runtime: if stop/reset races with running timer, return correct result

The timer code is careful to ensure that if stop/reset is called
while a timer is being run, we cancel the run. However, the code
failed to ensure that in that case stop/reset returned true,
meaning that the timer had been stopped. In the racing case
stop/reset could see that t.when had been set to zero,
and return false, even though the timer had not and never would fire.

Fix this by tracking whether a timer run is in progress,
and using that to reliably detect that the run was cancelled,
meaning that stop/reset should return true.

Fixes #69312

Change-Id: I78e870063eb96650638f12c056e32c931417c84a
Reviewed-on: https://go-review.googlesource.com/c/go/+/611496
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

1 files changed, 62 insertions, 0 deletions

diff --git a/src/time/sleep_test.go b/src/time/sleep_test.go
index 29f56ef752..5357ed23c8 100644
--- a/src/time/sleep_test.go
+++ b/src/time/sleep_test.go
@@ -785,6 +785,68 @@ func TestAdjustTimers(t *testing.T) {
 	}
 }
 
+func TestStopResult(t *testing.T) {
+	testStopResetResult(t, true)
+}
+
+func TestResetResult(t *testing.T) {
+	testStopResetResult(t, false)
+}
+
+// Test that when racing between running a timer and stopping a timer Stop
+// consistently indicates whether a value can be read from the channel.
+// Issue #69312.
+func testStopResetResult(t *testing.T, testStop bool) {
+	for _, name := range []string{"0", "1", "2"} {
+		t.Run("asynctimerchan="+name, func(t *testing.T) {
+			testStopResetResultGODEBUG(t, testStop, name)
+		})
+	}
+}
+
+func testStopResetResultGODEBUG(t *testing.T, testStop bool, godebug string) {
+	t.Setenv("GODEBUG", "asynctimerchan="+godebug)
+
+	stopOrReset := func(timer *Timer) bool {
+		if testStop {
+			return timer.Stop()
+		} else {
+			return timer.Reset(1 * Hour)
+		}
+	}
+
+	start := make(chan struct{})
+	var wg sync.WaitGroup
+	const N = 1000
+	wg.Add(N)
+	for range N {
+		go func() {
+			defer wg.Done()
+			<-start
+			for j := 0; j < 100; j++ {
+				timer1 := NewTimer(1 * Millisecond)
+				timer2 := NewTimer(1 * Millisecond)
+				select {
+				case <-timer1.C:
+					if !stopOrReset(timer2) {
+						// The test fails if this
+						// channel read times out.
+						<-timer2.C
+					}
+				case <-timer2.C:
+					if !stopOrReset(timer1) {
+						// The test fails if this
+						// channel read times out.
+						<-timer1.C
+					}
+				}
+			}
+		}()
+	}
+	close(start)
+	wg.Wait()
+}
+
 // Benchmark timer latency when the thread that creates the timer is busy with
 // other work and the timers must be serviced by other threads.
 // https://golang.org/issue/38860