Allow JSON data format to contain zero metrics (#3268)

(cherry picked from commit 22a9ffbb9d)

plugins/inputs/httpjson/httpjson_test.go

@@ -477,15 +477,13 @@ func TestHttpJsonBadJson(t *testing.T) {
 	assert.Equal(t, 0, acc.NFields())
 }
 
-// Test response to empty string as response objectgT
+// Test response to empty string as response object
 func TestHttpJsonEmptyResponse(t *testing.T) {
 	httpjson := genMockHttpJson(empty, 200)
 
 	var acc testutil.Accumulator
 	err := acc.GatherError(httpjson[0].Gather)
-
-	assert.Error(t, err)
-	assert.Equal(t, 0, acc.NFields())
+	assert.NoError(t, err)
 }
 
 // Test that the proper values are ignored or collected

plugins/parsers/json/parser.go

@@ -67,6 +67,10 @@ func (p *JSONParser) parseObject(metrics []telegraf.Metric, jsonOut map[string]i
 }
 
 func (p *JSONParser) Parse(buf []byte) ([]telegraf.Metric, error) {
+	buf = bytes.TrimSpace(buf)
+	if len(buf) == 0 {
+		return make([]telegraf.Metric, 0), nil
+	}
 
 	if !isarray(buf) {
 		metrics := make([]telegraf.Metric, 0)

plugins/parsers/json/parser_test.go

@@ -84,6 +84,16 @@ func TestParseValidJSON(t *testing.T) {
 		"b_c": float64(6),
 	}, metrics[0].Fields())
 	assert.Equal(t, map[string]string{}, metrics[0].Tags())
+
+	// Test that whitespace only will parse as an empty list of metrics
+	metrics, err = parser.Parse([]byte("\n\t"))
+	assert.NoError(t, err)
+	assert.Len(t, metrics, 0)
+
+	// Test that an empty string will parse as an empty list of metrics
+	metrics, err = parser.Parse([]byte(""))
+	assert.NoError(t, err)
+	assert.Len(t, metrics, 0)
 }
 
 func TestParseLineValidJSON(t *testing.T) {