Hi
On 02/04/2020 17:58, Elijah Newren wrote:
Hi,
On Thu, Apr 2, 2020 at 8:57 AM Philippe Blain
<levraiphilippeblain@xxxxxxxxx> wrote:
Hello,
I'm not sure if this has been reported yet (couldn't find it),
but I noticed a behavioral difference between the merge
and apply backends that is not mentioned in the documentation:
The 'apply' backend will run the post-checkout hook just after
moving HEAD to the commit we are rebasing onto;
this does not happen with the merge backend:
$ echo "echo \"Running post-checkout hook\"" > .git/hooks/post-checkout
$ git checkout -b <branch1> <commit2>
$ git rebase upstream/master --apply
First, rewinding head to replay your work on top of it...
Running post-checkout hook
Applying: <commit1>
Applying: <commit2>
$ git checkout -b <branch2> <commit2>
Successfully rebased and updated refs/heads/<branch2>
The merge backend runs `git checkout` to do the checkout and so does
actually run the post-checkout hook. However the invocation is wrapped
in run_command_silent_on_success() which means if the checkout is
successful you don't see the output from the hook (and if the checkout
fails you don't see the output from the hook because it is not run). You
can verify this by changing the first line of your example to
$ echo "echo \"Running post-checkout hook\" >>/tmp/post-checkout" >
.git/hooks/post-checkout
and examining /tmp/post-checkout
Cheers,
Philippe.
Interesting. We had a report about the post-commit hook, but not
about post-checkout. From the description in the githooks manpage
it's not clear to me that either rebase backend is correct or
incorrect here, but they should at least be consistent. Also, if
rebase should call post-checkout, should cherry-pick? If cherry-pick
should, should commit or revert call it?
cherry-pick/revert don't check anything out
What about reset or merge?
again they are not checking out an existing commit.
However the 'reset' command for rebase -r is checking out an existing
commit so if we decide to keep running the hook for the initial checkout
then I think we should do it after those commands as well as there's
nothing special about the initial checkout.
As for whether we should be running the hook I think it depends on what
people are using it for. If they have a pair of pre-commit and
post-checkout hooks to handle file metadata such as permissions and acls
they might want to run the post-checkout hook so the file metadata gets
updated but then there is no support for this when picking the rebased
commits so it's arguably pointless to do it on the initial checkout.
If they are using it to be notified of branch switches then maybe we
don't need to run it when checking out the onto commit (unless they want
to know that we've switched to a detached HEAD). Though we would still
want to do it for the branch switch in 'git rebase <upstream> <branch>'
As you can see I'm waffling and have no idea what's the right thing to do!
Best Wishes
Phillip
This hook to me seems to be rather badly defined because If any
operations other than checkout/switch should call it, then you've got
various angles of weirdness if you don't include all the others too.
(Also, digging through the history, the only reason that the apply
backend called the post-checkout hook was because it was written as a
script and invoked 'git checkout'. But then someone noticed that the
scripted version called the hook and thus ported it to the builtin.)
From another angle, maybe you could make the cutoff be only when HEAD
changes which symref it points to (meaning rebase would only call it
if you asked it to rebase a different branch than you were on), but
that has its own weirdness too. I think this hook is really weird,
unless we restrict it to only the things already explicitly mentioned
in the githooks manpage.
In any event, I guess that means we need to update this section of the
rebase manpage:
"""
Hooks
~~~~~
The apply backend has not traditionally called the post-commit hook,
while the merge backend has. However, this was by accident of
implementation rather than by design. Both backends should have the
same behavior, though it is not clear which one is correct.
"""
to also mention post-checkout, note that the situation about which
backend calls this hook is reversed relative to post-commit, and again
mention it's not clear which backend is correct.
My very rough opinion of the moment is:
* post-checkout should probably do as the manpage says, and only run
for checkout/switch plus new worktree situations (worktree add/clone).
rebase --apply should stop calling it.
* post-commit is horribly broken; it hardcodes an assumption of
exactly one commit. Also, it's a huge performance problem when things
want to create multiple commits. So, let's do as the githooks manpage
suggest and call this hook from a direct "git commit" -- and _only_
from there. Remove it from rebase, cherry-pick, revert, etc.
Normally, for consistency, I would say that merge should start calling
that hook (it only creates one commit, and "git commit" is used to
complete an interrupted merge anyway so why not uninterrupted ones),
but merge can be called by rebase --rebase-merges right now so that
pushes us back to the get it out of rebase side. The hook is kinda
broken anyway, so maybe limit exposure?
* Make a new post-batch-commit hook called by things that create new
commits, but only call it once per operation (e.g. rebase and
cherry-pick call it once rather than once per commit). Have commit,
merge, and revert all call this too. Not sure what to do about things
that change multiple refs simultaneously, e.g. fast-import (or things
like it, e.g. filter-branch).
CC'ing Jonathan and Emily since they've thought a lot about hooks, and
whom I thought might be making changes in this area[1][2].
Elijah
[1] https://lore.kernel.org/git/20200116203521.GA71299@xxxxxxxxxx/
[2] https://lore.kernel.org/git/20200115215922.GI181522@xxxxxxxxxx/
