[bug #60654] mdate.pl should use git modification times if possible

[G. Branden Robinson]

URL:
  <https://savannah.gnu.org/bugs/?60654>

                 Summary: mdate.pl should use git modification times if
possible
                 Project: GNU troff
            Submitted by: gbranden
            Submitted on: Sat 22 May 2021 03:18:42 AM UTC
                Category: None
                Severity: 3 - Normal
              Item Group: Build/Installation
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None

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Details:

Our mdate.pl script uses file modification times from the file system.  These
do not necessarily match the date of the last change to the file in git, and
frequently don't; for instance, if one is exploring git repo history with 'git
checkout' before returning to HEAD.

This leads to spurious differences being reported in generated man pages, and
possibly misleading content in our man pages--the modification time reported
in the man page footer is not the time the page was last changed in any
meaningful sense, but some ill-specified date probably close to the build time
of the distribution archive or package.  (Admittedly, the use of the third
argument to .TH as a document modification date is a convention, but one
preponderantly attested in history over any alternative semantics for it, as
far as I know.)

At the same time, mdate.pl is part of the distribution archive and is expected
to be run from trees that are not git repositories.

So it should work in both cases.

There's more on this type of issue at
<https://stackoverflow.com/questions/2458042/restore-a-files-modification-time-in-git>,
including recipes for resetting the file mtime to the date of the most recent
commit.

I propose that we should either (1) have mdate.pl try to determine whether it
is in a git repository, and if so, to reset the mtimes before using them to
determine the date; or (2) provide a separate script that a person (probably a
developer working in a git tree) can invoke it before performing a build.




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