write_ref_to_lockfile(): new function, extracted from write_ref_sha1()

This is the first step towards separating the checking and writing of
the new reference value to committing the change.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -3048,23 +3048,15 @@ int is_branch(const char *refname)
 }
 
 /*
- * Write sha1 into the ref specified by the lock. Make sure that errno
- * is sane on error.
+ * Write sha1 into the open lockfile, then close the lockfile. On
+ * errors, rollback the lockfile and set errno to reflect the problem.
  */
-static int write_ref_sha1(struct ref_lock *lock,
-	const unsigned char *sha1, const char *logmsg)
+static int write_ref_to_lockfile(struct ref_lock *lock,
+				 const unsigned char *sha1)
 {
 	static char term = '\n';
 	struct object *o;
 
-	if (!lock) {
-		errno = EINVAL;
-		return -1;
-	}
-	if (!lock->force_write && !hashcmp(lock->old_sha1, sha1)) {
-		unlock_ref(lock);
-		return 0;
-	}
 	o = parse_object(sha1);
 	if (!o) {
 		error("Trying to write ref %s with nonexistent object %s",
@@ -3089,6 +3081,28 @@ static int write_ref_sha1(struct ref_lock *lock,
 		errno = save_errno;
 		return -1;
 	}
+	return 0;
+}
+
+/*
+ * Write sha1 into the ref specified by the lock. Make sure that errno
+ * is sane on error.
+ */
+static int write_ref_sha1(struct ref_lock *lock,
+	const unsigned char *sha1, const char *logmsg)
+{
+	if (!lock) {
+		errno = EINVAL;
+		return -1;
+	}
+	if (!lock->force_write && !hashcmp(lock->old_sha1, sha1)) {
+		unlock_ref(lock);
+		return 0;
+	}
+
+	if (write_ref_to_lockfile(lock, sha1))
+		return -1;
+
 	clear_loose_ref_cache(&ref_cache);
 	if (log_ref_write(lock->ref_name, lock->old_sha1, sha1, logmsg) < 0 ||
 	    (strcmp(lock->ref_name, lock->orig_ref_name) &&