From 3432c68467d50ffc622fed230a37cd401d82d4bf Mon Sep 17 00:00:00 2001
From: Damien Neil <dneil@google.com>
Date: Mon, 2 Jun 2025 09:26:27 -0700
Subject: runtime: make bubbled timers more consistent with unbubbled

This CL makes two changes to reduce the predictability
with which bubbled timers fire.

When asynctimerchan=0 (the default), regular timers with an associated
channel are only added to a timer heap when some channel operation
is blocked on that channel. This allows us to garbage collect
unreferenced, unstopped timers. Timers in a synctest bubble, in
contrast, are always added to the bubble's timer heap.

This CL changes bubbled timers with a channel to be handled the
same as unbubbled ones, adding them to the bubble's timer heap only
when some channel operation is blocked on the timer's channel.
This permits unstopped bubbled timers to be garbage collected,
but more importantly it makes all timers past their deadline
behave identically, regardless of whether they are in a bubble.

This CL also changes timer scheduling to execute bubbled timers
immediately when possible rather than adding them to a heap.
Timers in a bubble's heap are executed when the bubble is idle.
Executing timers immediately avoids creating a predictable
order of execution.

For #73850
Fixes #73934

Change-Id: If82e441546408f780f6af6fb7f6e416d3160295d
Reviewed-on: https://go-review.googlesource.com/c/go/+/678075
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
---
 src/internal/synctest/synctest_test.go | 55 ++++++++++++++++++++++++++++++++--
 1 file changed, 53 insertions(+), 2 deletions(-)

(limited to 'src/internal')

diff --git a/src/internal/synctest/synctest_test.go b/src/internal/synctest/synctest_test.go
index 53c7c89716..c2f84be736 100644
--- a/src/internal/synctest/synctest_test.go
+++ b/src/internal/synctest/synctest_test.go
@@ -226,8 +226,8 @@ func TestTimerNondeterminism(t *testing.T) {
 		const iterations = 1000
 		var seen1, seen2 bool
 		for range iterations {
-			tm1 := time.NewTimer(0)
-			tm2 := time.NewTimer(0)
+			tm1 := time.NewTimer(1)
+			tm2 := time.NewTimer(1)
 			select {
 			case <-tm1.C:
 				seen1 = true
@@ -278,6 +278,57 @@ func TestSleepNondeterminism(t *testing.T) {
 	})
 }
 
+// TestTimerRunsImmediately verifies that a 0-duration timer sends on its channel
+// without waiting for the bubble to block.
+func TestTimerRunsImmediately(t *testing.T) {
+	synctest.Run(func() {
+		start := time.Now()
+		tm := time.NewTimer(0)
+		select {
+		case got := <-tm.C:
+			if !got.Equal(start) {
+				t.Errorf("<-tm.C = %v, want %v", got, start)
+			}
+		default:
+			t.Errorf("0-duration timer channel is not readable; want it to be")
+		}
+	})
+}
+
+// TestTimerRunsLater verifies that reading from a timer's channel receives the
+// timer fired, even when that time is in reading from a timer's channel receives the
+// time the timer fired, even when that time is in the past.
+func TestTimerRanInPast(t *testing.T) {
+	synctest.Run(func() {
+		delay := 1 * time.Second
+		want := time.Now().Add(delay)
+		tm := time.NewTimer(delay)
+		time.Sleep(2 * delay)
+		select {
+		case got := <-tm.C:
+			if !got.Equal(want) {
+				t.Errorf("<-tm.C = %v, want %v", got, want)
+			}
+		default:
+			t.Errorf("0-duration timer channel is not readable; want it to be")
+		}
+	})
+}
+
+// TestAfterFuncRunsImmediately verifies that a 0-duration AfterFunc is scheduled
+// without waiting for the bubble to block.
+func TestAfterFuncRunsImmediately(t *testing.T) {
+	synctest.Run(func() {
+		var b atomic.Bool
+		time.AfterFunc(0, func() {
+			b.Store(true)
+		})
+		for !b.Load() {
+			runtime.Gosched()
+		}
+	})
+}
+
 func TestChannelFromOutsideBubble(t *testing.T) {
 	choutside := make(chan struct{})
 	for _, test := range []struct {
-- 
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