Subject: Re: bug#1256: Race condition in vc-diff

From: Bob Rogers <[email-masked]>
Date: Sat, 25 Oct 2008 23:45:39 -0400

From: Stefan Monnier <[email-masked]>
   Date: Sat, 25 Oct 2008 22:31:18 -0400

   >    If you visit this file and do "C-x v =" in a single-window frame that
   > is more than about 30 lines tall, and if the backend diff command is
   > fast enough, the diff window will remain half the size of the screen,
   > rather than being shrunk to fit the buffer.  This is because
   > vc-exec-after finds that the buffer process has already finished, and
   > runs vc-diff-finish before the diff buffer has been made visible.  In
   > that case, vc-diff-finish assumes that the user has already buried it.

   >    I am not a big fan of this shrink-wrapping, but I do think vc-diff
   > ought to behave consistently (and it has worked this way for quite a
   > while now).  The attached patch is sufficient to fix it.

   As the comment indicates, this patch is not quite good enough because
   pop-to-buffer may change point.  We probably will need to manipulate
   point explicitly to work around the problem.  

Problem?  vc-diff-finish sets point in the diff buffer, so if it runs
immediately, it seems to me that it ought to override what pop-to-buffer
does.  But perhaps I don't understand what point-changing behavior you
mean.

   BTW you can make the "race-condition" deterministic by making the
   diff command synchronous (as is the case with the RCS backend, for
   example, where your problem should just *always* happen).

	   Stefan

It never occurred to me to try that.  However, the same argument would
seem to apply for (setq vc-disable-async-diff t), but that only makes
the bug appear intermittently for me, even with the identical test case.

					-- Bob