Resized animated GIF covers were converted to animated WebP by ffmpeg inside
the image request, bound to its 10s timeout. On slow CPUs large GIFs never
finished: ffmpeg was killed, nothing was cached, and every request repeated
the work. With ffmpeg 5.1 the conversion also failed mid-stream when reading
the GIF from a pipe, after the stream was already handed to the client, so
there was no fallback.
Requests now get a static stand-in right away, served with no-store and no
validators, while the conversion runs in the background. A single
process-wide slot bounds these conversions, since they outlive the request
throttle; when it is busy nothing is queued and a later request retries.
The result, or a static fallback if ffmpeg fails, is cached under the same
key. ffmpeg output is now buffered so mid-stream failures fall back to a
static resize. The file cache gains cache.Transient for loader results that
must be served but not stored. Without an available cache, conversion stays
inline.
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