worktrees: add find_shared_symref

Add a new function, find_shared_symref, which contains the heart of
die_if_checked_out, but works for any symref, not just HEAD.  Refactor
die_if_checked_out to use the same infrastructure as
find_shared_symref.

Soon, we will use find_shared_symref to protect notes merges in
worktrees.

Signed-off-by: David Turner <dturner@twopensource.com>
Reviewed-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -310,21 +310,23 @@ void remove_branch_state(void)
 	unlink(git_path("SQUASH_MSG"));
 }
 
-static void check_linked_checkout(const char *branch, const char *id)
+static char *find_linked_symref(const char *symref, const char *branch,
+				const char *id)
 {
 	struct strbuf sb = STRBUF_INIT;
 	struct strbuf path = STRBUF_INIT;
 	struct strbuf gitdir = STRBUF_INIT;
+	char *existing = NULL;
 
 	/*
-	 * $GIT_COMMON_DIR/HEAD is practically outside
-	 * $GIT_DIR so resolve_ref_unsafe() won't work (it
-	 * uses git_path). Parse the ref ourselves.
+	 * $GIT_COMMON_DIR/$symref (e.g. HEAD) is practically outside
+	 * $GIT_DIR so resolve_ref_unsafe() won't work (it uses
+	 * git_path). Parse the ref ourselves.
 	 */
 	if (id)
-		strbuf_addf(&path, "%s/worktrees/%s/HEAD", get_git_common_dir(), id);
+		strbuf_addf(&path, "%s/worktrees/%s/%s", get_git_common_dir(), id, symref);
 	else
-		strbuf_addf(&path, "%s/HEAD", get_git_common_dir());
+		strbuf_addf(&path, "%s/%s", get_git_common_dir(), symref);
 
 	if (!strbuf_readlink(&sb, path.buf, 0)) {
 		if (!starts_with(sb.buf, "refs/") ||
@@ -346,33 +348,53 @@ static void check_linked_checkout(const char *branch, const char *id)
 		strbuf_rtrim(&gitdir);
 	} else
 		strbuf_addstr(&gitdir, get_git_common_dir());
-	skip_prefix(branch, "refs/heads/", &branch);
 	strbuf_strip_suffix(&gitdir, ".git");
-	die(_("'%s' is already checked out at '%s'"), branch, gitdir.buf);
+
+	existing = strbuf_detach(&gitdir, NULL);
 done:
 	strbuf_release(&path);
 	strbuf_release(&sb);
 	strbuf_release(&gitdir);
+
+	return existing;
 }
 
-void die_if_checked_out(const char *branch)
+char *find_shared_symref(const char *symref, const char *target)
 {
 	struct strbuf path = STRBUF_INIT;
 	DIR *dir;
 	struct dirent *d;
+	char *existing;
 
-	check_linked_checkout(branch, NULL);
+	if ((existing = find_linked_symref(symref, target, NULL)))
+		return existing;
 
 	strbuf_addf(&path, "%s/worktrees", get_git_common_dir());
 	dir = opendir(path.buf);
 	strbuf_release(&path);
 	if (!dir)
-		return;
+		return NULL;
 
 	while ((d = readdir(dir)) != NULL) {
 		if (!strcmp(d->d_name, ".") || !strcmp(d->d_name, ".."))
 			continue;
-		check_linked_checkout(branch, d->d_name);
+		existing = find_linked_symref(symref, target, d->d_name);
+		if (existing)
+			goto done;
 	}
+done:
 	closedir(dir);
+
+	return existing;
+}
+
+void die_if_checked_out(const char *branch)
+{
+	char *existing;
+
+	existing = find_shared_symref("HEAD", branch);
+	if (existing) {
+		skip_prefix(branch, "refs/heads/", &branch);
+		die(_("'%s' is already checked out at '%s'"), branch, existing);
+	}
 }

branch.h

@@ -59,4 +59,12 @@ extern int read_branch_desc(struct strbuf *, const char *branch_name);
  */
 extern void die_if_checked_out(const char *branch);
 
+/*
+ * Check if a per-worktree symref points to a ref in the main worktree
+ * or any linked worktree, and return the path to the exising worktree
+ * if it is.  Returns NULL if there is no existing ref.  The caller is
+ * responsible for freeing the returned path.
+ */
+extern char *find_shared_symref(const char *symref, const char *target);
+
 #endif