PR/47703 CVS commit: src/tests/lib/libpthread

["Robert Elz" <kre%netbsd.org@localhost>]

The following reply was made to PR lib/47703; it has been noted by GNATS.

From: "Robert Elz" <kre%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/47703 CVS commit: src/tests/lib/libpthread
Date: Sun, 11 Dec 2022 10:02:54 +0000

 Module Name:	src
 Committed By:	kre
 Date:		Sun Dec 11 10:02:53 UTC 2022
 
 Modified Files:
 	src/tests/lib/libpthread: t_condwait.c
 
 Log Message:
 This test makes (made) a false assumption about the way that
 process scheduling works.   That a process (or in this case,
 a thread) is no longer blocked at time T does not mean that it
 will resume execution at time T.   The OS is free to devote
 resources to other processes/threads instead - all we should
 normally be able to expect is that if it is not unblocked before
 time T, that it will not start running before then.
 
 In general though, the pthread_cond_*wait() functions don't guarantee
 even that - but for this test, the possibility of something else
 randomly signalling the condvar isn't believable, so don't worry about
 that possibility (but do fail without calling strerror(0) on the off
 chance it does happen).
 
 Once we cease testing that the process resumed running before some
 particular time, we can stop dealing with qemu timekeeping issues,
 it might (seem to) take qemu twice as long as was requested before
 the thread resumes, but that's OK - the same thing could happen on
 a loaded system for other reasons.
 
 Beyond that, the test also has (had) a race condition.   When using
 CLOCK_REALTIME though that clock needed to have advanced to T before
 the ETIMEDOUT should happen, there is no guarantee that it will stay >T
 (CLOCK_REALTIME is allowed to be reset backwards).   So, only test
 that the current time (after ETIMEDOUT) >= T when we're using
 CLOCK_MONOTONIC - for CLOCK_REALTIME the time might have stepped
 back between when the ETIMEDOUT happened and when the thread
 obtains the current clock reading.  For that case, all we can test
 is that the ETIMEDOUT actually happens.
 
 With much of what was there now gone, the code can be simplified,
 we no longer need to do timespec arithmetic, just one comparison
 (simpler to test that Tend >= Tstart+period than Tend-Tstart > period
 as we need Tstart+period for the abstime value for the timeout anyway).
 
 Note that this still tests for the issue reported in PR lib/47703
 which is where the test came from in the first place.
 
 ps: we seem to be missing pthread_cond_clockwait() which is the same
 as pthread_cond_timedwait() except that the clock to use is passed
 as a parameter, rather than as an attribute of the condition variable.
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.9 -r1.10 src/tests/lib/libpthread/t_condwait.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.