[Wayland-bugs] [Bug 75303] New: Protocol: wl_buffer.release is racy

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=75303

          Priority: medium
            Bug ID: 75303
          Assignee: wayland-bugs at lists.freedesktop.org
           Summary: Protocol: wl_buffer.release is racy
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: ppaalanen at gmail.com
          Hardware: All
            Status: NEW
           Version: unspecified
         Component: wayland
           Product: Wayland

Consider a client doing this:
1. surface1.attach(buffer1)
2. surface1.commit
3. surface2.attach(buffer1)
4. surface2.commit

Then the client receives buffer1.release.

It is ambiguous, whether the release corresponds to step 2 or step 4. That is,
the client cannot know if the latter commit still holds the buffer reserved in
the server. The server may have had time to process and release the buffer
before step 4, in which case the buffer would be reserved again after step 4.

The problem is the same also, if surface1 and surface2 were both just surface1.
If the compositor has time to repaint in between the commits, the client may
get a release it most likely interprets as the release corresponding to step 4,
which would be wrong.

How should this be resolved?

A suggestion from Jason Ekstrand was to modify the protocol to guarantee one
release event for each commit that makes the buffer reserved. This would allow
clients to use simple reference counting.

Another possibility would be to require, that clients must not commit a
wl_buffer that is already reserved by the server. However, this seems quite
limiting, especially with the coming presentation extension that allows
queueing an arbitrary number of updates which could reference the same buffer
more than once (for e.g. simple cyclic animations like spinners or cursors).

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