checkout,clone: check return value of create_symref

It's unlikely that we would fail to create or update a
symbolic ref (especially HEAD), but if we do, we should
notice and complain. Note that there's no need to give more
details in our error message; create_symref will already
have done so.

While we're here, let's also fix a minor memory leak in
clone.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -661,7 +661,8 @@ static void update_refs_for_switch(const struct checkout_opts *opts,
 			describe_detached_head(_("HEAD is now at"), new->commit);
 		}
 	} else if (new->path) {	/* Switch branches. */
-		create_symref("HEAD", new->path, msg.buf);
+		if (create_symref("HEAD", new->path, msg.buf) < 0)
+			die("unable to update HEAD");
 		if (!opts->quiet) {
 			if (old->path && !strcmp(new->path, old->path)) {
 				if (opts->new_branch_force)

builtin/clone.c

@@ -636,9 +636,11 @@ static void update_remote_refs(const struct ref *refs,
 		struct strbuf head_ref = STRBUF_INIT;
 		strbuf_addstr(&head_ref, branch_top);
 		strbuf_addstr(&head_ref, "HEAD");
-		create_symref(head_ref.buf,
-			      remote_head_points_at->peer_ref->name,
-			      msg);
+		if (create_symref(head_ref.buf,
+				  remote_head_points_at->peer_ref->name,
+				  msg) < 0)
+			die("unable to update %s", head_ref.buf);
+		strbuf_release(&head_ref);
 	}
 }
 
@@ -648,7 +650,8 @@ static void update_head(const struct ref *our, const struct ref *remote,
 	const char *head;
 	if (our && skip_prefix(our->name, "refs/heads/", &head)) {
 		/* Local default branch link */
-		create_symref("HEAD", our->name, NULL);
+		if (create_symref("HEAD", our->name, NULL) < 0)
+			die("unable to update HEAD");
 		if (!option_bare) {
 			update_ref(msg, "HEAD", our->old_oid.hash, NULL, 0,
 				   UPDATE_REFS_DIE_ON_ERR);

t/t2011-checkout-invalid-head.sh

@@ -19,4 +19,10 @@ test_expect_success 'checkout master from invalid HEAD' '
 	git checkout master --
 '
 
+test_expect_success 'checkout notices failure to lock HEAD' '
+	test_when_finished "rm -f .git/HEAD.lock" &&
+	>.git/HEAD.lock &&
+	test_must_fail git checkout -b other
+'
+
 test_done