fix(fuse): a file read just as its create lands has its bytes (B179)

crates/pdfs-fuse/src/lib.rs

@@ -3225,8 +3225,19 @@ impl Core {
     /// `None` while it is still queued. Read from the row, which has it before
     /// the tree does.
     pub(crate) fn landed_row_uid(&self, ino: u64) -> Option<NodeUid> {
-        state::ino_lid(ino)
+        state::ino_lid(ino).and_then(|lid| self.landed_lid_uid(lid))
-            .and_then(|lid| self.db.uid_of_lid(lid).ok().flatten())
+    }
+
+    /// [`Core::landed_row_uid`] for a node known by its stand-in `uid`.
+    pub(crate) fn landed_stand_in_uid(&self, uid: &NodeUid) -> Option<NodeUid> {
+        pdfs_core::db::local_lid(&uid.to_string()).and_then(|lid| self.landed_lid_uid(lid))
+    }
+
+    fn landed_lid_uid(&self, lid: i64) -> Option<NodeUid> {
+        self.db
+            .uid_of_lid(lid)
+            .ok()
+            .flatten()
             .and_then(|uid| parse_node_uid(&uid))
             .filter(|uid| !is_local_uid(uid))
     }

crates/pdfs-fuse/src/reads.rs

@@ -499,9 +499,14 @@ impl Core {
         }
         // A node created offline and never written has no blob and no remote: it
         // is an empty file, and asking the API about a `local~` uid would only
-        // earn a 404 (offline.md Phase 3b).
+        // earn a 404 (offline.md Phase 3b). Unless its create landed since the
+        // read looked: the landing moved the queued write to the uid Drive
+        // gave it, and the row says which (`docs/BUGS.md` B179).
         if is_local_uid(uid) {
-            return Ok(Vec::new());
+            return match self.landed_stand_in_uid(uid) {
+                Some(real) => self.read_range(&real, mtime, fsize, offset, len, cache_blocks),
+                None => Ok(Vec::new()),
+            };
         }
         self.read_range_remote(uid, mtime, fsize, offset, len, cache_blocks)
     }

crates/pdfs-fuse/src/sim/daemon.rs

@@ -609,6 +609,41 @@ mod tests {
         let _ = std::fs::remove_dir_all(&dir);
     }
 
+    #[test]
+    #[ignore = "mounts FUSE: run with `cargo test -p pdfs-fuse sim:: -- --ignored`"]
+    fn a_file_read_as_its_create_lands_has_its_bytes() {
+        // The landing moved the queued write to the uid Drive gave the file.
+        // A read that still knew the file by its stand-in found no write
+        // queued under it, took the file for one never written and read it
+        // empty (B179).
+        let drive = FakeDrive::new();
+        let dir = scratch("read-as-create-lands");
+        let daemon = Daemon::start(&dir, drive.client(1, Faults::lan())).unwrap();
+        let mnt = daemon.mountpoint.clone();
+        let create = daemon.client.hold_reply_to_next_create();
+        std::fs::write(mnt.join("a.txt"), b"old").unwrap();
+        assert!(wait_until(Duration::from_secs(30), || create.reached()));
+        std::fs::write(mnt.join("a.txt"), b"newer").unwrap();
+        let uid = drive.lookup("a.txt").unwrap();
+        let landing = daemon.client.hold_next_landing(&uid);
+        drop(create);
+        assert!(wait_until(Duration::from_secs(30), || landing.reached()));
+        assert_eq!(std::fs::read(mnt.join("a.txt")).unwrap(), b"newer");
+        drop(landing);
+        assert!(
+            wait_until(Duration::from_secs(30), || {
+                daemon.pending().is_ok_and(|items| items.is_empty())
+                    && drive.tree().get("a.txt") == Some(&Entry::File(Arc::new(b"newer".to_vec())))
+            }),
+            "{:?} {:?}",
+            daemon.pending(),
+            drive.tree()
+        );
+
+        assert!(matches!(daemon.stop(), Ok(MountOutcome::Shutdown)));
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
     #[test]
     #[ignore = "mounts FUSE: run with `cargo test -p pdfs-fuse sim:: -- --ignored`"]
     fn a_file_moved_while_its_create_was_on_the_wire_keeps_its_new_name() {

docs/BUGS.md

@@ -12,6 +12,29 @@ Conventions:
 
 ---
 
+## B179 — A file read just as its create lands reads empty
+
+**Status:** Fixed (unverified).
+**Found:** 2026-10-04, by the account run on 378667f, on on-demand sync folders: the unusual
+names case read `<angle>|pipe` back as 0 bytes instead of 12, and B70's completed download as 0
+bytes instead of 147456. Both files had a write that landed during their create, which the
+daemon queued as a revision.
+
+**Where:** `Core::read_range` in `crates/pdfs-fuse/src/reads.rs`.
+
+**Cause.** A read takes the file's uid from the tree and reads on a transfer worker. A file made
+on the mount has its stand-in uid there until its create lands. When it lands, the queued write
+moves to the uid Drive gave the file. A read that still asked by the stand-in found no write
+queued under it, took the file for one created and never written, and returned nothing. The
+read waits on the transfer lane, which a run keeps busy, so the window is not small.
+
+**Fix.** A stand-in with no queued write is looked up by its local id. Once its create has
+landed, the read goes on under the uid Drive gave it.
+
+**Test:** `a_file_read_as_its_create_lands_has_its_bytes` in `crates/pdfs-fuse/src/sim/daemon.rs`.
+
+---
+
 ## B178 — Trashing a folder while sync is paused keeps a create in it as a trash
 
 **Status:** Fixed (unverified).

docs/CHANGELOG.md

@@ -83,6 +83,7 @@ by its local id, in the queue, the stored tree and pins alike.
   (B177).
 - **Deleting a folder while sync is paused no longer leaves a queued change behind** for a
   file in it that was never uploaded (B178).
+- **A file read just as it reaches Drive no longer reads empty** (B179).
 - **A file renamed just as its upload finished keeps its new name.** The mount no longer showed
   the old name, and a second rename no longer failed with "No such file or directory" (B152,
   B172).

docs/MILESTONE-3.0.0-STATUS.md

@@ -9,7 +9,7 @@ Tracks `docs/MILESTONE-3.0.0.md`. Update it with every commit that moves a phase
 - **Phase 2**: local ids, inode = local id, one tree for every mount (2.10.0).
 - **Phase 3, first part**: mkdir, create, rename, unlink and rmdir on the mount are recorded
   locally and sent from the queue; `"local_first": false` brings back the old path (5e0f30c).
-- Fixes from the simulation and account runs since then: B127 and B141 to B178.
+- Fixes from the simulation and account runs since then: B127 and B141 to B179.
 - **Sync issues, first cut** (§6.2): refusals sorted into issues, shown at once in the app,
   tray and CLI; Export button and `pdfs sync export`; `pdfs sync issues`.
 
@@ -124,7 +124,7 @@ Tracks `docs/MILESTONE-3.0.0.md`. Update it with every commit that moves a phase
          The account run on 378667f: a folder listed as its create landed asked Drive by
          its stand-in and answered `EIO` (B177, fixed). A folder trashed while sync was
          paused kept a create below it as a trash: a paused drain worker had claimed it
-         (B178, fixed).
+         (B178, fixed). A file read as its create landed read empty (B179, fixed).
 5. **Release**: 2.10.0 is tagged; the 2.9.0 tag still waits for the user's go-ahead.
 
 ## After 3.0.0