Three prod-observed download bugs, root-caused from a frozen prod queue with the
worker pinned at 100% CPU:
- ReDoS in normalize_title: _TRAILING_HASHTAGS had two overlapping unbounded
quantifiers around the required letter, so a Facebook title with a long hashtag
cluster that is NOT at the end ("... #a #b #c | Page") backtracked catastrophically.
It runs inside the yt-dlp progress hook holding the GIL, so one such download froze
the whole worker (and could stall feed enrichment too - sync/videos.py calls it).
Rewrite with a zero-width "has a letter" lookahead + a possessive tag body: one
unambiguous split, nothing to backtrack. 40-tag stress case: ~0.1ms.
- Reddit download killed by a cosmetic thumbnail convert: yt-dlp's
FFmpegThumbnailsConvertor runs "-bsf:v mjpeg2jpeg" on a JPEG-served-under-a-.png-name
(Reddit) and fails; being a postprocessor, it failed the WHOLE download even though
the media was fine. Drop it and produce the <base>.jpg poster/og:image ourselves via
editmod.thumbnail_to_jpg (plain ffmpeg decode->mjpeg, best-effort -> ensure_poster
cuts a frame on failure). The FB Messenger / MS Teams share preview keeps working
(posters verified as real JPEGs).
- classify: map "authentication is required" (Reddit's auth wall) to login_required
instead of unknown, so the UI explains it rather than offering a pointless retry.
Tests: ReDoS-timing + numeric-marker regression, the Reddit classify case, and
thumbnail_to_jpg (real JPEG out / None on bad input, never raises).
peter