singleflight: remove forgotten field

Port from CL 433315.

Change-Id: Iaf6ab4676dd10a73443c3c9981ffc77233b4631c
Reviewed-on: https://go-review.googlesource.com/c/sync/+/433555
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: cuonglm

singleflight/singleflight.go

@@ -52,10 +52,6 @@ type call struct {
 	val interface{}
 	err error
 
-	// forgotten indicates whether Forget was called with this call's key
-	// while the call was still in flight.
-	forgotten bool
-
 	// These fields are read and written with the singleflight
 	// mutex held before the WaitGroup is done, and are read but
 	// not written after the WaitGroup is done.
@@ -151,7 +147,7 @@ func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
 		c.wg.Done()
 		g.mu.Lock()
 		defer g.mu.Unlock()
-		if !c.forgotten {
+		if g.m[key] == c {
 			delete(g.m, key)
 		}
 
@@ -204,9 +200,6 @@ 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 c, ok := g.m[key]; ok {
-		c.forgotten = true
-	}
 	delete(g.m, key)
 	g.mu.Unlock()
 }