[release-branch.go1.9] expvar: make (*Map).Init clear existing keys

fixes #21619

Change-Id: I5bb513dfc8cac875b06a262eec40b5863ae23a4c
Reviewed-on: https://go-review.googlesource.com/62370
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/70973
Run-TryBot: Russ Cox <rsc@golang.org>

src/expvar/expvar.go

@@ -125,7 +125,17 @@ func (v *Map) String() string {
 	return b.String()
 }
 
-func (v *Map) Init() *Map { return v }
+// Init removes all keys from the map.
+func (v *Map) Init() *Map {
+	v.keysMu.Lock()
+	defer v.keysMu.Unlock()
+	v.keys = v.keys[:0]
+	v.m.Range(func(k, _ interface{}) bool {
+		v.m.Delete(k)
+		return true
+	})
+	return v
+}
 
 // updateKeys updates the sorted list of keys in v.keys.
 func (v *Map) addKey(key string) {

src/expvar/expvar_test.go

@@ -161,6 +161,28 @@ func BenchmarkStringSet(b *testing.B) {
 	})
 }
 
+func TestMapInit(t *testing.T) {
+	RemoveAll()
+	colors := NewMap("bike-shed-colors")
+	colors.Add("red", 1)
+	colors.Add("blue", 1)
+	colors.Add("chartreuse", 1)
+
+	n := 0
+	colors.Do(func(KeyValue) { n++ })
+	if n != 3 {
+		t.Errorf("after three Add calls with distinct keys, Do should invoke f 3 times; got %v", n)
+	}
+
+	colors.Init()
+
+	n = 0
+	colors.Do(func(KeyValue) { n++ })
+	if n != 0 {
+		t.Errorf("after Init, Do should invoke f 0 times; got %v", n)
+	}
+}
+
 func TestMapCounter(t *testing.T) {
 	RemoveAll()
 	colors := NewMap("bike-shed-colors")