fix(inputs/modbus) influxdata#11105 no empty grouped requests

Simple modification to
* allow starting the field list with an omitted field (useful if many requests needed)
* avoids requests where all fields are omitted

As a result, the total number of request is generally lower, also improving performance

plugins/inputs/modbus/modbus_test.go

@@ -1857,3 +1857,88 @@ func TestConfigurationPerRequestFail(t *testing.T) {
 		})
 	}
 }
+
+func TestRequestGroupingStartingWithOmit(t *testing.T) {
+	modbus := Modbus{
+		Name:              "Test",
+		Controller:        "tcp://localhost:1502",
+		ConfigurationType: "request",
+		Log:               testutil.Logger{},
+	}
+	modbus.Requests = []requestDefinition{
+		{SlaveID: 1,
+			ByteOrder:    "ABCD",
+			RegisterType: "holding",
+			Fields: []requestFieldDefinition{
+				{
+					Name:      "holding-0",
+					Address:   uint16(0),
+					InputType: "INT16",
+					Omit:      true,
+				},
+				{
+					Name:      "holding-1",
+					Address:   uint16(1),
+					InputType: "UINT16",
+					Omit:      true,
+				},
+				{
+					Name:        "holding-2",
+					Address:     uint16(2),
+					InputType:   "INT64",
+					Scale:       1.2,
+					OutputType:  "FLOAT64",
+					Measurement: "modbus",
+				},
+			},
+		},
+	}
+	require.NoError(t, modbus.Init())
+	require.NotEmpty(t, modbus.requests)
+	require.NotNil(t, modbus.requests[1])
+	require.Len(t, modbus.requests[1].holding, 1)
+	require.Equal(t, uint16(2), modbus.requests[1].holding[0].address)
+	require.Equal(t, uint16(4), modbus.requests[1].holding[0].length)
+}
+
+func TestRequestGroupingAllOmit(t *testing.T) {
+	modbus := Modbus{
+		Name:              "Test",
+		Controller:        "tcp://localhost:1502",
+		ConfigurationType: "request",
+		Log:               testutil.Logger{},
+	}
+	modbus.Requests = []requestDefinition{
+		{SlaveID: 1,
+			ByteOrder:    "ABCD",
+			RegisterType: "holding",
+			Fields: []requestFieldDefinition{
+				{
+					Name:      "holding-0",
+					Address:   uint16(0),
+					InputType: "INT16",
+					Omit:      true,
+				},
+				{
+					Name:      "holding-1",
+					Address:   uint16(1),
+					InputType: "UINT16",
+					Omit:      true,
+				},
+				{
+					Name:        "holding-2",
+					Address:     uint16(2),
+					InputType:   "INT64",
+					Scale:       1.2,
+					OutputType:  "FLOAT64",
+					Measurement: "modbus",
+					Omit:        true,
+				},
+			},
+		},
+	}
+	require.NoError(t, modbus.Init())
+	require.NotEmpty(t, modbus.requests)
+	require.NotNil(t, modbus.requests[1])
+	require.Len(t, modbus.requests[1].holding, 0)
+}

plugins/inputs/modbus/request.go

@@ -24,6 +24,10 @@ func newRequest(f field, tags map[string]string) request {
 	return r
 }
 
+func isNonEmptyRequest(c request) bool {
+	return !((len(c.fields) == 0) || ((len(c.fields) == 1) && c.fields[0].omit))
+}
+
 func groupFieldsToRequests(fields []field, tags map[string]string, maxBatchSize uint16) []request {
 	if len(fields) == 0 {
 		return nil
@@ -53,16 +57,23 @@ func groupFieldsToRequests(fields []field, tags map[string]string, maxBatchSize
 			current.length += f.length
 			if !f.omit {
 				// Omit adding the field but use it for constructing the request.
-				current.fields = append(current.fields, f)
+				if isNonEmptyRequest(current) {
+					current.fields = append(current.fields, f)
+				} else {
+					current = newRequest(f, tags)
+				}
 			}
 			continue
 		}
 
 		// Finish the current request, add it to the list and construct a new one
-		requests = append(requests, current)
+		if isNonEmptyRequest(current) {
+			requests = append(requests, current)
+		}
 		current = newRequest(f, tags)
 	}
-	requests = append(requests, current)
-
+	if isNonEmptyRequest(current) {
+		requests = append(requests, current)
+	}
 	return requests
 }