expvar: don't recursively acquire Map.RLock

Fixes #7575

LGTM=iant
R=dvyukov, iant
CC=golang-codereviews
https://golang.org/cl/77540044

src/pkg/expvar/expvar.go

@@ -108,7 +108,7 @@ func (v *Map) String() string {
 	var b bytes.Buffer
 	fmt.Fprintf(&b, "{")
 	first := true
-	v.Do(func(kv KeyValue) {
+	v.doLocked(func(kv KeyValue) {
 		if !first {
 			fmt.Fprintf(&b, ", ")
 		}
@@ -202,6 +202,12 @@ func (v *Map) AddFloat(key string, delta float64) {
 func (v *Map) Do(f func(KeyValue)) {
 	v.mu.RLock()
 	defer v.mu.RUnlock()
+	v.doLocked(f)
+}
+
+// doRLocked calls f for each entry in the map.
+// v.mu must be held for reads.
+func (v *Map) doLocked(f func(KeyValue)) {
 	for _, k := range v.keys {
 		f(KeyValue{k, v.m[k]})
 	}