getSafParameterForWrite doesn't work with -movflags faststart on Android

[gazlaws-dev]

Description
Creating an output video (.mp4) with the-movflags faststart doesn't work with getSafParameterForWrite (using Android's Storage Access Framework), presumably because it's not able to read the file for the second pass.

Expected behavior
An option to create the output file with read and write permissions. Maybe getSafParameter can be made public so we can pass 'rw' as openMode.

Current behavior
Output file is not created properly, error shown:

2021-10-03 15:27:24.849 24442-24657/x I/ffmpeg-kit: Output #0, mp4, to 'saf:105.mp4':
2021-10-03 15:27:28.770 24442-24657/x E/ffmpeg-kit: Error writing trailer of saf:105.mp4: Bad file descriptor

To Reproduce

inputVideoUrl = FFmpegKitConfig.getSafParameterForRead(requireContext(), inputVideoUri);
outputFilePath = FFmpegKitConfig.getSafParameterForWrite(requireContext(), resultVideoUri);

ffmpeg command (shortened):
  -y -i "saf:90.mp4" -vcodec libx264  -movflags faststart  saf:105.mp4

Logs

2021-10-03 15:27:18.726 24442-24442/x I/EditFragment: getContent: Input video absolute file path: saf:90.mp4
2021-10-03 15:27:24.547 24442-24657/x I/ffmpeg-kit: Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'saf:90.mp4':
2021-10-03 15:27:24.849 24442-24657/x I/ffmpeg-kit: Output #0, mp4, to 'saf:105.mp4':
2021-10-03 15:27:28.770 24442-24657/x E/ffmpeg-kit: Error writing trailer of saf:105.mp4: Bad file descriptor

Environment

• Platform: Android
• Architecture: arm64-v8a
• Version: v4.5

[tanersener]

Thanks for reporting this issue. Yeah, it seems like there is a problem there. Existing flags used internally ("r" and "w") are not enough to move the moov atom to the beginning of the file. We need to provide another method for it.

[tanersener]

My tests show that we have this problem on v4.4 and v4.5 because saf urls created by FFmpegKitConfig.getSafParameter methods are single-use. They cannot be used more than once. And, in your case, when -movflags faststart is added, ffmpeg tries to open the file second time to move the header to the beginning. But it fails and prints the standard error we have for re-using the same saf url: Bad file descriptor.

I fixed this on the development branch by re-implementing the whole saf protocol logic. SAF urls are reusable now. So, opening the file again to move the header is no longer a problem. I also changed the access modifier of FFmpegKitConfig.getSafParameter method. You can use it to pass your own open mode.

[tanersener]

The changes are released on v4.5.1. However, apps that use this feature must test it well. I saw two issues during my tests.

• Storage Access Framework handles open modes differently on old API levels. w mode that is enough to write an mp4 file on API Level 29+ devices doesn't work as expected on API Level 24 and 21. You need to use rw on them.
• Some Document providers do not support rw mode, e.g. Google Drive provider