_AM_FILESYSTEM_TIMESTAMP_RESOLUTION incorrect result (was Re: make check(s) pre-release problems)

[Zack Weinberg]

On Thu, Oct 6, 2022, at 4:19 PM, Zack Weinberg wrote:
> On Thu, Oct 6, 2022, at 1:04 PM, Zack Weinberg wrote:
>> On 2022-10-04 6:58 PM, Karl Berry wrote:
>>> Perhaps easier to debug: there are two targets to be run before making a
>>> release, check-no-trailing-backslash-in-recipes and check-cc-no-c-o,
>>> to try to ensure no reversion wrt these features. A special shell and
>>> compiler are configured, respectively (shell scripts that check the
>>> behavior).
>>
>> I'm running these targets now and will report what I get.
>
> No errors on RHEL7+autoconf2.71 (same environment I used for the Python fixes)
> from a serial "make check-no-trailing-backslash-in-recipes".  The other one is
> running now.

One failure from a serial "make check-cc-no-c-o":

FAIL: t/aclocal-autoconf-version-check
======================================

Running from installcheck: no
Test Protocol: none
...
configure: error: newly created file is older than distributed files!
Check your system clock
make: *** [config.status] Error 1

This doesn't appear to have anything to do with "make check-cc-no-c-o" mode, 
the problem is that the filesystem timestamp resolution was incorrectly 
detected:

configure:1965: checking whether sleep supports fractional seconds
configure:1979: result: true
configure:1982: checking the filesystem timestamp resolution
configure:2020: result: 0.1
configure:2024: checking whether build environment is sane
configure:2079: error: newly created file is older than distributed files!
Check your system clock

The filesystem I'm working in only records timestamps at second granularity. (I 
don't know why ls is printing .000000001 instead of .000000000 but it's always 
.000000001.)

$ touch A && sleep 0.1 && touch B && ls --full-time -t A B
-rw-r--r-- 1 zweinber users 0 2022-10-07 11:20:05.000000001 -0400 A
-rw-r--r-- 1 zweinber users 0 2022-10-07 11:20:05.000000001 -0400 B

I *think* this is a bug in _AM_FILESYSTEM_TIMESTAMP_RESOLUTION where, if it 
starts looping at *just the wrong time*, the first 0.1-second sleep will 
straddle a second boundary and we'll break out of the loop immediately and 
think we have 0.1-second timestamp resolution.

zw