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

From: Paul Eggert
Subject: Re: --set-mtime-command (was Re: posix atime/ctime changes despite mtime being set)
Date: Wed, 2 Aug 2023 08:09:37 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0

On 2023-08-01 06:36, Sergey Poznyakoff wrote:


Thanks for looking into this. My first reaction is that it's a bit complicated and I would omit --set-mtime-format as being more trouble than it's worth, and by default disable warnings about the command issuing an empty log message. I have what I hope is a better idea below.

1. When COMMAND returns empty output (e.g. when given a file that is
not in the repository), tar issues a warning message.

In the tzdb example this warning is unnecessary. Some files in a tarball are generated by 'Make' and these should use their own timestamps (with mtime truncated to integer seconds). We don't need to see a warning about this. I expect other projects using this approach to be similar.

The command ... implies additional complication, because it is executed in 

Possibly 'tar' should be using the -C option - though would this also apply to the mtime-determining subcommand? Anyway, I'm hoping for something simpler than this (see below).

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.

This is all getting a bit complicated.

Some time ago Bruno Haible suggested a simpler approach, which may be better for GNU tar's users. Here's the idea, which requires two passes over the input files, the first to collect timestamp info:

1. For each version-controlled file, obtain the mtime from your version-control system. Put this mtime into the tarball.

2. Let M be the maximum mtime determined in step (1), truncated to 1-second resolution. (If there are no such files, let M be zero.)

3. Sort each non-version-controlled file by time, putting them into a sorted list F1, F2, ... Fn. Put into the tarball the timestamps M+1, ..., M+n, respectively, for these files.

If F1, ..., Fn are all built from version-controlled files by a deterministic sequential process, the tarball's timestamps will be reproducible from the version-control system.

If GNU Tar had a --reproducible-mtime flag that did the above, that would be convenient to use. Ideally I wouldn't have to specify --set-mtime-command for the special case of Git; GNU Tar would just do the right thing by default for Git.

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