proton-drive-linux/docs/ARCHITECTURE.md

Proton Drive Linux Client: Architecture Specification

This document provides a deep, comprehensive architectural description of the Proton Drive Linux Client (pdfs). It specifies the design patterns, data flows, thread models, and subsystem dependencies that govern the virtual filesystem (FUSE), database persistence, cache management, and two-way sync engine.

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1. Subsystem Overview & Crate Topology

The application is modularized into four workspace crates, dividing core library logic, filesystem mounting, control-socket IPC, and front-ends.

graph TD
    %% Crates
    CLI["crates/pdfs-cli (CLI & Daemon Entrypoint)"]
    GUI["crates/pdfs-gui (GTK Front-end)"]
    FUSE["crates/pdfs-fuse (FUSE VFS & Sync Loop)"]
    CORE["crates/pdfs-core (DB, Cache & IPC Protocol)"]
    SDK["proton-sdk-rs (Proton Drive API & Cryptography)"]

    %% Dependencies
    CLI --> FUSE
    GUI --> CORE
    FUSE --> CORE
    CORE --> SDK
    CLI -.->|Unix Socket IPC| FUSE
    GUI -.->|Unix Socket IPC| FUSE

Crate Division & Responsibility Matrix

Crate	Primary Role	Key Components	State Management
pdfs-core	Core Infrastructure & Services	Cache bookkeeping, database migrations/schemas, IPC protocol payloads, and shared search relevance scoring.	Holds the unified SQLite DB (Db) connection and the on-disk cache metadata (ContentCache).
pdfs-fuse	VFS Layer & Reconciliation	FUSE callbacks, background upload queue (drain), two-way sync runner.	Manages in-memory inode maps (State), active descriptors (WriteHandle), and background task threads.
pdfs-cli	Command Line Interface	Command routing, daemon launcher, IPC client wrapper.	Stateless; communicates with daemon over IPC control socket.
pdfs-gui	Graphical Interface	GTK pages, tray, and the resident quick-search prompt.	Keeps UI state only; all durable state and Drive access remain behind the IPC socket.

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2. In-Memory VFS State & File Operations

The VFS layer implements FUSE via the fuser crate. Because the remote storage contains base64-encoded file keys and requires cryptographic envelope parsing, raw listings and inodes are virtualized and stored in a local state directory.

Inode and Path Resolution

• In-Memory Cache (State): Maps FUSE u64 inodes to Proton Drive NodeUids.
• Database Row Mapping (StoredNode): Stores directories, sizes, and timestamps.
• On-Demand Loading (ensure_children): If a directory is accessed, the daemon checks its database listed flag. If listed = 0, it triggers an API call to fetch remote nodes, populates the DB and in-memory caches, and returns.

sequenceDiagram
    autonumber
    actor User as Kernel (VFS Call)
    participant FUSE as pdfs-fuse VFS
    participant ST as State (In-Memory)
    participant DB as Db (SQLite)
    participant API as Proton API Client

    User->>FUSE: lookup(parent_ino, "report.pdf")
    FUSE->>ST: children.get(&parent_ino)
    alt Parent listing is resident in-memory
        ST-->>FUSE: returns child_ino
    else Listing missing in-memory
        FUSE->>DB: children_if_listed(parent_uid)
        alt Parent marked listed in DB
            DB-->>FUSE: returns child node metadata list
            FUSE->>ST: intern_from_db() and populate children cache
        else Parent not listed in DB
            FUSE->>API: enumerate_folder_children_node_uids()
            API-->>FUSE: list of UIDs
            FUSE->>API: enumerate_nodes(uids)
            API-->>FUSE: list of decrypted Nodes
            FUSE->>DB: upsert_nodes() & set_listed(true)
            FUSE->>ST: intern_batch() and populate children cache
        end
    end
    FUSE-->>User: returns child inode metadata (attributes, TTL)

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3. Read Path & Block Caching Pipeline

Read requests are parallelized and served in blocks of size BLOCK_SIZE (4 MiB).

• Unpinned files (Streaming): Avoids writing full files to disk to preserve storage. Instead, blocks are kept in a fixed-size stream_ring (in-memory ring cache) and evicted immediately.
• Pinned files (Persistent): Block downloads are saved directly to ContentCache on disk.
• Read-Ahead: The reader thread spawns asynchronous tasks to pre-fetch upcoming blocks.

sequenceDiagram
    autonumber
    actor Kernel as Kernel Read (offset, size)
    participant FUSE as pdfs-fuse VFS
    participant Cache as ContentCache (Local Disk)
    participant Ring as StreamRing (In-Memory)
    participant API as Proton API Client

    Kernel->>FUSE: read(ino, fh, offset, size)
    FUSE->>FUSE: calculate block indices [first_block..last_block]
    loop For each block index
        alt Block in disk Cache (Pinned/Cache hit)
            FUSE->>Cache: read_block(block_idx)
            Cache-->>FUSE: block bytes
        else Block in stream ring cache
            FUSE->>Ring: get(block_idx)
            Ring-->>FUSE: block bytes
        else Block Cache Miss
            FUSE->>API: download_range(offset, len)
            API-->>FUSE: decrypted block bytes
            alt caching_enabled
                FUSE->>Cache: store_block(block_idx)
            else streaming_only
                FUSE->>Ring: insert(block_idx)
            end
        end
    end
    FUSE->>FUSE: stitch blocks and slice to offset/size
    FUSE-->>Kernel: return data buffer

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4. Write Path & Staging/Draining Pipeline

Because Proton Drive does not support partial byte writes, modified files must be uploaded as whole new revisions.

1. Staging writes (WriteHandle): Writes are stored locally in a scratch file. The daemon tracks modified regions using Intervals (which holds ranges of edited bytes).
2. Close/Release (queue_revision): When the application closes the file descriptor, the daemon:
   • Fetches any untouched gaps from the remote base file to compile the full file.
   • Durably publishes the scratch data and authored-range sidecar into staging under a {uid}-{millis}-{counter} name. Temporary data and metadata are synced before atomic rename, and their directory is synced before the source is removed.
   • Transactionally queues or supersedes a pending database operation (PendingOp), so insertion failure cannot erase the previously acknowledged upload. For OP_REVISION ops, execution is debounced by 2 seconds (DRAIN_REVISION_DEBOUNCE = 2s) to give rapid follow-up writes (e.g. aria2c preallocation followed by writing) time to supersede the staged blob before network transmission.
3. Async Drain Thread (run_pending_drain): The background drain worker picks up the database operations queue, handles revision uploads, resolves conflicts, and cleans up staging files. Upon landing a revision upload (refresh_after_upload), it rebaselines both still-queued ops (rebaseline_pending) and open write handles targeting the same node to prevent false self-conflict copies on subsequent writes.

sequenceDiagram
    autonumber
    actor Kernel as Kernel Write (fh, offset, data)
    participant FUSE as pdfs-fuse VFS
    participant WH as WriteHandle (Scratch File)
    participant DB as Db (SQLite Queue)
    participant DR as Drain Thread
    participant API as Proton API Client

    Kernel->>FUSE: write(ino, fh, offset, data)
    FUSE->>WH: write_at(offset, data)
    FUSE->>WH: update written intervals
    FUSE-->>Kernel: return bytes_written

    Note over Kernel, FUSE: Application closes file (close(2))
    Kernel->>FUSE: release(fh)
    FUSE->>FUSE: fill_gaps() (fetch untouched remote ranges)
    FUSE->>FUSE: move scratch file to staging directory
    FUSE->>DB: enqueue_op(OP_REVISION, staged_path, meta)
    FUSE->>FUSE: record_pending_write() (update size/mtime in memory & DB)
    FUSE-->>Kernel: return success (async release)
    
    Note over DB, DR: Background Queue Processing
    DR->>DB: next_due_op()
    DB-->>DR: return OP_REVISION
    DR->>API: upload_new_revision_from(staged_path)
    API-->>DR: return new node revision metadata
    DR->>DB: delete_op()
    DR->>FUSE: refresh_after_upload() (sync local metadata with server time)

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5. Sync Engine (Two-Way Reconciliation)

The sync engine handles offline-capable, bidirectional synchronization between the local disk and Proton Drive for directories marked in mirror mode.

Lifecycle of a Sync Pass

1. Walk Local: Walks the local directory tree recursively, scanning sizes and modification times while carrying a completeness result. A readdir, metadata, permission, or transient I/O failure makes the pass non-destructive instead of turning omitted paths into deletions.
2. Walk Remote: Walks the remote database representation. If remote file modification times are updated, it calls the API to decrypt their sizes.
3. Load Baseline: Loads the sync_entry database table, which contains the snapshot of both sides during the last successful sync.
4. Permutation Diffing: The loop compares the three states (local, remote, baseline) to classify items:

graph TD
    %% States
    Classify{"Classify (Local, Remote, Baseline)"}

    %% Logic Rules
    Classify -->|Both Sides Match| Match["No-Op (In Sync)"]
    Classify -->|Local Changed, Remote Untouched| Upload["Upload Revision"]
    Classify -->|Remote Changed, Local Untouched| Download["Download Revision"]
    Classify -->|Both Sides Changed| Conflict["Conflict Copy (Local renamed to 'sync-conflict', remote downloaded)"]
    Classify -->|Local Deleted, Remote Untouched| RemoteDelete["Trash Remote Node"]
    Classify -->|Remote Deleted, Local Untouched| LocalDelete["Delete Local File"]
    Classify -->|New Local File, No Remote/Baseline| UploadNew["Upload New Node"]
    Classify -->|New Remote File, No Local/Baseline| DownloadNew["Download New Node"]

5. Depth-Ascending Batching: Folders are processed first to ensure hierarchies exist before files are placed. Work is executed concurrently up to a set limit.
6. Safety Gate and Settle: Destructive plans are rejected when the scan is incomplete. Total-wipe protection applies to every non-empty baseline, including a single-entry folder. On success, baseline entries are upserted, timestamps updated, and any pending mode switches (e.g. going on-demand) are evaluated. Failed local deletion or conflict preservation retains the previous baseline and prevents remote content from overwriting the local source.

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6. IPC Socket Protocol

The CLI and GUI front-ends do not access database files or make network calls directly. They communicate with the background daemon process over a Unix domain socket.

• Transport: IPC over Unix Stream Socket.
• Framing: Line-delimited JSON payloads.
• Resource limits: Requests are limited to 1 MiB, must end with a newline, and must arrive within 10 seconds. At most 64 connection handlers are active at once.
• Control Protocol:
  • Client sends a single JSON line (Request).
  • Daemon parses, handles the request, and replies with a single JSON line (Response).
  • Timeout durations are separated: 2 seconds for writes (avoids hangs on defunct sockets) and 120 seconds for reads (accommodates heavy transfers).

Unified Search

SearchV2 is the shared search boundary for the resident prompt. One request carries the query, result limit, requested sources (Drive, Local), and content kind. The daemon queries Drive metadata and the local home-directory index, then applies pdfs_core::search::relevance_score to both result sets. Exact and prefix matches rank above substring, abbreviation, and bounded typo matches; all query terms must match either the basename or parent path. Returning scores on one scale lets the GUI merge both sources into a deterministic Best matches list.

The prompt is a single-instance GTK application. It retains and hides its window between activations, resets its query state when summoned, and ignores stale asynchronous replies. Folder and streamable audio/video results open through the FUSE mount so applications can issue range reads; ordinary Drive files use the daemon's materialize-then-open path.

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7. Subsystem Interaction & Thread Map

The background daemon relies on the following thread topology:

1. Main Thread / Dispatch Loop: Blocks on fuser::Session loop. Reads kernel FUSE events and hands off network-bound VFS work to the FUSE workers pool.
2. FUSE Workers Pool (11 threads, two lanes): Bounded thread pool handling network operations. Split into 3 threads reserved for metadata (lookup, readdir) and 8 general threads that serve transfers (block reads) and fall back to metadata when no transfer is waiting. Reserved threads never accept a transfer — that is what keeps a directory listing from queuing behind saturated downloads (audit A6).
3. IPC listener Thread: Listens on the Unix socket and admits at most 64 concurrent, timeout-bound handlers for front-end status/configuration requests.
4. Sync Engine Loop Thread: Serializes sync runs. Wakes on debounced local inotify filesystem changes, remote polling intervals, or manual user requests.
5. Drain Queue Worker Thread: Processes staged writes (PendingOp) sequentially, uploading revisions and retrying with exponential backoff on failures.

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8. Threat Model: What This Client Writes to Disk in Plaintext

Proton Drive is zero-knowledge: the server never holds the keys to your content. That property ends at this daemon. Serving a remote file through a POSIX filesystem means producing plaintext, and serving it quickly means keeping some of that plaintext around. This section states exactly what lands on disk, because the guarantee users infer from "zero-knowledge" is stronger than the one a files-on-demand client can offer locally.

The short version: the cache and state directories hold decrypted content and decrypted metadata, and this client assumes the disk underneath them is encrypted (LUKS, or an encrypted home). On an unencrypted disk, an attacker with the powered-off machine can read cached file content and the full name/structure of your Drive without ever touching your password.

8.1 Decrypted content

Everything under $XDG_CACHE_HOME/<app>/content/ is plaintext:

Path	Holds	Lifetime
<uid> blobs	Whole decrypted files (pinned files, opened files)	Until LRU eviction or budget purge
blocks/	Decrypted 4 MiB block ranges of partially-read files	Until LRU eviction
thumbs/	Decrypted thumbnails and previews	Until LRU eviction
scratch/	In-progress writes from open file handles	Until release, or rescued at next open
staging/	Released writes awaiting upload	Until the upload lands
recovery/	fsynced writes rescued from an unclean shutdown	Until replayed into staging/

staging/ and recovery/ deserve separate attention: unlike the cache directories, they are not a copy of something the server already has. They hold user-authored content that may exist nowhere else yet, which is why they are deliberately never cleared on startup (§4, and audit A2). They are simultaneously the most sensitive thing on disk and the thing that must not be deleted to reclaim space.

8.2 Decrypted metadata

$XDG_STATE_HOME/<app>/cache.db is a plain SQLite database containing decrypted node names, the folder hierarchy, sizes, timestamps, a trigram full-text index over those names, the activity log, and the photos timeline. It is not evictable and not budgeted — it is the persistence layer the in-memory tree rehydrates from (§2).

Filename and directory-structure confidentiality is an explicit part of Proton Drive's model (each folder's manifest is encrypted server-side). This database is where that property is spent locally: it is a queryable, plaintext index of your entire Drive, and it survives cache purges. A PurgeCache clears content, not this.

8.3 What is not written in plaintext

Credentials. The session blob — access and refresh tokens, and the key material needed to resume unattended — lives only in the OS keyring via libsecret (auth.rs), never on disk in cleartext. config.json and pins.json hold settings and node uids, no secrets.

Note that the control socket is a credential of a different kind: anything that can connect to control.sock can drive the daemon — list and read paths, upload, trash, create share links — without touching the keyring at all. See §8.5.

8.4 Memory, swap, and the page cache

Two exposures this client does not currently mitigate, stated plainly rather than left implied:

• Swap. Content keys, session keys, and decrypted buffers live in ordinary heap memory. Nothing calls mlock(2), so under memory pressure they may be paged out. Raising LimitMEMLOCK in the systemd unit would not change this — there is no locking to permit. The effective mitigation is encrypted swap (dm-crypt / systemd-cryptsetup), which is standard on a LUKS install.
• Kernel page cache. Plaintext returned through FUSE is cached by the kernel like any other file data, and is likewise swappable. Defeating this would mean direct_io on every read, forfeiting the readahead and caching that make the mount usable. The trade is taken deliberately in favour of performance.

8.5 Enforced ownership and file modes

On every start, AppDirs::ensure verifies that the state, cache, and configuration paths are real directories owned by the effective user, rejects symlink substitution or the wrong owner, and enforces mode 0700. Both Unix sockets are changed to 0600 immediately after binding; failure is fatal rather than falling back to an unguarded daemon.

These checks protect the artifacts even when the surrounding home or XDG parent is traversable:

Artifact	Why access is restricted
content/	Another local user can read cached plaintext file content
cache.db	Another local user can read the full decrypted name/structure index
control.sock	Another local user can drive the daemon: enumerate, read, upload, trash, create public share links

The socket is the sharpest of the three, because it is an authority boundary rather than a data one — connecting to it confers the daemon's authenticated session without any credential.

Configuration publication follows the same fail-closed model: a restricted temporary file is written and synced, atomically renamed, and followed by a directory sync. A malformed existing configuration is reported and preserved rather than overwritten with defaults.

See RECOVERY.md for what a lost machine means for the plaintext described in this section, and what to revoke.

8.6 Implications for deployment

• Treat the cache and state directories as being as sensitive as the Drive contents themselves.
• Restored or manually copied profiles must remain owned by the user. The daemon refuses to start against wrong-owner or symlinked sensitive directories.
• Purging the cache (pdfs settings, or PurgeCache over IPC) removes content but not cache.db, and deliberately never removes undrained staging/ or recovery/ blobs.

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9. Human Verification (CAPTCHA) Flow

When logging in from an unfamiliar IP address or VPN, the Proton API may gate the sign-in with a human verification challenge (CAPTCHA). This client handles this asynchronously and interactively.

9.1 Sequence of Verification and Re-Authentication

sequenceDiagram
    autonumber
    actor User as User (GUI)
    participant Core as pdfs-core (Auth)
    participant UI as pdfs-gui (Login Page)
    participant Web as WebKitWebView Dialog
    participant API as Proton API Server

    User->>UI: Enter credentials & click Sign In
    UI->>Core: login_interactive()
    Core->>API: auth/v4 (Initial SRP Handshake)
    API-->>Core: HTTP 422 (Error 9001: CAPTCHA Challenge URL)
    Core->>UI: Error::HumanVerificationRequired(hv)
    UI->>UI: Block login thread, dispatch to GTK main loop
    UI->>Web: Create dialog & load verification URL
    Note over User, Web: User completes CAPTCHA in embedded WebView
    Web->>UI: window.postMessage(HUMAN_VERIFICATION_SUCCESS)
    UI->>UI: Extract token (with double-serialization safety)
    UI->>UI: Close dialog & send token to blocked login thread
    UI->>Core: login_verified(with verification token)
    Core->>API: auth/v4 (SRP retry with x-pm-human-verification-token)
    API-->>Core: Returns session tokens
    Core-->>UI: Login Successful

9.2 Key Technical Design Decisions

1. Weak Reference UI Binding: To prevent memory leaks and strong reference cycles between the parent dialog, the child WebKitWebView, the script message manager, and the connection callback, the dialog is downgraded to a WeakRef inside the callback:

   let dlg_weak = dialog.downgrade();
   content.connect_script_message_received(Some("hv"), move |_, value| {
       // ...
       if let Some(dlg) = dlg_weak.upgrade() {
           dlg.close();
       }
   });
   

2. Double-Serialization Tolerance: The JavaScript message listener forwards event data as a JSON string to the native handler. Since the underlying page may post either JS objects or pre-serialized JSON strings, the Rust side performs dual-phase parsing:

   let mut value = serde_json::from_str(raw).ok()?;
   if let Some(inner) = value.as_str() {
       if let Ok(parsed) = serde_json::from_str(inner) {
           value = parsed;
       }
   }
   

   This ensures compatibility with all versions of Proton's client verification scripts.
3. SRP Handshake Reset: Because a gated login burns the SRP handshake on the API side, the client cannot simply resume the previous request. Instead, auth::login_interactive restarts the SRP process from scratch with the verification credentials attached, keeping the complex handshake details isolated from the front-end.
4. CLI Fallback: Since the CLI has no native web browser engine, hitting the CAPTCHA gate fails immediately with a user-friendly message directing the user to sign in once via the GUI (pdfs-app) to persist the authenticated session keys to the system keyring.