worktree: refactor find_linked_symref function

Refactoring will help transition this code to provide additional useful
worktree functions.

Signed-off-by: Michael Rappazzo <rappazzo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

worktree.c

@@ -3,6 +3,64 @@
 #include "strbuf.h"
 #include "worktree.h"
 
+/*
+ * read 'path_to_ref' into 'ref'.  Also if is_detached is not NULL,
+ * set is_detached to 1 (0) if the ref is detatched (is not detached).
+ *
+ * $GIT_COMMON_DIR/$symref (e.g. HEAD) is practically outside $GIT_DIR so
+ * for linked worktrees, `resolve_ref_unsafe()` won't work (it uses
+ * git_path). Parse the ref ourselves.
+ *
+ * return -1 if the ref is not a proper ref, 0 otherwise (success)
+ */
+static int parse_ref(char *path_to_ref, struct strbuf *ref, int *is_detached)
+{
+	if (is_detached)
+		*is_detached = 0;
+	if (!strbuf_readlink(ref, path_to_ref, 0)) {
+		/* HEAD is symbolic link */
+		if (!starts_with(ref->buf, "refs/") ||
+				check_refname_format(ref->buf, 0))
+			return -1;
+	} else if (strbuf_read_file(ref, path_to_ref, 0) >= 0) {
+		/* textual symref or detached */
+		if (!starts_with(ref->buf, "ref:")) {
+			if (is_detached)
+				*is_detached = 1;
+		} else {
+			strbuf_remove(ref, 0, strlen("ref:"));
+			strbuf_trim(ref);
+			if (check_refname_format(ref->buf, 0))
+				return -1;
+		}
+	} else
+		return -1;
+	return 0;
+}
+
+static char *find_main_symref(const char *symref, const char *branch)
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct strbuf path = STRBUF_INIT;
+	struct strbuf gitdir = STRBUF_INIT;
+	char *existing = NULL;
+
+	strbuf_addf(&path, "%s/%s", get_git_common_dir(), symref);
+	if (parse_ref(path.buf, &sb, NULL) < 0)
+		goto done;
+	if (strcmp(sb.buf, branch))
+		goto done;
+	strbuf_addstr(&gitdir, get_git_common_dir());
+	strbuf_strip_suffix(&gitdir, ".git");
+	existing = strbuf_detach(&gitdir, NULL);
+done:
+	strbuf_release(&path);
+	strbuf_release(&sb);
+	strbuf_release(&gitdir);
+
+	return existing;
+}
+
 static char *find_linked_symref(const char *symref, const char *branch,
 				const char *id)
 {
@@ -11,36 +69,20 @@ static char *find_linked_symref(const char *symref, const char *branch,
 	struct strbuf gitdir = STRBUF_INIT;
 	char *existing = NULL;
 
-	/*
-	 * $GIT_COMMON_DIR/$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/%s", get_git_common_dir(), id, symref);
-	else
-		strbuf_addf(&path, "%s/%s", get_git_common_dir(), symref);
+	if (!id)
+		die("Missing linked worktree name");
 
-	if (!strbuf_readlink(&sb, path.buf, 0)) {
-		if (!starts_with(sb.buf, "refs/") ||
-		    check_refname_format(sb.buf, 0))
-			goto done;
-	} else if (strbuf_read_file(&sb, path.buf, 0) >= 0 &&
-	    starts_with(sb.buf, "ref:")) {
-		strbuf_remove(&sb, 0, strlen("ref:"));
-		strbuf_trim(&sb);
-	} else
+	strbuf_addf(&path, "%s/worktrees/%s/%s", get_git_common_dir(), id, symref);
+
+	if (parse_ref(path.buf, &sb, NULL) < 0)
 		goto done;
 	if (strcmp(sb.buf, branch))
 		goto done;
-	if (id) {
-		strbuf_reset(&path);
-		strbuf_addf(&path, "%s/worktrees/%s/gitdir", get_git_common_dir(), id);
-		if (strbuf_read_file(&gitdir, path.buf, 0) <= 0)
-			goto done;
-		strbuf_rtrim(&gitdir);
-	} else
-		strbuf_addstr(&gitdir, get_git_common_dir());
+	strbuf_reset(&path);
+	strbuf_addf(&path, "%s/worktrees/%s/gitdir", get_git_common_dir(), id);
+	if (strbuf_read_file(&gitdir, path.buf, 0) <= 0)
+		goto done;
+	strbuf_rtrim(&gitdir);
 	strbuf_strip_suffix(&gitdir, ".git");
 
 	existing = strbuf_detach(&gitdir, NULL);
@@ -59,7 +101,7 @@ char *find_shared_symref(const char *symref, const char *target)
 	struct dirent *d;
 	char *existing;
 
-	if ((existing = find_linked_symref(symref, target, NULL)))
+	if ((existing = find_main_symref(symref, target)))
 		return existing;
 
 	strbuf_addf(&path, "%s/worktrees", get_git_common_dir());