Validate json parse for TargetGroup Unmarshal (#3614)

Using DisallowUnknownFields in golang 1.10 to forbid unknown fields in targetGroups
added the license header for the targetGroup test

discovery/targetgroup/targetgroup.go

@@ -14,6 +14,7 @@
 package targetgroup
 
 import (
+	"bytes"
 	"encoding/json"
 
 	"github.com/prometheus/common/model"
@@ -77,7 +78,10 @@ func (tg *Group) UnmarshalJSON(b []byte) error {
 		Targets []string       `json:"targets"`
 		Labels  model.LabelSet `json:"labels"`
 	}{}
-	if err := json.Unmarshal(b, &g); err != nil {
+
+	dec := json.NewDecoder(bytes.NewReader(b))
+	dec.DisallowUnknownFields()
+	if err := dec.Decode(&g); err != nil {
 		return err
 	}
 	tg.Targets = make([]model.LabelSet, 0, len(g.Targets))

discovery/targetgroup/targetgroup_test.go

@@ -0,0 +1,48 @@
+// Copyright 2018 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package targetgroup
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/prometheus/prometheus/util/testutil"
+)
+
+func TestTargetGroupStrictJsonUnmarshal(t *testing.T) {
+	tests := []struct {
+		json          string
+		expectedReply error
+	}{
+		{
+			json: `	{"labels": {},"targets": []}`,
+			expectedReply: nil,
+		},
+		{
+			json: `	{"label": {},"targets": []}`,
+			expectedReply: errors.New("json: unknown field \"label\""),
+		},
+		{
+			json: `	{"labels": {},"target": []}`,
+			expectedReply: errors.New("json: unknown field \"target\""),
+		},
+	}
+	tg := Group{}
+
+	for _, test := range tests {
+		actual := tg.UnmarshalJSON([]byte(test.json))
+		testutil.Equals(t, test.expectedReply, actual)
+	}
+
+}