operations: Return correct error code when merge fails

When the UserMergeToRef fails because of a conflict, then it's currently
not using any gRPC error code but instead directly returns an internal
error, where the Ruby implementation returned a PreconditionFailed error
instead.

Fix this discrepancy by also returning a FailedPrecondition error.
Adjust a test to pin down this error case.

2 files changed, 2 insertions, 1 deletions

diff --git a/internal/gitaly/service/operations/merge.go b/internal/gitaly/service/operations/merge.go
index 194d48786..cd8163d35 100644
--- a/internal/gitaly/service/operations/merge.go
+++ b/internal/gitaly/service/operations/merge.go
@@ -460,7 +460,7 @@ func (s *server) userMergeToRef(ctx context.Context, request *gitalypb.UserMerge
 			return nil, helper.ErrInvalidArgument(err)
 		}
 		//nolint:stylecheck
-		return nil, fmt.Errorf("Failed to create merge commit for source_sha %s and target_sha %s at %s", sourceRef, string(request.TargetRef), refName)
+		return nil, helper.ErrPreconditionFailed(fmt.Errorf("Failed to create merge commit for source_sha %s and target_sha %s at %s", sourceRef, string(request.TargetRef), refName))
 	}
 
 	// ... and move branch from target ref to the merge commit. The Ruby
diff --git a/internal/gitaly/service/operations/merge_test.go b/internal/gitaly/service/operations/merge_test.go
index d165c6f27..19d9b6c04 100644
--- a/internal/gitaly/service/operations/merge_test.go
+++ b/internal/gitaly/service/operations/merge_test.go
@@ -681,6 +681,7 @@ func testConflictsOnUserMergeToRefRequest(t *testing.T, ctx context.Context) {
 
 		_, err := client.UserMergeToRef(ctx, request)
 		require.Error(t, err)
+		testhelper.RequireGrpcError(t, err, codes.FailedPrecondition)
 	})
 }