[PATCH 1/2] worktree repair: refactor and reduce .git file reads

@@ -637,6 +637,14 @@ int other_head_refs(struct repository *repo,
 	return ret;
 }
 
+static const char *get_worktree_id(const char *dotgit_contents)
+{
+	const char *slash = find_last_dir_sep(dotgit_contents);
+	if (!slash)
+		return "";
+	return slash + 1;
+}
+
 /*
  * Repair worktree's /path/to/worktree/.git file if missing, corrupt, or not
  * pointing at <repo>/worktrees/<id>.

@@ -798,30 +806,20 @@ static int is_main_worktree_path(struct repository *repo, const char *path)
  * Returns -1 on failure and strbuf.len on success.
  */
 static ssize_t infer_backlink(struct repository *repo,
-			      const char *gitfile,
+			      const char *dotgit_contents,
 			      struct strbuf *inferred)
 {
-	struct strbuf actual = STRBUF_INIT;
 	const char *id;
 
-	if (strbuf_read_file(&actual, gitfile, 0) < 0)
-		goto error;
-	if (!starts_with(actual.buf, "gitdir:"))
-		goto error;
-	if (!(id = find_last_dir_sep(actual.buf)))
-		goto error;
-	strbuf_trim(&actual);
-	id++; /* advance past '/' to point at <id> */
+	id = get_worktree_id(dotgit_contents);
 	if (!*id)
 		goto error;
 	repo_common_path_replace(repo, inferred, "worktrees/%s", id);
 	if (!is_directory(inferred->buf))
 		goto error;
 
-	strbuf_release(&actual);
 	return inferred->len;
 error:
-	strbuf_release(&actual);
 	strbuf_reset(inferred); /* clear invalid path */
 	return -1;
 }

@@ -840,7 +838,8 @@ void repair_worktree_at_path(struct repository *repo,
 	struct strbuf inferred_backlink = STRBUF_INIT;
 	struct strbuf gitdir = STRBUF_INIT;
 	struct strbuf olddotgit = STRBUF_INIT;
-	char *dotgit_contents = NULL;
+	struct strbuf contents = STRBUF_INIT;
+	const char *dotgit_contents = NULL;
 	const char *repair = NULL;
 	int err;
 

@@ -856,51 +855,49 @@ void repair_worktree_at_path(struct repository *repo,
 		goto done;
 	}
 
-	infer_backlink(repo, dotgit.buf, &inferred_backlink);
-	strbuf_realpath_forgiving(&inferred_backlink, inferred_backlink.buf, 0);
-	dotgit_contents = xstrdup_or_null(read_gitfile_gently(dotgit.buf, &err));
-	if (dotgit_contents) {
-		strbuf_addstr(&backlink, dotgit_contents);
-	} else if (err == READ_GITFILE_ERR_NOT_A_FILE ||
-			err == READ_GITFILE_ERR_IS_A_DIR) {
+	err = read_gitfile_raw(&contents, dotgit.buf);
+	if (err == READ_GITFILE_ERR_NOT_A_FILE ||
+	    err == READ_GITFILE_ERR_IS_A_DIR) {
 		fn(1, dotgit.buf, _("unable to locate repository; .git is not a file"), cb_data);
 		goto done;
-	} else if (err == READ_GITFILE_ERR_NOT_A_REPO) {
-		if (inferred_backlink.len) {
-			/*
-			 * Worktree's .git file does not point at a repository
-			 * but we found a .git/worktrees/<id> in this
-			 * repository with the same <id> as recorded in the
-			 * worktree's .git file so make the worktree point at
-			 * the discovered .git/worktrees/<id>.
-			 */
-			strbuf_swap(&backlink, &inferred_backlink);
-		} else {
-			fn(1, dotgit.buf, _("unable to locate repository; .git file does not reference a repository"), cb_data);
-			goto done;
-		}
-	} else {
+	} else if (err) {
 		fn(1, dotgit.buf, _("unable to locate repository; .git file broken"), cb_data);
 		goto done;
 	}
 
+	dotgit_contents = contents.buf;
+	infer_backlink(repo, dotgit_contents, &inferred_backlink);
+	strbuf_realpath_forgiving(&inferred_backlink, inferred_backlink.buf, 0);
+
+	if (is_absolute_path(dotgit_contents)) {
+		strbuf_addstr(&backlink, dotgit_contents);
+	} else {
+		strbuf_addbuf(&backlink, &dotgit);
+		strbuf_strip_suffix(&backlink, ".git");
+		strbuf_addstr(&backlink, dotgit_contents);
+		strbuf_realpath_forgiving(&backlink, backlink.buf, 0);
+	}
+
+	if (!is_git_directory(backlink.buf) && !inferred_backlink.len) {
+		fn(1, dotgit.buf, _("unable to locate repository; .git file does not reference a repository"), cb_data);
+		goto done;
+	}
+
 	/*
 	 * If we got this far, either the worktree's .git file pointed at a
-	 * valid repository (i.e. read_gitfile_gently() returned success) or
+	 * valid repository (i.e. is_git_directory() returned true) or
 	 * the .git file did not point at a repository but we were able to
 	 * infer a suitable new value for the .git file by locating a
 	 * .git/worktrees/<id> in *this* repository corresponding to the <id>
 	 * recorded in the worktree's .git file.
 	 *
-	 * However, if, at this point, inferred_backlink is non-NULL (i.e. we
-	 * found a suitable .git/worktrees/<id> in *this* repository) *and* the
-	 * worktree's .git file points at a valid repository *and* those two
-	 * paths differ, then that indicates that the user probably *copied*
-	 * the main and linked worktrees to a new location as a unit rather
-	 * than *moving* them. Thus, the copied worktree's .git file actually
-	 * points at the .git/worktrees/<id> in the *original* repository, not
-	 * in the "copy" repository. In this case, point the "copy" worktree's
-	 * .git file at the "copy" repository.
+	 * Even if the worktree's .git file pointed at a valid repository,
+	 * it doesn't always mean that the backlink is correct. For example,
+	 * the user might have *copied* the main and linked worktrees to a
+	 * new location as a unit rather than *moving* them (the copied
+	 * worktree's .git file actually points at the .git/worktrees/<id>
+	 * in the *original* repository, not in the "copy" repository).
+	 * Therefore, we prioritize inferred_backlink over backlink.
 	 */
 	if (inferred_backlink.len && fspathcmp(backlink.buf, inferred_backlink.buf))
 		strbuf_swap(&backlink, &inferred_backlink);

@@ -926,12 +923,12 @@ void repair_worktree_at_path(struct repository *repo,
 					     gitdir.buf, use_relative_paths);
 	}
 done:
-	free(dotgit_contents);
 	strbuf_release(&olddotgit);
 	strbuf_release(&backlink);
 	strbuf_release(&inferred_backlink);
 	strbuf_release(&gitdir);
 	strbuf_release(&dotgit);
+	strbuf_release(&contents);
 }
 
 int should_prune_worktree(struct repository *repo,