fix: avoid pipeline builder panic when first op is disabled (#3500)

* fix: avoid pipeline builder panic if first op is disabled

* chore: use pipeline builder test to trigger getOperator panic

---------

Co-authored-by: Nityananda Gohain <nityanandagohain@gmail.com>

pkg/query-service/app/logparsingpipeline/pipelineBuilder.go

@@ -58,7 +58,7 @@ func getOperators(ops []model.PipelineOperator) []model.PipelineOperator {
 	filteredOp := []model.PipelineOperator{}
 	for i, operator := range ops {
 		if operator.Enabled {
-			if i > 0 {
+			if len(filteredOp) > 0 {
 				filteredOp[len(filteredOp)-1].Output = operator.ID
 			}
 			filteredOp = append(filteredOp, operator)

pkg/query-service/app/logparsingpipeline/pipelineBuilder_test.go

@@ -163,6 +163,29 @@ var prepareProcessorTestData = []struct {
 			},
 		},
 	},
+	{
+		Name: "first op disabled",
+		Operators: []model.PipelineOperator{
+			{
+				ID:      "move_filename",
+				Output:  "move_function",
+				Enabled: false,
+				Name:    "move_filename",
+			},
+			{
+				ID:      "move_function",
+				Enabled: true,
+				Name:    "move_function",
+			},
+		},
+		Output: []model.PipelineOperator{
+			{
+				ID:      "move_function",
+				Enabled: true,
+				Name:    "move_function",
+			},
+		},
+	},
 }
 
 func TestPreparePipelineProcessor(t *testing.T) {

pkg/query-service/tests/integration/logparsingpipeline_test.go

@@ -231,8 +231,8 @@ func (tb *LogPipelinesTestBed) PostPipelinesToQSExpectingStatusCode(
 
 	if response.StatusCode != expectedStatusCode {
 		tb.t.Fatalf(
-			"Received response status %d after posting log pipelines. Expected: %d",
+			"Received response status %d after posting log pipelines. Expected: %d\nResponse body:%s\n",
-			response.StatusCode, expectedStatusCode,
+			response.StatusCode, expectedStatusCode, string(responseBody),
 		)
 	}