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c05fbd0edb
The mp3 muxer writes the Xing frame that carries the frame count only after rewinding to the start of the stream, which it cannot do on a pipe (-f mp3 -). An uncached transcode therefore declares its length nowhere: no Xing frame, and no Content-Length unless the client asks for the estimate. A decoder has to guess the duration from the bytes it has so far, and an offline copy stays unseekable, the same defect #6105 fixed for FLAC. Transcode now inserts a Xing frame ahead of the first audio frame, carrying the frame count derived from the track duration. Only the frame count is declared: a piped encode cannot know its own byte size, and a wrong size is worse than an absent one. Decoded audio is unchanged, since decoders skip the tag frame. Measured on a 20s 128kbps transcode: ffprobe reports 20.010s instead of 20.036s, and Firefox reports 19.998s at loadedmetadata instead of 2.04s climbing over nine durationchange events. This does not make an uncached transcode resumable; that needs range support, which only the cache path has. Reported in #6170.