Transient network drops and Wi-Fi timeouts no longer promote a download
straight to a hard Failed state. A shared retry engine classifies the
error, emits a transient `Retrying` state, and runs a 3-strike
exponential backoff (2s / 5s / 10s) while preserving the active download
allocation (semaphore permit / worker slot). Resumability primitives are
reused on every retry so no downloaded bytes are discarded.
Engine core (src-tauri/src/retry.rs, new):
- BACKOFF_SCHEDULE = [2s, 5s, 10s], MAX_RETRIES = 3
- backoff_for(strike): schedule index with graceful clamp beyond range
- is_transient_network_error(msg): string classifier covering reqwest,
yt-dlp, and aria2c phrasing; permanent conditions (HTTP 401/403/404/
410/451, not-found, permission denied, out-of-disk) checked first so
they always fail fast
- backoff_and_emit / backoff_and_emit_cancel: cancel-safe sleep helpers
driving a Retrying state emit before each delay
- 9 unit tests covering schedule math, clamping, transient vs permanent
classification, and permanent-wins-over-transient precedence
State model (src-tauri/src/ipc.rs):
- DownloadStatus::Retrying variant (serialized "retrying")
- DownloadStateEvent::retrying(id, reason) constructor
- Regenerated TypeScript binding: src/bindings/DownloadStatus.ts
Native reqwest backend (src-tauri/src/download.rs):
- DownloadEvent::Retrying variant (headless mirror)
- CoordinatorEventSink::emit_retrying() drives the production
download-state channel with status "retrying"
- download_file retry core rewritten: transient -> emit_retrying ->
backoff_for sleep inside a control_rx select (pause/cancel honored
mid-backoff) -> re-issue Range header; permanent errors or strike
exhaustion advance to the next URL then hard Failed
yt-dlp media backend (src-tauri/src/lib.rs):
- child spawn+stream loop wrapped in a 3-strike re-spawn loop; stderr
tail classified and, if transient, the process is re-spawned after
backoff (--continue resumes); --retry-wait=2 added to aria2c downloader
aria2c backend (src-tauri/src/queue.rs):
- retry-wait=2 option so aria2's internal retries are not rapid-fire
- handle_aria2_download_error: intercepts transient onDownloadError,
backs off, re-issues addUri, and rotates the stale gid -> id mapping
via rotate_aria2_gid (fresh addUri mints a new gid; not rotating would
detach subsequent WS events and leak the semaphore permit permanently)
- aria2_payloads / aria2_retry_strikes tracking with cleanup on terminal
outcomes
Verification: cargo build clean; cargo test --lib 37 passed / 0 failed.
This commit replaces the frontend-imperative download dispatcher with a centralized backend `QueueManager`. It acts as the sole concurrency gatekeeper using a single `tokio::sync::Semaphore` across all download paths (aria2 RPC, native HTTP, yt-dlp media).
Backend Changes:
- **queue**: Added `QueueManager` to manage an ordered `VecDeque` of tasks, semaphore permits, and retirement debt (CAS resize).
- **commands**: Replaced direct start commands with `enqueue_download`, `enqueue_many`, `move_in_queue`, and `remove_from_queue`.
- **ipc**: Exported `DownloadStateEvent` and `QueueDirection` to the frontend.
- **tests**: Added 11 integration tests covering idle-parking, idempotent releases, CAS underflow prevention, and gid-completion races.
Frontend Changes:
- **store**: Made `useDownloadStore` reactive to the backend via the `download-state` event.
- **store**: Removed `processQueue` and introduced `pendingOrder` to track the accurate sequence of queued items.
- **ui**: Updated `DownloadItem` with queue visuals (clock icon, position badge).
- **ui**: Added Move Up/Down controls to interact with the backend queue reordering API.
NimBold