Pass through the exit code of post-checkout

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

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;
 }
 

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