worktree: return allocated string from `get_worktree_git_dir()`

The `get_worktree_git_dir()` function returns a string constant that
does not need to be free'd by the caller. This string is computed for
three different cases:

  - If we don't have a worktree we return a path into the Git directory.
    The returned string is owned by `the_repository`, so there is no
    need for the caller to free it.

  - If we have a worktree, but no worktree ID then the caller requests
    the main worktree. In this case we return a path into the common
    directory, which again is owned by `the_repository` and thus does
    not need to be free'd.

  - In the third case, where we have an actual worktree, we compute the
    path relative to "$GIT_COMMON_DIR/worktrees/". This string does not
    need to be released either, even though `git_common_path()` ends up
    allocating memory. But this doesn't result in a memory leak either
    because we write into a buffer returned by `get_pathname()`, which
    returns one out of four static buffers.

We're about to drop `git_common_path()` in favor of `repo_common_path()`,
which doesn't use the same mechanism but instead returns an allocated
string owned by the caller. While we could adapt `get_worktree_git_dir()`
to also use `get_pathname()` and print the derived common path into that
buffer, the whole schema feels a lot like premature optimization in this
context. There are some callsites where we call `get_worktree_git_dir()`
in a loop that iterates through all worktrees. But none of these loops
seem to be even remotely in the hot path, so saving a single allocation
there does not feel worth it.

Refactor the function to instead consistently return an allocated path
so that we can start using `repo_common_path()` in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/builtin/fsck.c b/builtin/fsck.c
index c12203e012..eea1d43647 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -1057,7 +1057,7 @@ int cmd_fsck(int argc,
 			struct worktree *wt = *p;
 			struct index_state istate =
 				INDEX_STATE_INIT(the_repository);
-			char *path;
+			char *path, *wt_gitdir;
 
 			/*
 			 * Make a copy since the buffer is reusable
@@ -1065,9 +1065,13 @@ int cmd_fsck(int argc,
 			 * while we're examining the index.
 			 */
 			path = xstrdup(worktree_git_path(the_repository, wt, "index"));
-			read_index_from(&istate, path, get_worktree_git_dir(wt));
+			wt_gitdir = get_worktree_git_dir(wt);
+
+			read_index_from(&istate, path, wt_gitdir);
 			fsck_index(&istate, path, wt->is_current);
+
 			discard_index(&istate);
+			free(wt_gitdir);
 			free(path);
 		}
 		free_worktrees(worktrees);