VAAPI: fix use after free in CVAAPIContext::DestroyContext

[anssih]

Commit cfd5959 ("VAAPI: add error and info callbacks") added calls
into vaSetInfoCallback() and vaSetErrorCallback() in
CVAAPIContext::DestroyContext().

However, those calls are done after vaTerminate(m_display) which frees
the context, causing use after free.

Fix that by only performing the callback removal if vaTerminate() fails.

Also, set m_display to NULL after a successful vaTerminate() to avoid
similar issues being hidden in the future.

[fritsch]

Thanks much.

I am not a friend of the "indent" when using #ifdef - but there are arguments for it (debugging code where it is evaluated to true) and against.

[lrusak]

should these callbacks be done before vaTerminate instead?

[anssih]

I think that would work as well.

I believe it is only a cosmetic difference as the context the callbacks are stored in is destroyed anyway. But I guess I could be wrong :) And we would avoid the additional conditional.

[lrusak]

I think that would work as well.

I believe it is only a cosmetic difference as the context the callbacks are stored in is destroyed anyway. But I guess I could be wrong :) And we would avoid the additional conditional.

It's up to you. I'm fine either way. I think I did it this way originally because I wanted to catch any issues with vaTerminate in the log (I'm not sure if there is anything logged in that call though).

[anssih]

I think that would work as well.
I believe it is only a cosmetic difference as the context the callbacks are stored in is destroyed anyway. But I guess I could be wrong :) And we would avoid the additional conditional.

It's up to you. I'm fine either way. I think I did it this way originally because I wanted to catch any issues with vaTerminate in the log (I'm not sure if there is anything logged in that call though).

Ah, good point, that's probably why I kept the calling order. So I do prefer it this way after all :)

I also doubt vaTerminate() ever logs anything, but that depends on the backend as well so one could make a backend that does log.