[PATCH 0/1] Pass through the exit code of post-checkout

[PATCH 0/1] Pass through the exit code of post-checkout

[Johannes Schindelin via GitGitGadget]

This is my attempt to revive an old discussion
[https://public-inbox.org/git/20180314003816.GE147135@aiede.svl.corp.google.com/] 
(related to this StackOverflow thread
[https://stackoverflow.com/questions/25561485/git-rebase-i-with-squash-cannot-detach-head]
).

> [...] is this the right behavior for "git checkout" to have? I.e. is it
useful for "git checkout" to fail when the post-checkout hook fails, or
would it be better for it to e.g. simply print a message and exit with
status 0?


To answer Jonathan's question, at long last, yes, it is useful. A hook is
not only an opportunity to run code at given points in Git's life cycle, but
also an opportunity to stop Git in its tracks. Further, if you script the
operation, it may very well be useful to discern between an exit code from
Git's operation from an exit code produced by your hook.

If you don't want your git checkout/git switch/git restore to fail due to a 
post-checkout failure, just make sure that that hook does not fail ;-) (This
could easily be achieved by trap EXIT { exit 0; }, I believe.

I discovered, however, that the original patch contribution missed that a 
git checkout -b <branch> will not pass through the exit code, but instead
return exit code 1. As part of my contribution, I fix this, and also
introduce a regression test to document the now-consistent behavior.

Johannes Schindelin (1):
  switch/restore: consistently pass through exit code of `post-checkout`

 Documentation/githooks.txt    | 3 ++-
 builtin/checkout.c            | 8 +++++---
 t/t5403-post-checkout-hook.sh | 9 +++++++++
 3 files changed, 16 insertions(+), 4 deletions(-)


base-commit: 70bf0b755af4d1e66da25b7805cac0e481a082e4
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-385%2Fdscho%2Fpost-checkout-hook-exit-code-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-385/dscho/post-checkout-hook-exit-code-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/385
-- 
gitgitgadget

[PATCH 1/1] switch/restore: consistently pass through exit code of `post-checkout`

[Johannes Schindelin via GitGitGadget]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

Currently, the exit code of the `post-checkout` hook becomes the exit
code of the `git restore` (or the equivalent `git checkout`) command,
_iff_ no error occurred during the `restore` operation.

That allows versatile scripting where the `post-checkout` failure modes
can be discerned from the `git restore` failure modes.

There is a problem, though: if that `git restore` operation fails
_before_ the `post-checkout` hook is run, -1 is returned from the
`checkout_paths()` function, which yields the misleading exit code 127
(which is reserved to indicate "command not found").

Another problem: a non-zero exit code of the `post-checkout` hook as run
at the end of a `git switch` run will always causes that command to exit
with exit code 1.

This is inconsistent. Let's pass through the exit code of the
`post-checkout` hook verbatim (unless the Git operation fails, in which
case we want to exit with code 1, even if the hook returns another exit
code).

Document this, and add a regression test for this behavior, too.

Inspired-by-a-patch-by: Magne Land <magne.land@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 Documentation/githooks.txt    | 3 ++-
 builtin/checkout.c            | 8 +++++---
 t/t5403-post-checkout-hook.sh | 9 +++++++++
 3 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 50365f2914..12c756a37b 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -193,7 +193,8 @@ worktree.  The hook is given three parameters: the ref of the previous HEAD,
 the ref of the new HEAD (which may or may not have changed), and a flag
 indicating whether the checkout was a branch checkout (changing branches,
 flag=1) or a file checkout (retrieving a file from the index, flag=0).
-This hook cannot affect the outcome of `git switch` or `git checkout`.
+If this hook exits with an exit code other than 0, it causes the calling
+`git switch` or `git checkout` command to fail with the same exit code.
 
 It is also run after linkgit:git-clone[1], unless the `--no-checkout` (`-n`) option is
 used. The first parameter given to the hook is the null-ref, the second the
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 1283727761..56afc3c1a8 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -400,7 +400,7 @@ static int checkout_paths(const struct checkout_opts *opts,
 	static char *ps_matched;
 	struct object_id rev;
 	struct commit *head;
-	int errs = 0;
+	int errs = 0, ret;
 	struct lock_file lock_file = LOCK_INIT;
 	int checkout_index;
 
@@ -548,8 +548,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 	read_ref_full("HEAD", 0, &rev, NULL);
 	head = lookup_commit_reference_gently(the_repository, &rev, 1);
 
-	errs |= post_checkout_hook(head, head, 0);
-	return errs;
+	ret = post_checkout_hook(head, head, 0);
+	return !errs ? ret : (errs < 0 ? 1 : errs);
 }
 
 static void show_local_changes(struct object *head,
@@ -1062,6 +1062,8 @@ static int switch_branches(const struct checkout_opts *opts,
 
 	ret = post_checkout_hook(old_branch_info.commit, new_branch_info->commit, 1);
 	free(path_to_free);
+	if (ret > 0)
+		return ret;
 	return ret || writeout_error;
 }
 
diff --git a/t/t5403-post-checkout-hook.sh b/t/t5403-post-checkout-hook.sh
index a39b3b5c78..60ce81f6db 100755
--- a/t/t5403-post-checkout-hook.sh
+++ b/t/t5403-post-checkout-hook.sh
@@ -73,4 +73,13 @@ test_expect_success 'post-checkout hook is triggered by clone' '
 	test -f clone3/.git/post-checkout.args
 '
 
+test_expect_success 'exit code of post-checkout hook is passed through' '
+	mkdir -p exit-code &&
+	echo "exit 123" | write_script exit-code/post-checkout &&
+	test_expect_code 123 \
+	git -c core.hooksPath="$PWD/exit-code" switch -c trigger-exit-code &&
+	test_expect_code 123 \
+	git -c core.hooksPath="$PWD/exit-code" restore .
+'
+
 test_done
-- 
gitgitgadget

Re: [PATCH 0/1] Pass through the exit code of post-checkout

[Junio C Hamano]

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> To answer Jonathan's question, at long last, yes, it is useful. A hook is
> not only an opportunity to run code at given points in Git's life cycle, but
> also an opportunity to stop Git in its tracks.

In general that is correct, and especially so for pre_$git_action
hooks, which are about interfering and vetoing an action that is
being carried out.

On the other hand, post_$git_action hooks are specifically defined
as a way to trigger an extra action and documented that they cannot
affect the outcome of the operation (in other words, they trigger at
a point in the operation flow that is too late to affect the
outcome).

Now, it is somewhat debatable how the "outcome" should be defined.
A post-rebase hook that drops the last commit in the sequence, for
example, should not be allowed (the rebase has rebuilt a sequence
and that final sequence of commits should be left), but should it be
allowed to signal back to "git rebase" and affect its exit status?

I am not 100% sure if it is a good idea to allow post-checkout to
affect the exit status of "git checkout" or its friends.  If one
codepath in "git checkout" or its friends lets post-checkout to
influence the exit status while another codepath ignores, it makes
sense to discuss if the inconsistency needs to be removed, but in
such a case, I think it would make sense to unify in the direction
of ignoring (i.e. not allowing post-* hook to affect the outcome).