Expect sql.ErrNoRows when querying read_errors table

differ/differ.go

@@ -537,7 +537,9 @@ func (d *Differ) produceEntriesToKafka(cluster types.ClusterEntry, ruleContent t
 }
 
 // checkReadError function checks whether reading from read_errors table was
-// successful
+// successful. This log gives important context as the insertion into the
+// read_error will fail if there's already a row but we couldn't read it for
+// whatever reason.
 func checkReadError(err error) {
 	if err != nil {
 		log.Err(err).Msg("read_errors read access error")

differ/storage.go

@@ -564,6 +564,10 @@ func (storage DBStorage) ReadErrorExists(
 
 	// check for any error during query
 	if err != nil {
+		// errNoRows is expected
+		if err == sql.ErrNoRows {
+			return false, nil
+		}
 		return false, err
 	}
 

differ/storage_test.go

@@ -411,6 +411,38 @@ func TestReadErrorOnError(t *testing.T) {
 	checkAllExpectations(t, mock)
 }
 
+// TestReadErrorOnErrorNoRows checks if Storage.ReadErrorExists returns
+// expected results when no rows are found
+func TestReadErrorOnErrorNoRows(t *testing.T) {
+	// error to be thrown
+	mockedError := sql.ErrNoRows
+
+	// prepare new mocked connection to database
+	connection, mock := mustCreateMockConnection(t)
+
+	// expected query performed by tested function
+	expectedQuery := "SELECT exists\\(SELECT 1 FROM read_errors WHERE org_id=\\$1 and cluster=\\$2 and updated_at=\\$3\\);"
+
+	// let's raise an error!
+	mock.ExpectQuery(expectedQuery).WillReturnError(mockedError)
+	mock.ExpectClose()
+
+	// prepare connection to mocked database
+	storage := differ.NewFromConnection(connection, 1)
+
+	// call the tested method
+	_, err := storage.ReadErrorExists(1, "123", types.Timestamp(time.Now()))
+
+	// error is expected to be returned from called method
+	assert.Nil(t, err, "no error is expected if no row is found in read_errors table")
+
+	// connection to mocked DB needs to be closed properly
+	checkConnectionClose(t, connection)
+
+	// check if all expectations were met
+	checkAllExpectations(t, mock)
+}
+
 // TestWriteReadError function checks the method
 // Storage.WriteReadError.
 func TestWriteReadError(t *testing.T) {