[bug#60807] [PATCH v2] tests: reuse am_cv_filesystem_timestamp_resolution

[Zack Weinberg]

On Sat, Jan 14, 2023, at 7:18 PM, Mike Frysinger wrote:
> Rather than assume such coarse delays, re-use existing logic for
> probing the current filesystem resolution.  This speeds up the
> testsuite significantly.  On my system, it speeds -j1 up quite a
> lot -- by ~30%.  While I didn't gather many samples to produce a
> statistically significant distribution, my runs seem to be fairly
> consistent with the values below with deviations of <1 minute.

No objection to this patch in itself, but I want to make sure you're
aware that the "existing logic for probing the current filesystem
resolution" has a bug where, if you start running the script at just
the wrong time, it will erroneously detect a finer timestamp resolution
than the system actually supports.   For instance, if we can sleep for
0.1 second, the filesystem's timestamp resolution is 2s, and the sleep
loop happens to start executing at 0h00m59.9s, then it'll tick over to
0h01m00.0s and conftest.file.a and conftest.file.b will have distinct
timestamps.  This happens to me quite reliably: whenever I try to
run the Automake test suite inside AFS, I'll get a couple of spurious
test failures because of this bug.

zw