refs/files: use correct error type when lock exists

When fetching references into a repository, if a lock for a particular
reference exists, then `lock_raw_ref()` throws:

    - REF_TRANSACTION_ERROR_CASE_CONFLICT: when there is a conflict
    because the transaction contains conflicting references while being
    on a case-insensitive filesystem.

    - REF_TRANSACTION_ERROR_GENERIC: for all other errors.

The latter causes the entire set of batched updates to fail, even in
case sensitive filessystems.

Instead, return a 'REF_TRANSACTION_ERROR_CREATE_EXISTS' error. This
allows batched updates to reject the individual update which conflicts
with the existing file, while updating the rest of the references.

Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak

refs/files-backend.c

@@ -797,9 +797,24 @@ static enum ref_transaction_error lock_raw_ref(struct files_ref_store *refs,
 			goto retry;
 		} else {
 			unable_to_lock_message(ref_file.buf, myerr, err);
-			if (myerr == EEXIST && ignore_case &&
-			    transaction_has_case_conflicting_update(transaction, update))
-				ret = REF_TRANSACTION_ERROR_CASE_CONFLICT;
+			if (myerr == EEXIST) {
+				if (ignore_case &&
+				    transaction_has_case_conflicting_update(transaction, update)) {
+					/*
+					 * In case-insensitive filesystems, ensure that conflicts within a
+					 * given transaction are handled. Pre-existing refs on a
+					 * case-insensitive system will be overridden without any issue.
+					 */
+					ret = REF_TRANSACTION_ERROR_CASE_CONFLICT;
+				} else {
+					/*
+					 * Pre-existing case-conflicting reference locks should also be
+					 * specially categorized to avoid failing all batched updates.
+					 */
+					ret = REF_TRANSACTION_ERROR_CREATE_EXISTS;
+				}
+			}
+
 			goto error_return;
 		}
 	}

t/t5510-fetch.sh

@@ -1546,6 +1546,32 @@ test_expect_success CASE_INSENSITIVE_FS,REFFILES 'existing references in a case
 	)
 '
 
+test_expect_success REFFILES 'existing reference lock in repo' '
+	test_when_finished rm -rf base repo &&
+	(
+		git init --ref-format=reftable base &&
+		cd base &&
+		echo >file update &&
+		git add . &&
+		git commit -m "updated" &&
+		git branch -M main &&
+
+		git update-ref refs/heads/foo @ &&
+		git update-ref refs/heads/branch @ &&
+		cd .. &&
+
+		git init --ref-format=files --bare repo &&
+		cd repo &&
+		git remote add origin ../base &&
+		touch refs/heads/foo.lock &&
+		test_must_fail git fetch -f origin "refs/heads/*:refs/heads/*" 2>err &&
+		test_grep "error: fetching ref refs/heads/foo failed: reference already exists" err &&
+		git rev-parse refs/heads/main >expect &&
+		git rev-parse refs/heads/branch >actual &&
+		test_cmp expect actual
+	)
+'
+
 . "$TEST_DIRECTORY"/lib-httpd.sh
 start_httpd