Fix parsing of JSON with a UTF8 BOM in httpjson (#3267)

plugins/inputs/httpjson/httpjson.go

 package httpjson
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"net/http"
@@ -15,6 +16,10 @@ import (
 	"github.com/influxdata/telegraf/plugins/parsers"
 )
 
+var (
+	utf8BOM = []byte("\xef\xbb\xbf")
+)
+
 // HttpJson struct
 type HttpJson struct {
 	Name            string
@@ -170,7 +175,6 @@ func (h *HttpJson) gatherServer(
 	serverURL string,
 ) error {
 	resp, responseTime, err := h.sendRequest(serverURL)
-
 	if err != nil {
 		return err
 	}
@@ -266,6 +270,7 @@ func (h *HttpJson) sendRequest(serverURL string) (string, float64, error) {
 	if err != nil {
 		return string(body), responseTime, err
 	}
+	body = bytes.TrimPrefix(body, utf8BOM)
 
 	// Process response
 	if resp.StatusCode != http.StatusOK {

plugins/inputs/httpjson/httpjson_test.go

@@ -560,3 +560,18 @@ func TestHttpJsonArray200Tags(t *testing.T) {
 		}
 	}
 }
+
+var jsonBOM = []byte("\xef\xbb\xbf[{\"value\":17}]")
+
+// TestHttpJsonBOM tests that UTF-8 JSON with a BOM can be parsed
+func TestHttpJsonBOM(t *testing.T) {
+	httpjson := genMockHttpJson(string(jsonBOM), 200)
+
+	for _, service := range httpjson {
+		if service.Name == "other_webapp" {
+			var acc testutil.Accumulator
+			err := acc.GatherError(service.Gather)
+			require.NoError(t, err)
+		}
+	}
+}