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, 2 insertions, 2 deletions

diff --git a/builtin/init-db.c b/builtin/init-db.c
index 944ec77fe1..5bf61a7e05 100644
--- a/builtin/init-db.c
+++ b/builtin/init-db.c
@@ -356,12 +356,12 @@ int init_db(const char *git_dir, const char *real_git_dir,
 		if (!exist_ok && !stat(real_git_dir, &st))
 			die(_("%s already exists"), real_git_dir);
 
-		set_git_dir(real_path(real_git_dir));
+		set_git_dir(real_git_dir, 1);
 		git_dir = get_git_dir();
 		separate_git_dir(git_dir, original_git_dir);
 	}
 	else {
-		set_git_dir(real_path(git_dir));
+		set_git_dir(git_dir, 1);
 		git_dir = get_git_dir();
 	}
 	startup_info->have_repository = 1;