Two subsystems post-process the same completed transfer: the browser-poll
status endpoint (web_server) and the background download monitor. Both watch
the same slskd/streaming transfers and each launches the verification
pipeline. When one path quarantines + requeues the next-best candidate
(clearing username/filename, status -> 'searching'), the monitor's
already-submitted run_post_processing_worker then runs, finds no source info,
and falsely marks the task 'failed' ("missing file or source information") —
clobbering the in-flight retry while a parallel attempt imports the song.
Fix: a single atomic claim (downloading/queued -> post_processing under
tasks_lock) so exactly one path processes each download.
- runtime_state: new claim_for_post_processing() helper
- post_processing: race guard — worker bails (no fail/notify) if the task is
no longer 'post_processing' when it runs
- web_server: both poll paths (Soulseek + streaming) claim before launching;
claim is released on thread-launch failure
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
05b36704c3