[PATCH] nptl: Fix racy pipe closing in tst-cancel{20,21}

[Rich Felker]

On Wed, Oct 14, 2015 at 04:03:15PM -0300, Adhemerval Zanella wrote:
> The tst-cancel20 open two pipes and creates a thread which blocks
> reading the first pipe.  It then issues a signal to activate an
> handler which also blocks reading the second pipe.  Finally the
> cancellation cleanup-up handlers are tested by first closing the
> all the pipe ends and issuing a pthread_cancel. The tst-cancel21 
> have a similar behavior, but use an extra fork after the test itself.
> 
> The race condition occurs if the cancellation handling acts after the
> pipe close: in this case read will return EOF (indicating side-effects)
> and thus the cancellation must not act.  However current GLIBC
> cancellation behavior acts regardless the syscalls returns with
> sid-effects.
> 
> This patch adjust the test by moving the pipe closing after the
> cancellation handling.  This avoid spurious cancellation for the
> case described.
> 
> Checked on x86_64 and i386.

I was involved in the discussion of this and believe that the fix is
correct. The only reason the tests "worked" before was that
cancellation was wrongly being acted upon after read succeeded in
reading EOF.

Note that, with this change, the tests will now timeout if read fails
to act on cancellation, rather than exiting with a reportable error.
This could be fixed with some very complicated machinery involving an
additional signal handler and AS-safe synchronization mechanisms to
control the ordering of close with respect to interruption of read,
but as long as timeout is an acceptable way of detecting test failure,
I see no reason to complicate the test logic like that.

Rich