[Xcb] [RFC PATCH] New XCB socket handoff mechanism for Xlib/XCB and other libraries

Jamey Sharp jamey at minilop.net
Tue Mar 18 12:34:52 PDT 2008

On Sat, Mar 15, 2008 at 09:36:37PM -0700, Jamey Sharp wrote:
> Please review and provide comments. We hope to commit these patches and
> release new versions of XCB and Xlib/XCB soon.

Here are the bugs we currently know about that are keeping us from
committing these patches as-is. We still believe they're suitable for
testing right now, as the bugs are in rarely-encountered corner cases.
In fact I'm quite pleased that our testing methodology caught these. :-)

- KDE's taskbar, "kicker", after many hours, syncs too late and hangs.

  I can reliably reproduce this by leaving my desktop along for 5.5
  hours or so, but Josh and I are baffled by the problem. We already
  fixed a bug that looks identical, and can't see why the fix might be
  failing. If somebody else could reproduce the problem and maybe spot
  the cause, we'd appreciate it.

- The glue between Xlib and XCB is not 64-bit clean.

  XCB uses "unsigned int" for sequence numbers, but Xlib uses "unsigned
  long". These types are part of the ABI of both libraries, making them
  difficult to change. There are a few ways we could address this, and
  we haven't decided on one yet. Patches welcomed! :-)

  One side effect of this bug: on a 64-bit system, after 2**32 requests,
  if Xlib notices that 2**16 requests have passed without a response
  from the server, its attempt to sync will infinitely recurse. So it
  crashes when it runs out of stack space.

- Xlib has a multi-threaded race condition involving XAllocID.

  This race applies with or without XCB, and with or without the new
  patches. However, it can only affect applications which call Xlib from
  more than one thread concurrently, and I'm aware of about a half dozen
  of those worldwide. This bug is much more likely to show up when Xlib
  is built using XCB. We're currently exploring ideas for fixing it.

- After running out of its initial allocation of XIDs, XCB assert-fails.

  We found and fixed this bug. Our new code assert()ed something that
  wasn't true. Our next patchset will have the corrected code.

  Since it takes a long time to run out of two million XIDs in normal
  usage, I don't feel the new patches are urgent.

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