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

[Jacob Bachmeyer]

Paul Eggert wrote:
> On 2023-02-04 16:02, Jacob Bachmeyer wrote:
> > 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.
>
> That's fine. All we want is the exact file timestamp. If the file 
> system timestamp resolution is only 2 s, then we want that multiple of 
> 2. Admittedly we can't get the exact file timestamp on many modern 
> file systems since Time::HiRes is precise only to the nearest ~238 ns 
> for today's timestamps, but the idea is to get what we easily can.

How often is Perl built to use long doubles these days?  (That was an 
option beginning with Perl 5.6.)

> > 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. 
>
> Sorry, I don't remember seeing that suggestion. I guess it was in 
> another thread. Could you resend that patch to 61240@debbugs.gnu.org 
> and cc me? Preferably a patch against the latest Automake, in "git 
> format-patch" format; see 
> <https://savannah.gnu.org/git/?group=automake> for how to get 
> bleeding-edge Automake. The idea would be to port bleeding-edge 
> Automake to Perl < 5.10 when that's easy.

It was bug#60807.  Looking at the patch, I suspect that I will need to 
take a closer look at the code:  the existing code before Mike's patch 
used File::stat, which I suspect may not be compatible with 
Time::HiRes::stat (not compatible in the sense that File::stat's 
override of stat() probably will not use Time::HiRes's override of 
stat()).  If so, then "use Time::HiRes qw(stat);" will not do what you 
want because either stat() will either cease to return an object or will 
not have subsecond resolution.

Looking at the code, commit 01bf65daf6f6627b56fbe78fc436fd877ccd3537 
appears fine, all I am asking is that commit 
4e3744a15c4d8bdb46c11ead2fb56c5f591b714b be reverted.  The current 
Automake Git master should actually work on Perl 5.6 if Time::HiRes has 
been installed, which was possible with 5.6 although it was bundled with 
Perl beginning with the 5.7.3 development release.

In other words, bleeding-edge Automake is already ported to Perl < 5.10, 
in fact it should run on 5.8 out-of-the-box and 5.6 if Time::HiRes has 
been installed from CPAN (or perhaps BackPAN today, but Time::HiRes was 
on CPAN when 5.6 was current).

> Please bear in mind that I stopped coding in Perl 30 years ago and so 
> am a bit rusty.

Yes, very---30 years ago would have been before Perl 5.6!  Much of 
modern Perl did not exist then...


-- Jacob