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

[Duy Nguyen <pclouds@gmail.com>]

On Wed, Apr 25, 2018 at 5:18 PM, Marc Branchaud <marcnarc@xiplink.com> 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.

I am revisiting this because I'm not even happy with my
post-checkout-modified hook suggestion, so..

>
> Naively, it's simply
>
>         for F in `git diff --name-only $1 $2`; do touch "$F"; done
>
> But consider:
>
> * Symlinks can cause the wrong file to be touched.  (Granted, Michał's
> proposed patch also doesn't deal with symlinks.)  Let's assume that a hook
> can be crafted will all possible sophistication.  There are still some
> fundamental problems:

OK so this one could be tricky to get right, but it's possible.

>
> * 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.

This is a limitation of the current post-checkout hook. $3==0 from the
hook lets us know this is not a branch switch, but it does not really
tell you the affected paths. If it somehow passes all the given
pathspec to you, then you should be able to do "git ls-files --
$pathspec" which gives you the exact same set of paths that
git-checkout updates. We could do this by setting $4 to "--" and put
all the pathspecs in $5+ [1] e.g. "HEAD@{1} HEAD 0 -- path/to/file" in
the above example.

There is  third case here, if you do "git checkout <tree-ish> --
path/to/file" then it cannot be covered by the current design. I guess
we could set $3 to '2' (retrieve from a tree) to indicate this in
addition to 0 (from index) and 1 (from switching branch) and then $1
could be the tree in question (pathspecs are passed the same way
above)

[1] I wonder if we could have a more generic approach to pass
pathspecs via environment, which could work for more than just this
one hook. Not sure if it's a good idea though.

> * The hook has to be set up in every repo and submodule (at least until
> something like Ævar's experiments come to fruition).

Either --template or core.hooksPath would solve this, or I'll try to
get my "hooks.* config" patch in. I think that one is a good thing to
do anyway because it allows more flexible hook management (and it
could allow multiple hooks of the same name too). With this, we could
avoid adding more command-specific config like core.fsmonitor or
uploadpack.packObjectsHook which to me are hooks.

Another option is extend core.hooksPath for searching multiple places
instead of just one like AEvar suggested.

> * 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.)

Both --template and config-based hooks should let you handle this case.

So, I think if we improve the hook system to give more information
(which is something we definitely should do), we could do this without
adding special cases in git. I'm not saying that we should never add
special cases, but at least this lets us play with different kinds of
post-checkout activities before we decide which one would be best
implemented in git.
-- 
Duy