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[bug#61240] improve high-res file timestamp in Automake

From: Jacob Bachmeyer
Subject: [bug#61240] improve high-res file timestamp in Automake
Date: Sat, 04 Feb 2023 18:02:56 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20090807 SeaMonkey/1.1.17 Mnenhy/

Paul Eggert wrote:
On 2023-02-03 18:27, Jacob Bachmeyer wrote:
Where are you actually using a 5.10 feature?

Where lib/Automake/ says "use Time::HiRes qw(stat);". This does not work with Perl 5.6.

Time::HiRes is (perhaps was) installable from CPAN and is definitely /not/ a 5.10 feature. I have a Perl 5.8 with it installed, and while my memories that far back are a bit fuzzy, I seem to remember installing Time::HiRes on a Perl 5.6 installation some years ago. Things like "our" variables, PerlIO-by-default, and the defined-or operator are Perl features (those in 5.6, 5.8, and 5.10 if I remember correctly), modules are (with rare exceptions) not Perl features.

The correct solution if you do not want to provide for the case where Time::HiRes is not available is to simply "use Time::HiRes qw(stat);" and *let* *that* *fail* if Time::HiRes is not available or cannot export stat(). Time::HiRes was on CPAN long before it was bundled with Perl, so the Perl version does /not/ tell you if it is available. (If Automake::FileUtils is not immediately loaded, put "use Time::HiRes;" somewhere that is. That will be enough to ensure that Time::HiRes is available.)

For why we bumped the version to 5.10, please see:

Please do not arbitrarily bump version requirements just to bump version requirements.

That's not what was done here. The abovementioned URL says version requirements were bumped from 5.6 to 5.10 because the feature is not present in 5.6 (2000), is present in 5.10 (2007), and we lacked access to the museum pieces in the middle. If you are sure that a version number lower than 5.10 will do, please let us know.

This was arbitrary because Time::HiRes is not actually dependent on the Perl version in that way. (You should also be able to get access to the museum pieces in the middle fairly easily using perlbrew.)

I am fairly sure that Time::HiRes could be installed from CPAN on at least Perl 5.6 and later, and possibly even on Perls older than 5.6. (To add it to a 5.6 installation today may require retrieving it from BackPAN.) I definitely know that it can be installed on 5.8, as I still have a Perl 5.8 that has it, and "use Time::HiRes qw(stat);" succeeds in that Perl. If the requirement is Time::HiRes, then the proper course of action is to "require Time::HiRes;" rather than requiring some version of Perl that you know shipped it as a core module. Doing the latter /is/ arbitrarily bumping the version requirement.

In fact, according to my copy of Module::CoreList (or, to be precise, its corelist(1) command-line frontend), Time::HiRes was first bundled with Perl 5.7.3. Note that I ran that query against a Perl 5.34 installation; the Perl community takes backwards compatibility like this very seriously. Note also that I said "bundled"; it could be installed from CPAN in older Perls, so requiring 5.8 (since 5.7 was a development series) is /not/ correct here.

There are also the no-runtime-overhead options of using "eval { use Time::HiRes qw(stat) };" which will replace stat() with the hi-res version if it is available and continue with the regular stat() builtin if not, or "use constant HAVE_Time_HiRes => eval { use Time::HiRes };" and a conditional "if (HAVE_Time_HiRes) { ... } else { ... }" as I suggested as an improvement to Mike Frysinger's patch.

In any case, you will still need to account for the possibility that Time::HiRes::stat() might not actually have higher resolution, depending on the filesystem. FAT, in particular, is notorious for its 2-second timestamp resolution. (This issue is actually mentioned in the Time::HiRes::stat documentation.)

-- Jacob

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