(unmount_error) = mount.teardown() {
+                    warn!(id, error = %unmount_error, "secondary teardown after control startup failure failed");
+                }
+            }
+            if let Err(unmount_error) = teardown_session(&core.session_live, || {
+                umount_session_unblocked(bg, main_conn)
+            }) {
+                warn!(error = %unmount_error, "unmount after control startup failure failed");
+            }
+            let _ = std::fs::remove_file(control_socket);
+            return Err(error);
+        }
+    }
+    // Re-establish on-demand mounts only after the primary session is live.
+    // Keep the worker so shutdown can close publication, wait for any in-flight
+    // fetch to observe that closure, and then drain a stable registry.
+    let restore_worker = {
+        let restore_core = core.clone();
+        match std::thread::Builder::new()
+            .name("pdfs-restore-ondemand".into())
+            .spawn(move || restore_core.restore_ondemand_mounts())
+        {
+            Ok(worker) => Some(worker),
+            Err(error) => {
+                warn!(%error, "start on-demand restore worker failed");
+                None
+            }
+        }
+    };
     rt.spawn(run_event_sync(client, scope, core.clone()));
+    if online {
+        let repair_core = core.clone();
+        let root_uid = core.primary_root_uid.clone();
+        if let Err(error) = std::thread::Builder::new()
+            .name("pdfs-share-id".into())
+            .spawn(move || repair_core.repair_primary_share_id(&root_uid))
+        {
+            warn!(%error, "start My Files share-id repair failed");
+        }
+    }
 
     // Stop signals (SIGTERM from `systemctl --user stop`, SIGINT from Ctrl-C)
     // are delivered onto the async runtime; bridge them onto a sync channel so
@@ -396,7 +571,10 @@ pub fn mount(
         match sig_rx.recv_timeout(Duration::from_millis(500)) {
             Ok(()) => {
                 info!("stop requested; unmounting");
-                if let Err(e) = umount_session_unblocked(bg, main_conn) {
+                core.mounts.lock().close();
+                if let Err(e) = teardown_session(&core.session_live, || {
+                    umount_session_unblocked(bg, main_conn)
+                }) {
                     warn!(error = %e, "umount_and_join failed");
                 }
                 break MountOutcome::Shutdown;
@@ -404,7 +582,8 @@ pub fn mount(
             Err(std::sync::mpsc::RecvTimeoutError::Timeout) => {
                 if bg.guard.is_finished() {
                     info!("mount ended externally");
-                    if let Err(e) = bg.join() {
+                    core.mounts.lock().close();
+                    if let Err(e) = teardown_session(&core.session_live, || bg.join()) {
                         warn!(error = %e, "session join failed");
                     }
                     break MountOutcome::Unmounted;
@@ -412,7 +591,8 @@ pub fn mount(
             }
             Err(std::sync::mpsc::RecvTimeoutError::Disconnected) => {
                 // Signal task gone (failed to install); fall back to join.
-                let _ = bg.join();
+                core.mounts.lock().close();
+                let _ = teardown_session(&core.session_live, || bg.join());
                 break MountOutcome::Unmounted;
             }
         }
@@ -420,9 +600,14 @@ pub fn mount(
 
     // Unmount every on-demand sync folder too, or the kernel mounts linger as
     // stale and the next start fails with EBUSY (devices.md Phase 3).
-    let secondaries: Vec<_> = core.mounts.lock().drain().collect();
-    for (id, (session, conn)) in secondaries {
-        if let Err(e) = umount_session_unblocked(session, conn) {
+    if let Some(worker) = restore_worker
+        && worker.join().is_err()
+    {
+        warn!("on-demand restore worker panicked");
+    }
+    let secondaries = core.mounts.lock().close_and_drain();
+    for (id, mount) in secondaries {
+        if let Err(e) = mount.teardown() {
             warn!(id, error = %e, "unmount on-demand folder failed");
         }
     }
@@ -430,3 +615,41 @@ pub fn mount(
     let _ = std::fs::remove_file(control_socket);
     Ok(outcome)
 }
+
+#[cfg(test)]
+mod tests {
+    use std::sync::atomic::{AtomicBool, Ordering};
+
+    use super::{SecondaryInsertRejection, SecondaryMountRegistry, teardown_session};
+
+    #[test]
+    fn session_is_unroutable_before_teardown_starts() {
+        let live = AtomicBool::new(true);
+        teardown_session(&live, || {
+            assert!(
+                !live.load(Ordering::Acquire),
+                "teardown must never run while the registry still reports mounted"
+            );
+        });
+    }
+
+    #[test]
+    fn secondary_registry_rejects_closed_and_duplicate_insertions() {
+        let mut mounts = SecondaryMountRegistry::default();
+        assert_eq!(
+            mounts.insert(7, "closed"),
+            Err((SecondaryInsertRejection::Closed, "closed"))
+        );
+
+        mounts.open();
+        assert_eq!(mounts.insert(7, "first"), Ok(()));
+        assert_eq!(
+            mounts.insert(7, "duplicate"),
+            Err((SecondaryInsertRejection::Duplicate, "duplicate"))
+        );
+        assert!(mounts.contains_key(&7));
+
+        assert_eq!(mounts.close_and_drain(), vec![(7, "first")]);
+        assert!(!mounts.is_accepting());
+    }
+}

crates/pdfs-fuse/src/profile.rs

@@ -12,6 +12,7 @@ use std::sync::OnceLock;
 use std::sync::atomic::{AtomicBool, Ordering};
 
 use pdfs_core::control::RestorableFolder;
+use pdfs_core::mounts::MountMode;
 use pdfs_core::profile::{PROFILE_FILE_NAME, PROFILE_VERSION, Profile, ProfileFolder, ProfilePin};
 use pdfs_core::{CoreError, CoreResult};
 use proton_drive_rs::proton_sdk::ids::NodeUid;
@@ -133,7 +134,7 @@ impl Core {
                 local_path: f.local_path,
                 // A queued switch is where the user asked the folder to be, so
                 // that is what the next machine should set up.
-                mode: f.pending_mode.unwrap_or(f.mode),
+                mode: MountMode::from(f.pending_mode.unwrap_or(f.mode).as_str()),
             })
             .collect();
         let pins = self
@@ -267,7 +268,7 @@ impl Core {
                     name: n.name,
                     local_path: proposed.to_string_lossy().to_string(),
                     mode: saved
-                        .map(|f| f.mode.clone())
+                        .map(|f| f.mode.to_string())
                         .unwrap_or_else(|| "mirror".into()),
                 }
             })
@@ -319,10 +320,10 @@ impl Core {
                 skipped.push(format!("{}: {e}", item.local_path));
                 continue;
             }
-            let mode = match item.mode.as_str() {
-                "mirror" | "ondemand" => item.mode.as_str(),
-                other => {
-                    skipped.push(format!("{}: unknown mode {other}", item.local_path));
+            let mode = match restore_mode(&item.mode) {
+                Some(mode) => mode,
+                None => {
+                    skipped.push(format!("{}: unknown mode {}", item.local_path, item.mode));
                     continue;
                 }
             };
@@ -333,7 +334,7 @@ impl Core {
             // `ondemand` is applied through the normal request path, which
             // mounts the FUSE session and handles the folder being busy — the
             // restore does not get its own copy of that logic.
-            if mode == "ondemand" {
+            if mode == MountMode::OnDemand {
                 let _ = self.request_sync_folder_mode(id, "ondemand");
             }
             let _ = self.sync_tx.send(sync::SyncMsg::Reconcile(id));
@@ -350,6 +351,17 @@ impl Core {
     }
 }
 
+/// Accept only modes this build can safely restore. `MountMode` itself is
+/// tolerant so a future value does not reject the whole profile; the folder
+/// carrying that value is skipped here with the message assembled above.
+fn restore_mode(value: &str) -> Option<MountMode> {
+    match MountMode::from(value) {
+        MountMode::Mirror => Some(MountMode::Mirror),
+        MountMode::OnDemand => Some(MountMode::OnDemand),
+        MountMode::Unknown => None,
+    }
+}
+
 /// Whether a path recorded on another machine can be reused verbatim here.
 ///
 /// Requires the *parent* to exist: `~/Documents` on a machine with the same
@@ -382,6 +394,14 @@ mod tests {
         assert!(!path_is_usable_here(Path::new("relative/path")));
     }
 
+    #[test]
+    fn unknown_profile_mode_is_skipped_without_rejecting_known_modes() {
+        assert_eq!(restore_mode("mirror"), Some(MountMode::Mirror));
+        assert_eq!(restore_mode("ondemand"), Some(MountMode::OnDemand));
+        assert_eq!(restore_mode("streaming"), None);
+        assert_eq!(restore_mode("unknown"), None);
+    }
+
     /// A path whose parent exists but which does not itself is exactly the
     /// restore case — the directory is about to be created.
     #[test]