Review pass on the idle-unload feature found two material concurrency
bugs; both fixed here with a regression guard:
1. Unload-vs-use null deref (HIGH): _transcribe_local re-read the
module global _local_model at the transcribe call site. An idle
unload firing between the model load and transcribe() evaluated
None.transcribe → AttributeError → user-visible 'Local
transcription failed'. The window was real: the idle timer was only
touched AFTER a successful transcription, so a voice note arriving
exactly as the timeout expired raced the watcher directly.
Fix: bind a strong local reference under the model lock and use it
for the whole transcription (the watcher can null the global at any
time; this in-flight call keeps its instance — the generator holds
self, so no use-after-free). Also touch the idle timer at the START
of transcription so a long in-flight transcribe can't be counted as
idle time. The CUDA-fallback retry path gets the same treatment
(locked global write, local ref use).
2. Watcher replacement race + response-path join (MEDIUM/HIGH): the
old design stopped and re-started the watcher after EVERY
transcription with an unlocked set/join(5)/clear/start sequence on
shared globals. Two concurrent voice messages could interleave to
leave TWO live watchers (one with a stale, shorter timeout — a
raised unload_after_idle_seconds could still unload on the old
value), and the join(timeout=5) sat on the user-visible response
path (a watcher blocked on _local_model_lock during a concurrent
multi-second model load stalls the reply up to 5s).
Fix: single long-lived watcher under a management lock — started
only when none is alive (per-transcription cost: one lock + one
is_alive check), re-reads the configured timeout from config every
cycle (config edits now apply within one 30s interval, without
waiting for the next voice message — previously undocumented), and
stands down without unloading when the timeout is set to 0
mid-idle.
Tests: 17 now — idempotent start (same thread, no churn), config
re-read + stand-down-when-disabled, and the race guard
(unload firing mid-transcription must not fail the in-flight call).
The race guard is mutation-verified: reverting the fix (re-reading
the global at the call site) makes it fail with the exact NoneType
error; the fixed code passes.
The local faster-whisper model singleton (_local_model) is loaded once
and never released — the 'base' model holds ~370 MB of RAM/VRAM for
the entire lifetime of the process, even when no voice messages arrive
for hours or days. On long-running gateway processes (especially with
local LLMs competing for the same GPU) this is wasteful.
Add a config-driven idle unload: after stt.local.unload_after_idle_seconds
(default 0 = never) of no transcription activity, a lightweight daemon
thread sets _local_model = None so the Python GC can reclaim the
ctranslate2 objects. The next voice message reloads the model
transparently (the existing lazy-load path handles it).
The watcher:
- Checks every 30s whether idle time exceeds the configured threshold
- Acquires _local_model_lock before unloading (prevents races with
concurrent transcriptions that are mid-load)
- Exits immediately if the model is already None (unloaded by another
path, e.g. the CUDA fallback eviction)
- Is restarted by each transcription with the current config value,
so changing stt.local.unload_after_idle_seconds in config.yaml takes
effect on the next voice message without a process restart
Default is 0 (never unload) — zero behavior change for existing users.
Recommended value for gateway processes: 300 (5 minutes).
15 tests: config resolution (garbage/negative/None fallbacks), unload
safety (already-None, lock acquisition), touch timestamp, watcher
lifecycle (unload after timeout, no unload within timeout, exits when
model already None, stopped on new start). Existing STT test suite
unchanged.
kshitij