set_git_dir: fix crash when used with real_path()

`real_path()` returns result from a shared buffer, inviting subtle
reentrance bugs. One of these bugs occur when invoked this way:
    set_git_dir(real_path(git_dir))

In this case, `real_path()` has reentrance:
    real_path
    read_gitfile_gently
    repo_set_gitdir
    setup_git_env
    set_git_dir_1
    set_git_dir

Later, `set_git_dir()` uses its now-dead parameter:
    !is_absolute_path(path)

Fix this by using a dedicated `strbuf` to hold `strbuf_realpath()`.

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/environment.c b/environment.c
index e72a02d..c436de3 100644
--- a/environment.c
+++ b/environment.c
@@ -345,11 +345,20 @@ static void update_relative_gitdir(const char *name,
 	free(path);
 }
 
-void set_git_dir(const char *path)
+void set_git_dir(const char *path, int make_realpath)
 {
+	struct strbuf realpath = STRBUF_INIT;
+
+	if (make_realpath) {
+		strbuf_realpath(&realpath, path, 1);
+		path = realpath.buf;
+	}
+
 	set_git_dir_1(path);
 	if (!is_absolute_path(path))
 		chdir_notify_register(NULL, update_relative_gitdir, NULL);
+
+	strbuf_release(&realpath);
 }
 
 const char *get_log_output_encoding(void)