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Fingerprint truncated MP4/M4A files with overrun mdat size - #4
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Truncated .m4a files (interrupted download/copy) keep an mdat box header advertising the original size, which now runs past EOF. Symphonia's isomp4 demuxer discards such an mdat and fails with "no atom pending read", even though the audio present decodes fine. Detect a final mdat whose declared end exceeds the file length and rewrite its size field to the bytes on disk via a MediaSource wrapper, so the available audio is fingerprinted (symphonia then stops at a clean EOF). Also break on a demuxer DecodeError instead of failing the whole file.
zas
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Jul 10, 2026
zas
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Thanks for your patch, overall it looks good, but I think we can improve it slightly, see my comments.
Address review feedback on PR zas#4: - Move the mdat scan and PatchedSource into a new src/container_repair/ module with a format-specific mp4 submodule, a shared Patch type, and a single detect_repairs(file, path, len) entry point, so future format repairs are just a new submodule and one match arm. - Pass the already-open File into the scan instead of reopening the path, removing the redundant open and the race between the two opens. The scan rewinds the shared cursor when done. - Add unit tests for the 32-bit and 64-bit mdat size repairs.
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Some MP4/M4A files declare an
mdatbox size that overruns the actual file length (e.g. truncated downloads). These previously failed to fingerprint. This handles the overrun by clamping to the real file size so the audio can still be decoded and fingerprinted.