Re: [RFC PATCH] checkout: Force matching mtime between files

["Robin H. Johnson" <robbat2@gentoo.org>]

On Wed, Apr 25, 2018 at 11:18:26AM -0400, Marc Branchaud wrote:
> > The best approach to do so is to have those people do the "touch"
> > thing in their own post-checkout hook.  People who use Git as the
> > source control system won't have to pay runtime cost of doing the
> > touch thing, and we do not have to maintain such a hook script.
> > Only those who use the "feature" would.
> 
> The post-checkout hook approach is not exactly straightforward.
> 
> Naively, it's simply
> 
> 	for F in `git diff --name-only $1 $2`; do touch "$F"; done
Even this naive attempt gets it wrong: successive files have increasing
times.  You need to capture the time at the start, and use it as the time
for the files.
  touch /tmp/ref && \
  for F in `git diff --name-only $1 $2`; do touch -r /tmp/ref "$F"; done && \
  rm /tmp/ref
(or pass a fixed time into touch).

> But consider:
> 
> * Symlinks can cause the wrong file to be touched.  (Granted, Michał's 
> proposed patch also doesn't deal with symlinks.) 
Yes, it blindly touches the file, and rather than trying to do
utimensat's AT_SYMLINK_NOFOLLOW flag.

> Let's assume that a 
> hook can be crafted will all possible sophistication.  There are still 
> some fundamental problems:
>
> * In a "file checkout" ("git checkout -- path/to/file"), $1 and $2 are 
> identical so the above loop does nothing.  Offhand I'm not even sure how 
> a hook might get the right files in this case.
Yes, this would need to be a new hook that knows more than post-checkout
presently does.

post-checkout right now runs AFTER the worktree has been updated, and
only gets the refs of old/new HEAD and if the branch was changed.
It does NOT know which files were actually modified, and since it

If a hook is added for that, then this behavior can be implemented in
the hook. Alternatively adding a pre-checkout hook to absorb some state
of the unmodified worktree (this could be a bit expensive).

> * The hook has to be set up in every repo and submodule (at least until 
> something like Ævar's experiments come to fruition).
> 
> * A fresh clone can't run the hook.  This is especially important when 
> dealing with submodules.  (In one case where we were bit by this, make 
> though that half of a fresh submodule clone's files were stale, and 
> decided to re-autoconf the entire thing.)
The fresh clone case really matters for my usage, where new clones are
firing in CI/CD processes.

> I just don't think the hook approach can completely solve the problem.
> 
> I appreciate Ævar's concern that there are more than just two mtime 
> requests floating around.  But I think git's users are best served by a 
> built-in approach, with a config setting to control the desired mtime 
> handling (defaulting to the current behaviour).  People who want a 
> different mtime solution will at least have a clear place in the code to 
> propose a patch.
+1 as long as we can set the behavior during the clone.

-- 
Robin Hugh Johnson
Gentoo Linux: Dev, Infra Lead, Foundation Treasurer
E-Mail   : robbat2@gentoo.org
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