singleflight: fix duplicate deleting key when Forget called

When Forget was called, we delete key associated with current call from
map. When that call is done, it does delete key again, causing the same
key set by other call after Forget lost.

To fix it, adding a boolean value to check whether the call is forgotten,
the call only does delete key if Forget is not called.

Fixes golang/go#31420

Change-Id: I9708352ca3ff76c77f659916b37a496fdeb480d2
Reviewed-on: https://go-review.googlesource.com/c/sync/+/171897
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

singleflight/singleflight.go

@@ -17,6 +17,10 @@ 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.
@@ -94,7 +98,9 @@ func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
 	c.wg.Done()
 
 	g.mu.Lock()
-	delete(g.m, key)
+	if !c.forgotten {
+		delete(g.m, key)
+	}
 	for _, ch := range c.chans {
 		ch <- Result{c.val, c.err, c.dups > 0}
 	}
@@ -106,6 +112,9 @@ 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 c, ok := g.m[key]; ok {
+		c.forgotten = true
+	}
 	delete(g.m, key)
 	g.mu.Unlock()
 }