Re: POSIX ruling on up-to-date vs. identical timestamps

[Ray Donnelly]

On Thu, Aug 21, 2014 at 4:57 PM, Ray Donnelly <address@hidden> wrote:
> On Thu, Aug 21, 2014 at 4:32 PM, Eric Blake <address@hidden> wrote:
>> Make folks:
>> You may want to check out http://austingroupbugs.net/view.php?id=857 and
>> add comments and/or change GNU make behavior accordingly.  There, the
>> argument is made that HP-UX make behavior is nicer than GNU's current
>> behavior when two files have identical timestamps: HP-UX considers the
>> file as out-of-date, while GNU make considers it up-to-date.  A strict
>> reading of POSIX can argue that GNU's behavior was required, but this
>> reading has been called into question.
>>
>> GNU's behavior is an optimization that avoids needless churn on file
>> systems with course timestamps (well, FAT still exists, but these days,
>> MOST file systems have sub-second resolution, so it is harder to get
>> files with identical timestamps without actually touch'ing them that
>> way) - but it risks leaving a tree in an incomplete state.  HP-UX
>> behavior guarantees the rules are run, even if they were not strictly
>> necessary, but has the nice property that the tree is never left in an
>> incomplete state due to unfortunate timing on a file system with course
>> timestamps.
>>
>> The POSIX recommendation was therefore that GNU should change its
>> behavior to act like HP-UX, and consider identical timestamps as
>> out-of-date, because the standard will be fixed to allow HP-UX behavior.
>>
>> Autoconf folks:
>> The section of the autoconf manual that discusses this should probably
>> be modernized, particularly if changes to POSIX and/or GNU make result
>> from this discussion.
>> https://www.gnu.org/software/autoconf/manual/autoconf.html#Timestamps-and-Make
>>
>
> Some projects that want to have deterministic builds by using the
> gitian builder [1] use libfaketime to set the timestamps to a known
> constant, and gnu make trips up on this, occasionally causing broken
> builds and othertimes autoconf declares suck a build system "insane".

Freudian slip? I meant "such", not "suck".

> There was a bug in libfaketime so that the nanosecond field wasn't
> cleared and this allowed you to avoid both these bugs if the planets
> were aligned correctly (which has since been fixed). I agree that
> these issues should be fixed in make and autoconf.
>
> [1] https://gitian.org/
>
>> --
>> Eric Blake   eblake redhat com    +1-919-301-3266
>> Libvirt virtualization library http://libvirt.org
>>
>>
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