expvar: Ensure strings are written as valid JSON.

Change-Id: I5147dbf4e85cf42cd1f32c57861e4c16d9dbd049
Reviewed-on: https://go-review.googlesource.com/21529Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/expvar/expvar.go

@@ -219,8 +219,10 @@ type String struct {
 
 func (v *String) String() string {
 	v.mu.RLock()
-	defer v.mu.RUnlock()
-	return strconv.Quote(v.s)
+	s := v.s
+	v.mu.RUnlock()
+	b, _ := json.Marshal(s)
+	return string(b)
 }
 
 func (v *String) Set(value string) {

src/expvar/expvar_test.go

@@ -142,8 +142,14 @@ func TestString(t *testing.T) {
 		t.Errorf("name.s = %q, want \"Mike\"", name.s)
 	}
 
-	if s := name.String(); s != "\"Mike\"" {
-		t.Errorf("reqs.String() = %q, want \"\"Mike\"\"", s)
+	if s, want := name.String(), `"Mike"`; s != want {
+		t.Errorf("from %q, name.String() = %q, want %q", name.s, s, want)
+	}
+
+	// Make sure we produce safe JSON output.
+	name.Set(`<`)
+	if s, want := name.String(), "\"\\u003c\""; s != want {
+		t.Errorf("from %q, name.String() = %q, want %q", name.s, s, want)
 	}
 }