x/sync/singleflight: Add lazy map init to Forget

Forget is missing lazy map initialization. If Forget is called before any other method it will panic.

Author: CAFxX

singleflight/singleflight.go

@@ -204,6 +204,9 @@ func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
 // an earlier call to complete.
 func (g *Group) Forget(key string) {
 g.mu.Lock()
+if g.m == nil {
+g.m = make(map[string]*call)
+}
 if c, ok := g.m[key]; ok {
 c.forgotten = true
 }

singleflight/singleflight_test.go

@@ -318,3 +318,8 @@ func TestPanicDoSharedByDoChan(t *testing.T) {
 t.Errorf("Test subprocess failed, but the crash isn't caused by panicking in Do")
 }
 }
+
+func TestForgetEarly(t *testing.T) {
+var g Group
+g.Forget("key") // calling Forget before Do/DoChan should not panic
+}