--set-mtime-command (was Re: posix atime/ctime changes despite mtime being set)

[Sergey Poznyakoff]

Hi Paul,

Here is a tentative implementation of the proposed --set-mtime-command
option: [1].

Two new options are implemented: --set-mtime-command=COMMAND, which
defines the shell command which outputs mtime for the archive
member being added, and --set-mtime-format=FMT, which defines the
format of its output, in the sense of strptime(3).  If the latter
is not specified, parse_gettime will be used to parse it.

Here is the modified version of your Makefile that uses this feature:

  https://github.com/graygnuorg/tz/blob/main/Makefile

I've taken care to make the new feature optional, so it continues to
work with older tar versions as well.  Produced archives are equivalent
in both cases.

Notice several problems with this approach:

1. When COMMAND returns empty output (e.g. when given a file that is
not in the repository), tar issues a warning message.  Perhaps a
correspondig --warning control will be needed to disable it.

2. The command [2]

implies additional complication, because it is executed in subdirectory.
Two additional variables are defined because of this: TARMTIMEFLAGS, which
defines the --set-mtime-command option to use when calling tar from the
git local tree, and TARUPDIRMTIMEFLAGS, defining the option to use
when running tar in a subdirectory.  A better solution would be possible
if instead of of simply passing the file name to the command,
--set-mtime-command used some kind of variable expansion, e.g.

  --set-mtime-command="git log -1 --format='tformat:%cI' ../$file"

3. Command execution timeout should be implemented, perhaps.  

Opinions?

Regards,
Sergey

[1] 
http://git.savannah.gnu.org/cgit/tar.git/commit/?id=9a30bb267430eb1b69151f947cf4613803182c61
[2] 
https://github.com/graygnuorg/tz/blob/e1e0f321af3b5e2cf7134455ca7a4151cbfb5d70/Makefile#L1177