Display an error from update-ref if target ref name is invalid.

Alex Riesen (raa.lkml@gmail.com) recently observed that git branch
would fail with no error message due to unexpected situations with
regards to refs.  For example, if .git/refs/heads/gu is a file but
"git branch -b refs/heads/gu/fixa HEAD" was invoked by the user
it would fail silently due to refs/heads/gu being a file and not
a directory.

This change adds a test for trying to create a ref within a directory
that is actually currently a file, and adds error printing within
the ref locking routine should the resolve operation fail.

The error printing code probably belongs at this level of the library
as other failures within the ref locking, writing and logging code
are also currently at this level of the code.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

refs.c

@@ -294,6 +294,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *path,
 	int plen,
 	const unsigned char *old_sha1, int mustexist)
 {
+	const char *orig_path = path;
 	struct ref_lock *lock;
 	struct stat st;
 
@@ -303,7 +304,11 @@ static struct ref_lock *lock_ref_sha1_basic(const char *path,
 	plen = strlen(path) - plen;
 	path = resolve_ref(path, lock->old_sha1, mustexist);
 	if (!path) {
+		int last_errno = errno;
+		error("unable to resolve reference %s: %s",
+			orig_path, strerror(errno));
 		unlock_ref(lock);
+		errno = last_errno;
 		return NULL;
 	}
 	lock->lk = xcalloc(1, sizeof(struct lock_file));

t/t1400-update-ref.sh

@@ -14,6 +14,8 @@ D=4444444444444444444444444444444444444444
 E=5555555555555555555555555555555555555555
 F=6666666666666666666666666666666666666666
 m=refs/heads/master
+n_dir=refs/heads/gu
+n=$n_dir/fixes
 
 test_expect_success \
 	"create $m" \
@@ -25,6 +27,16 @@ test_expect_success \
 	 test $B = $(cat .git/$m)'
 rm -f .git/$m
 
+test_expect_success \
+	"fail to create $n" \
+	'touch .git/$n_dir
+	 git-update-ref $n $A >out 2>err
+	 test $? = 1 &&
+	 test "" = "$(cat out)" &&
+	 grep "error: unable to resolve reference" err &&
+	 grep $n err'
+rm -f .git/$n_dir out err
+
 test_expect_success \
 	"create $m (by HEAD)" \
 	'git-update-ref HEAD $A &&