Regression: failure to take screenshot with hwdec=vaapi

[kevmitch]

mpv version and platform

mpv 0.27.0-534-g522bfe5be1
ffmpeg 2d131fc31
libva1 1.8.3-1
Intel Ivy Bridge

Reproduction steps

mpv <file> --hwdec=vaapi
press s for screen shot

Expected behavior

png screenshot taken

Actual behavior

[ffmpeg] AVHWFramesContext: Failed to read image from surface 0x400002b: 16 (surface is in use).
Taking screenshot failed.

Log file

log

Bisect

first bad commit 522bfe5 (vo_gpu: don't re-render hwdec frames when repeating frames)

[jeeb]

I guess you mean 5261d1b for the bad commit.

[kevmitch]

I guess you mean 5261d1b for the bad commit.

ugh, yes

[grantwwu]

I have also observed this bug.

[ghost]

Not sure why this happens, when it apparently never happened with our old vaapi code. I don't think we try to access the image while it's written to, but surely it can happen while it's read from (like being used as reference frame during decoding, or when being displayed via GL). But surely, doing read accesses while it's used for other read accesses, it should work.

@fhvwy any ideas or comments?

Do we need some nonsense like staging surfaces, and could the libavutil code do this?