Sync Engine

Sync Engine

DE: Deutsch → Sync-Engine

The sync engine lives in src-tauri/src/sync.rs and is owned by AppState (sync: Arc<SyncEngine>). All file transfer reuses the WebDAV helpers in nextcloud/webdav.rs (put_file, get_file, delete, make_collection).

Concepts

• Folder — a local directory bound to a remote directory /FlutLink/<name> for a specific account. The folder id is a hex unix nanosecond timestamp; the account key is "{username}@{instance_url}".
• Journal — a JSON file sync-journal-<id>.json in the app-data dir. Per relative path it stores the last-synced { size, mtime } fingerprint of the local and the remote side (plus an is_dir flag). It is the source of truth for deciding what changed on either side. Statuses are seeded from the journal on startup, so sync folders show their persisted state immediately.
• Pass — one planner run that scans local (iterative directory walk) and remote (BFS PROPFIND) state, produces a bounded list of actions (MAX_OPS_PER_PASS = 200), and executes them in dependency-safe order: EnsureDir before uploads, parents before children.

Decision rules (decide)

Local	Remote	Journal	Action
new	—	none	Upload
—	new	none	Download
changed	unchanged	synced	Upload
unchanged	changed	synced	Download
missing	unchanged	synced	DeleteRemote
unchanged	missing	synced	DeleteLocal
changed	changed	synced	UploadConflict (name (conflict copy).ext)
unchanged	unchanged	synced	Skip
empty dir	—	—	EnsureDir
—	empty dir	synced (dir)	DeleteRemoteDir
dir	file (unchanged)	—	MoveRemoteConflict
file (unchanged)	dir	—	MoveLocalConflict

Deletions only propagate when the other side is unchanged since the journal, which prevents accidental loss. Skipped patterns: dotfiles, ~$*, trailing ~, Thumbs.db, symlinks.

Directories & type conflicts

• Empty local folders are mirrored remotely (EnsureDir); the journal records them as directory entries so a later remote deletion can propagate.
• Remote-only empty folders are only deleted when the journal knows them as synced directories (DeleteRemoteDir), and never when the remote folder still has children.
• Local folder vs. remote file and local file vs. remote folder are treated as conflicts, not as deletions: the colliding remote side is moved aside (MoveRemoteConflict / MoveLocalConflict) into a unique conflict copy.

Conflict names

Conflicts use name (conflict copy).ext for files. Within one pass every target is unique (conflict_name_n): if the copy target already exists (from an earlier pass or a prior conflict), a numeric suffix (conflict copy 2), (conflict copy 3), … is added so no copy overwrites another.

Executors

• exec_upload — streams the file with X-OC-MTime set to the local mtime so the server timestamp matches, then records the journal.
• exec_download — streams to a temp file and atomically renames into place.
• exec_delete_remote / exec_delete_local — 404s are tolerated.
• exec_upload_conflict — uploads the local copy under a conflict name and records both sides, so the conflict never re-triggers.
• exec_mkdir — creates a remote folder and journals it as a directory entry.
• exec_delete_remote_dir — deletes a synced empty remote folder and removes its journal entry.
• exec_move_remote_conflict / exec_move_local_conflict — rename the colliding side aside (WebDAV MOVE / local rename) and record the new path in the journal.

Statuses & events

SyncFolderStatus carries state (idle | syncing | paused | error), pending counts, failures, last error and lastSyncedAt. The worker emits a sync-status event (payload: all statuses) only when something changed. A paused folder is skipped by passes; resuming sets the state back to idle immediately.

Worker

spawn_worker starts a tokio task that waits on the engine's tokio::sync::Notify (fired by sync_trigger, sync_add, CLI --sync, and any status change) and a 10-second interval, running run_all (all unpaused folders) whenever a signal arrives. The CLI --path handler emits sync-folders-changed, which the frontend listens for to reload the folder list.

IPC

Backend: sync_list, sync_add (canonicalizes the path, rejects duplicate remote paths), sync_remove, sync_set_paused, sync_trigger in commands.rs. Frontend: wrappers in src/lib/ipc.ts, reactive state in src/stores/sync.ts, UI in src/components/SyncPanel.vue (folder picker via tauri-plugin-dialog).