runtime: don't clear timerModifiedEarliest if adjustTimers is 0

This avoids a race when a new timerModifiedEarlier timer is created by
a different goroutine.

Fixes #47329

Change-Id: I6f6c87b4a9b5491b201c725c10bc98e23e0ed9d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/336432
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/runtime2.go

@@ -681,7 +681,7 @@ type p struct {
 	// timerModifiedEarlier status. Because the timer may have been
 	// modified again, there need not be any timer with this value.
 	// This is updated using atomic functions.
-	// This is 0 if the value is unknown.
+	// This is 0 if there are no timerModifiedEarlier timers.
 	timerModifiedEarliest uint64
 
 	// Per-P GC state

src/runtime/time.go

@@ -668,11 +668,6 @@ func adjusttimers(pp *p, now int64) {
 		if verifyTimers {
 			verifyTimerHeap(pp)
 		}
-		// There are no timers to adjust, so it is safe to clear
-		// timerModifiedEarliest. Do so in case it is stale.
-		// Everything will work if we don't do this,
-		// but clearing here may save future calls to adjusttimers.
-		atomic.Store64(&pp.timerModifiedEarliest, 0)
 		return
 	}
 

src/time/sleep_test.go

@@ -527,6 +527,40 @@ func TestZeroTimer(t *testing.T) {
 	}
 }
 
+// Test that rapidly moving a timer earlier doesn't cause it to get dropped.
+// Issue 47329.
+func TestTimerModifiedEarlier(t *testing.T) {
+	past := Until(Unix(0, 0))
+	count := 1000
+	fail := 0
+	for i := 0; i < count; i++ {
+		timer := NewTimer(Hour)
+		for j := 0; j < 10; j++ {
+			if !timer.Stop() {
+				<-timer.C
+			}
+			timer.Reset(past)
+		}
+
+		deadline := NewTimer(10 * Second)
+		defer deadline.Stop()
+		now := Now()
+		select {
+		case <-timer.C:
+			if since := Since(now); since > 8*Second {
+				t.Errorf("timer took too long (%v)", since)
+				fail++
+			}
+		case <-deadline.C:
+			t.Error("deadline expired")
+		}
+	}
+
+	if fail > 0 {
+		t.Errorf("%d failures", fail)
+	}
+}
+
 // 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