# Concurrency model
Pre-refactor concurrency was emergent + unbounded:
- The worker's `_run` thread called `_scan_cycle` every 60s,
processing candidates synchronously in a for-loop.
- The `/api/auto-import/scan-now` endpoint spawned a fresh
`threading.Thread(target=_scan_cycle)` per click — extra parallel
scan cycles on top of the timer.
- Multiple "Scan Now" clicks during in-flight processing → multiple
threads racing on `_processing_paths` / `_folder_snapshots` state,
no upper bound on concurrent scanners.
- `stop()` didn't wait for in-flight processing — could leave file
moves / tag writes / DB inserts mid-flight.
Refactor to the pattern Cin uses elsewhere (`missing_download_executor`,
`sync_executor`, `import_singles_executor` all use
`ThreadPoolExecutor(max_workers=3, thread_name_prefix=...)`):
- **One scan thread** — both timer + manual triggers go through
`trigger_scan()`, gated by a non-blocking `_scan_lock`. Duplicate
triggers no-op instead of stacking parallel scanners.
- **Bounded executor** — `ThreadPoolExecutor` (default 3 workers,
configurable via `auto_import.max_workers`) runs per-candidate
work. Each candidate runs to completion in its own pool thread;
up to N candidates run in parallel.
- `_scan_and_submit()` is fast — just enumeration + executor submit,
returns immediately, doesn't block on per-candidate work.
- `_process_one_candidate(candidate)` holds the per-candidate logic
identical to the old for-loop body, lifted into a method so the
pool can run multiple instances concurrently.
- `_submitted_hashes` set + lock dedupes candidates across the
timer + manual triggers so a candidate already queued / running
doesn't get re-submitted.
- `stop()` calls `executor.shutdown(wait=True)` — clean shutdown,
no orphaned file ops.
# Per-candidate UI state isolation
The executor refactor opened two concurrency holes that the old
sequential model masked. Both fixed in this commit:
1. **Scalar UI fields stomped across pool workers.** Pre-refactor
`_current_folder` / `_current_status` / `_current_track_*` were
safe under the sequential model — only one candidate processed
at a time, so the fields tracked the in-flight one. With three
pool workers writing the same fields, the polling UI saw garbage
like "Processing AlbumA, track 7/14: SongFromAlbumB".
Replaced with `_active_imports: Dict[hash, _ActiveImport]` keyed
on folder_hash, gated by `_active_lock`. Each pool worker owns
its own entry. Helpers `_register_active` / `_update_active` /
`_unregister_active` / `_snapshot_active` are the only API.
2. **Stats counters not thread-safe.** `self._stats[k] += 1` is
read-modify-write — under load, parallel pool workers drop
increments. New `_stats_lock` + `_bump_stat()` helper wraps every
mutation. `get_status()` reads under the same lock and returns
a copy.
# Endpoint change
`/api/auto-import/scan-now` no longer spawns its own scan thread —
calls `auto_import_worker.trigger_scan()` (which routes through the
shared lock + executor). Multiple clicks while a scan is in flight
no-op deterministically. Endpoint still wraps the call in a daemon
thread so the HTTP response returns immediately even if the staging
walk is slow.
# Backward compat
The scalar `_current_folder` / `_current_status` / `_current_track_*`
fields are preserved as **read-only properties** that resolve to the
FIRST active import. The existing `get_status()` payload still
includes those fields populated from the first entry — single-import
UIs (and the test fixture) keep working unchanged. New
`active_imports` array exposes the full multi-candidate state for
parallel-aware UIs.
# Behavior preserved
- Per-candidate identify / match / process logic byte-identical
- Live-progress state preserved (per candidate now)
- Stability gate / already-processed dedup preserved
- `_record_in_progress` / `_finalize_result` UI rows preserved
- Tag-based loose-file grouping unchanged
# Behavior changes
- Multiple albums process IN PARALLEL up to `max_workers`
- "Scan Now" while scan in progress no-ops (was: spawned another)
- `stop()` waits for in-flight pool work via `shutdown(wait=True)`
- Auto-import card now lists each in-flight album (one line per
active import) instead of a single shared progress line
# UI
`webui/static/stats-automations.js`:
- Progress widget reads `active_imports` array, renders one line
per in-flight album with per-candidate status / track index
- Falls back to the legacy summary line when payload doesn't
carry `active_imports` (older backend)
- Per-row "live processing" lookup now matches by `folder_hash`
through the array instead of by `folder_name` against scalars
# Tests added (`tests/imports/test_auto_import_executor.py`)
- Pool config: default max_workers=3, configurable via constructor
+ via `auto_import.max_workers` config, floors at 1
- Scan lock: 5 concurrent `trigger_scan()` calls run only 1 scan
while lock held; releases properly so subsequent triggers run
- Executor dispatch: 5 candidates → 5 process calls via the pool
- Bounded parallelism: max_workers=3 caps at 3 concurrent;
max_workers=2 caps at 2
- Cross-trigger dedup: candidate submitted in scan A doesn't get
re-submitted by scan B while still in-flight
- Graceful shutdown: `stop()` blocks until in-flight pool work
finishes
- Per-candidate state isolation: 2 parallel workers updating their
own candidate state don't interfere — each candidate's
track_index / track_name / folder_name reads back exactly as
written for that hash
- `get_status()` returns coherent `active_imports` array with
one entry per in-flight candidate; aggregate top-level
`current_status` is 'processing' when any entry is processing
- Unregister removes only that candidate, others stay visible
- Stats counter thread-safety: 1000 parallel bumps land at 1000
(the read-modify-write race regresses without the lock)
- `get_status()` stats snapshot is a copy, not a live reference
# Verification
- 17 new tests pass (executor + state isolation)
- 2347 full suite passes (1 pre-existing flaky test —
`test_watchdog_warns_about_stuck_workers` — passes in isolation,
unrelated)
- Ruff clean
Broque Thomas