net: avoid relying on singleflight.Group.DoChan to detect hook called

So next CLs can revert changes to "internal/singleflight" in CL #82795,
then replace it with "golang.org/x/sync/singleflight" instead.

For #31697

Change-Id: I873ce30d7e051539aa6dc5d4f21e558869a6d132
Reviewed-on: https://go-review.googlesource.com/c/go/+/423654
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/net/lookup.go

@@ -316,14 +316,15 @@ func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IP
 
 	lookupKey := network + "\000" + host
 	dnsWaitGroup.Add(1)
-	ch, called := r.getLookupGroup().DoChan(lookupKey, func() (any, error) {
-		defer dnsWaitGroup.Done()
+	ch, _ := r.getLookupGroup().DoChan(lookupKey, func() (any, error) {
 		return testHookLookupIP(lookupGroupCtx, resolverFunc, network, host)
 	})
-	if !called {
-		dnsWaitGroup.Done()
-	}
 
+	dnsWaitGroupDone := func(ch <-chan singleflight.Result, cancelFn context.CancelFunc) {
+		<-ch
+		dnsWaitGroup.Done()
+		cancelFn()
+	}
 	select {
 	case <-ctx.Done():
 		// Our context was canceled. If we are the only
@@ -335,11 +336,9 @@ func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IP
 		// See issues 8602, 20703, 22724.
 		if r.getLookupGroup().ForgetUnshared(lookupKey) {
 			lookupGroupCancel()
+			go dnsWaitGroupDone(ch, func() {})
 		} else {
-			go func() {
-				<-ch
-				lookupGroupCancel()
-			}()
+			go dnsWaitGroupDone(ch, lookupGroupCancel)
 		}
 		ctxErr := ctx.Err()
 		err := &DNSError{
@@ -352,6 +351,7 @@ func (r *Resolver) lookupIPAddr(ctx context.Context, network, host string) ([]IP
 		}
 		return nil, err
 	case r := <-ch:
+		dnsWaitGroup.Done()
 		lookupGroupCancel()
 		err := r.Err
 		if err != nil {