Bugfix-773 show better error message on JSON decoding errors

instead of dumping entire JSON, offset, line number and column numbers are displayed

js/modules/k6/http/response_test.go

@@ -80,6 +80,11 @@ const jsonData = `{"glossary": {
 	  "GlossSeeAlso": ["GML","XML"]},
 	"GlossSee": "markup"}}}}}`
 
+const invalidJSONData = `{			
+	"a":"apple",
+	"t":testing"
+}`
+
 func myFormHandler(w http.ResponseWriter, r *http.Request) {
 	var body []byte
 	var err error
@@ -110,6 +115,14 @@ func jsonHandler(w http.ResponseWriter, r *http.Request) {
 	_, _ = w.Write(body)
 }
 
+func invalidJSONHandler(w http.ResponseWriter, r *http.Request) {
+	body := []byte(invalidJSONData)
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Content-Length", fmt.Sprintf("%d", len(body)))
+	w.WriteHeader(200)
+	_, _ = w.Write(body)
+}
+
 func TestResponse(t *testing.T) {
 	tb, state, samples, rt, _ := newRuntime(t)
 	defer tb.Cleanup()
@@ -118,6 +131,7 @@ func TestResponse(t *testing.T) {
 
 	tb.Mux.HandleFunc("/myforms/get", myFormHandler)
 	tb.Mux.HandleFunc("/json", jsonHandler)
+	tb.Mux.HandleFunc("/invalidjson", invalidJSONHandler)
 
 	t.Run("Html", func(t *testing.T) {
 		_, err := common.RunString(rt, sr(`
@@ -178,7 +192,14 @@ func TestResponse(t *testing.T) {
 
 		t.Run("Invalid", func(t *testing.T) {
 			_, err := common.RunString(rt, sr(`http.request("GET", "HTTPBIN_URL/html").json();`))
-			assert.EqualError(t, err, "GoError: invalid character '<' looking for beginning of value")
+			//nolint:lll
+			assert.EqualError(t, err, "GoError: cannot parse json due to an error at line 1, character 2 , error: invalid character '<' looking for beginning of value")
+		})
+
+		t.Run("Invalid", func(t *testing.T) {
+			_, err := common.RunString(rt, sr(`http.request("GET", "HTTPBIN_URL/invalidjson").json();`))
+			//nolint:lll
+			assert.EqualError(t, err, "GoError: cannot parse json due to an error at line 3, character 9 , error: invalid character 'e' in literal true (expecting 'r')")
 		})
 	})
 	t.Run("JsonSelector", func(t *testing.T) {

lib/netext/httpext/response.go

@@ -24,6 +24,9 @@ import (
 	"context"
 	"crypto/tls"
 	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httputil"
 
 	"github.com/loadimpact/k6/lib/netext"
 	"github.com/pkg/errors"
@@ -55,6 +58,17 @@ const (
 	ResponseTypeNone
 )
 
+type jsonError struct {
+	line      int
+	character int
+	err       error
+}
+
+func (j jsonError) Error() string {
+	errMessage := "cannot parse json due to an error at line"
+	return fmt.Sprintf("%s %d, character %d , error: %v", errMessage, j.line, j.character, j.err)
+}
+
 // ResponseTimings is a struct to put all timings for a given HTTP response/request
 type ResponseTimings struct {
 	Duration       float64 `json:"duration"`
@@ -127,7 +141,6 @@ func (res *Response) JSON(selector ...string) (interface{}, error) {
 		}
 
 		if hasSelector {
-
 			if !res.validatedJSON {
 				if !gjson.ValidBytes(body) {
 					return nil, nil
@@ -144,11 +157,35 @@ func (res *Response) JSON(selector ...string) (interface{}, error) {
 		}
 
 		if err := json.Unmarshal(body, &v); err != nil {
+			if syntaxError, ok := err.(*json.SyntaxError); ok {
+				err = checkErrorInJSON(body, int(syntaxError.Offset), err)
+			}
 			return nil, err
 		}
 		res.validatedJSON = true
 		res.cachedJSON = v
 	}
 	return res.cachedJSON, nil
+}
+
+func checkErrorInJSON(input []byte, offset int, err error) error {
+	lf := '\n'
+	str := string(input)
+
+	// Humans tend to count from 1.
+	line := 1
+	character := 0
+
+	for i, b := range str {
+		if b == lf {
+			line++
+			character = 0
+		}
+		character++
+		if i == offset {
+			break
+		}
+	}
 
+	return jsonError{line: line, character: character, err: err}
 }