time: add a regression test for time.AfterFunc goroutine starvation

The test is skipped on wasm platforms for now, because it
successfully detects a starvation bug on those platforms.

For golang#65178.

Change-Id: I05d28f1c7be99fcab67ec4dfaa38f412e11fd3cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/557038
Auto-Submit: Bryan Mills <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

src/time/sleep_test.go

@@ -83,6 +83,50 @@ func TestAfterStress(t *testing.T) {
 	stop.Store(true)
 }
 
+func TestAfterFuncStarvation(t *testing.T) {
+	// Start two goroutines ping-ponging on a channel send.
+	// At any given time, at least one of these goroutines is runnable:
+	// if the channel buffer is full, the receiver is runnable,
+	// and if it is not full, the sender is runnable.
+	//
+	// In addition, the AfterFunc callback should become runnable after
+	// the indicated delay.
+	//
+	// Even if GOMAXPROCS=1, we expect the runtime to eventually schedule
+	// the AfterFunc goroutine instead of the runnable channel goroutine.
+	// However, in https://go.dev/issue/65178 this was observed to live-lock
+	// on wasip1/wasm and js/wasm after <10000 runs.
+
+	if runtime.GOARCH == "wasm" {
+		testenv.SkipFlaky(t, 65178)
+	}
+
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
+
+	var (
+		wg   sync.WaitGroup
+		stop atomic.Bool
+		c    = make(chan bool, 1)
+	)
+
+	wg.Add(2)
+	go func() {
+		for !stop.Load() {
+			c <- true
+		}
+		close(c)
+		wg.Done()
+	}()
+	go func() {
+		for range c {
+		}
+		wg.Done()
+	}()
+
+	AfterFunc(1*Microsecond, func() { stop.Store(true) })
+	wg.Wait()
+}
+
 func benchmark(b *testing.B, bench func(n int)) {
 
 	// Create equal number of garbage timers on each P before starting