[RFC] Non-zero exit code when encoding is incomplete

Dear FFmpeg developers,

I would like to propose an improvement to FFmpeg's exit code behavior.

Problem

When encoding fails partway through (e.g., due to a corrupted input frame), FFmpeg still returns exit code 0. The output file exists with non-zero size, but is incomplete. This makes it impossible for programs calling FFmpeg to reliably detect encoding failure.

Current workarounds and their limitations:
Method
Problem

Exit code
Returns 0 even on incomplete output

Output file exists
File exists even when incomplete

File size > 0
File has content even when incomplete

Duration comparison
Unreliable when -t, -ss, or trimming is used

stderr parsing
Different encoders produce different summary lines (e.g., "Qavg" for AAC, "muxing overhead" for muxer, "frame=" for video). The output order is also inconsistent across modules.


Example: encoding a corrupted input file produces an incomplete output with exit code 0:
ffmpeg -i corrupted_input.mp4 -c:v libx264 -c:a aac output.mp4 echo $?   # returns 0, but output.mp4 is incomplete 


Proposal

Return a non-zero exit code when encoding does not complete as expected (e.g., end of input stream not reached, or the duration specified by -t not achieved).

This would make exit code a reliable indicator of encoding completeness, consistent with standard CLI conventions where exit code 0 means "the task completed successfully."

Additional Observation

The stderr output order is inconsistent across modules. For example:
[out#0/mp4 ...] muxing overhead: 0.326370%     ← muxer finishes first frame=64238 fps=484 ... time=00:42:49.55 ...   ← then frame summary [aac @ ...] Qavg: 65531.836                     ← then encoder summary 


There is no unified, ordered, machine-readable completion indicator in stderr either.

I understand this may be a design choice rather than a bug, but I believe reliable exit codes would greatly improve FFmpeg's usability as a programmatic tool.

Thank you for your time and consideration.
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