lock_ref_sha1_basic(): improve diagnostics for ref D/F conflicts

If there is a failure to lock a reference that is likely caused by a
D/F conflict (e.g., trying to lock "refs/foo/bar" when reference
"refs/foo" already exists), invoke verify_refname_available() to try
to generate a more helpful error message.

That function might not detect an error. For example, some
non-reference file might be blocking the deletion of an
otherwise-empty directory tree, or there might be a race with another
process that just deleted the offending reference. In such cases,
generate the strerror-based error message like before.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

refs.c

@@ -2369,8 +2369,12 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 		ref_file = git_path("%s", orig_refname);
 		if (remove_empty_directories(ref_file)) {
 			last_errno = errno;
-			strbuf_addf(err, "there are still refs under '%s'",
-				    orig_refname);
+
+			if (!verify_refname_available(orig_refname, extras, skip,
+						      get_loose_refs(&ref_cache), err))
+				strbuf_addf(err, "there are still refs under '%s'",
+					    orig_refname);
+
 			goto error_return;
 		}
 		refname = resolve_ref_unsafe(orig_refname, resolve_flags,
@@ -2380,8 +2384,12 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 	    *type_p = type;
 	if (!refname) {
 		last_errno = errno;
-		strbuf_addf(err, "unable to resolve reference %s: %s",
-			    orig_refname, strerror(errno));
+		if (last_errno != ENOTDIR ||
+		    !verify_refname_available(orig_refname, extras, skip,
+					      get_loose_refs(&ref_cache), err))
+			strbuf_addf(err, "unable to resolve reference %s: %s",
+				    orig_refname, strerror(last_errno));
+
 		goto error_return;
 	}
 	/*

t/t1404-update-ref-df-conflicts.sh

@@ -36,7 +36,7 @@ test_expect_success 'existing loose ref is a simple prefix of new' '
 
 	prefix=refs/1l &&
 	test_update_rejected $prefix "a c e" false "b c/x d" \
-		"unable to resolve reference $prefix/c/x: Not a directory"
+		"$Q$prefix/c$Q exists; cannot create $Q$prefix/c/x$Q"
 
 '
 
@@ -52,7 +52,7 @@ test_expect_success 'existing loose ref is a deeper prefix of new' '
 
 	prefix=refs/2l &&
 	test_update_rejected $prefix "a c e" false "b c/x/y d" \
-		"unable to resolve reference $prefix/c/x/y: Not a directory"
+		"$Q$prefix/c$Q exists; cannot create $Q$prefix/c/x/y$Q"
 
 '
 
@@ -68,7 +68,7 @@ test_expect_success 'new ref is a simple prefix of existing loose' '
 
 	prefix=refs/3l &&
 	test_update_rejected $prefix "a c/x e" false "b c d" \
-		"there are still refs under $Q$prefix/c$Q"
+		"$Q$prefix/c/x$Q exists; cannot create $Q$prefix/c$Q"
 
 '
 
@@ -84,7 +84,7 @@ test_expect_success 'new ref is a deeper prefix of existing loose' '
 
 	prefix=refs/4l &&
 	test_update_rejected $prefix "a c/x/y e" false "b c d" \
-		"there are still refs under $Q$prefix/c$Q"
+		"$Q$prefix/c/x/y$Q exists; cannot create $Q$prefix/c$Q"
 
 '