Fix "aliases vs pre-command hook" issue

It was reported that the pre-command hook is not called as expected when
running aliases in subdirectories.

Turns out that handle_alias() nastily initializes the Git directory, and
then tries (incompletely) to restore the state beforehand.

Let's do a more thorough job at restoring the state.

An alternative approach would be to avoid calling
setup_git_directory_gently() *just* to read the alias' expansion, using
discover_git_directory_gently(). But that method is fraught with peril, as
e.g. "external" aliases, i.e. aliases starting with an exclamation point
expect to be called in the top-level directory of the current worktree
(and set GIT_PREFIX accordingly), if any.

Signed-off-by: Johannes Schindelin <johasc@microsoft.com>

cache.h

@@ -485,6 +485,8 @@ extern const char *get_git_namespace(void);
 extern const char *strip_namespace(const char *namespaced_ref);
 extern const char *get_super_prefix(void);
 extern const char *get_git_work_tree(void);
+extern void save_git_dir_before_alias(void);
+extern void restore_git_dir_after_alias(void);
 
 /*
  * Return true if the given path is a git directory; note that this _just_

environment.c

@@ -102,7 +102,7 @@ static size_t namespace_len;
 
 static const char *super_prefix;
 
-static const char *git_dir, *git_common_dir;
+static const char *git_dir, *git_common_dir, *saved_git_dir;
 static char *git_object_dir, *git_index_file, *git_graft_file;
 int git_db_env, git_index_env, git_graft_env, git_common_dir_env;
 
@@ -196,6 +196,16 @@ static void setup_git_env(void)
 		set_alternate_shallow_file(shallow_file, 0);
 }
 
+void save_git_dir_before_alias(void)
+{
+	saved_git_dir = git_dir;
+}
+
+void restore_git_dir_after_alias(void)
+{
+	git_dir = saved_git_dir;
+}
+
 int is_bare_repository(void)
 {
 	/* if core.bare is not 'false', let's see if there is a work tree */

git.c

@@ -26,13 +26,16 @@ static const char *env_names[] = {
 };
 static char *orig_env[4];
 static int save_restore_env_balance;
+static struct startup_info orig_startup_info;
 
 static void save_env_before_alias(void)
 {
 	int i;
 
 	assert(save_restore_env_balance == 0);
 	save_restore_env_balance = 1;
+	orig_startup_info = *startup_info;
+	save_git_dir_before_alias();
 	orig_cwd = xgetcwd();
 	for (i = 0; i < ARRAY_SIZE(env_names); i++) {
 		orig_env[i] = getenv(env_names[i]);
@@ -46,6 +49,8 @@ static void restore_env(int external_alias)
 
 	assert(save_restore_env_balance == 1);
 	save_restore_env_balance = 0;
+	*startup_info = orig_startup_info;
+	restore_git_dir_after_alias();
 	if (!external_alias && orig_cwd && chdir(orig_cwd))
 		die_errno("could not move to %s", orig_cwd);
 	free(orig_cwd);

t/t0400-pre-command-hook.sh

@@ -40,4 +40,17 @@ test_expect_success 'in a subdirectory' '
 	test_path_is_file sub/i-was-here
 '
 
+test_expect_success 'in a subdirectory, using an alias' '
+	git reset --hard &&
+	echo "echo \"\$@; \$(pwd)\" >>log" |
+	write_script .git/hooks/pre-command &&
+	mkdir -p sub &&
+	(
+		cd sub &&
+		git -c alias.r="rebase HEAD" r
+	) &&
+	test_path_is_missing log &&
+	test_line_count = 2 sub/log
+'
+
 test_done