runtime: prevent time.Timer.Reset(0) from deadlocking testing/synctest tests

In Go 1.23+, timer channels behave synchronously. When we have a timer
channel (i.e. !async && t.isChan) we would lock the
runtime.timer.sendLock mutex at the beginning of
runtime.timer.modify()'s execution.

Calling time.Timer.Reset(0) within a testing/synctest test,
unfortunately, causes it to hang indefinitely. This is because the
runtime.timer.sendLock mutex ends up being locked twice before it could
be unlocked:

- When calling time.Timer.Reset(), runtime.timer.modify() would lock the
  mutex per usual.
- Due to the 0 argument, runtime.timer.modify() would also try to
  execute the bubbled timer immediately rather than adding them to a
  heap. However, in doing so, it uses runtime.timer.unlockAndRun(),
  which also locks the same mutex.

This CL solves this issue by making sure that a locked
runtime.timer.sendLock mutex is unlocked first, whenever we try to
execute bubbled timer immediately in the stack.

Fixes #76052

Change-Id: I66429b9bf6971400de95dcf2d5dc9670c3135492
Reviewed-on: https://go-review.googlesource.com/c/go/+/716883
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Nicholas Husin <nsh@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/synctest/synctest_test.go

@@ -5,6 +5,7 @@
 package synctest_test
 
 import (
+	"context"
 	"fmt"
 	"internal/synctest"
 	"internal/testenv"
@@ -329,6 +330,31 @@ func TestAfterFuncRunsImmediately(t *testing.T) {
 	})
 }
 
+// TestTimerResetZeroDoNotHang verifies that using timer.Reset(0) does not
+// cause the test to hang indefinitely. See https://go.dev/issue/76052.
+func TestTimerResetZeroDoNotHang(t *testing.T) {
+	synctest.Run(func() {
+		timer := time.NewTimer(0)
+		ctx, cancel := context.WithCancel(context.Background())
+
+		go func() {
+			for {
+				select {
+				case <-ctx.Done():
+					return
+				case <-timer.C:
+				}
+			}
+		}()
+
+		synctest.Wait()
+		timer.Reset(0)
+		synctest.Wait()
+		cancel()
+		synctest.Wait()
+	})
+}
+
 func TestChannelFromOutsideBubble(t *testing.T) {
 	choutside := make(chan struct{})
 	for _, test := range []struct {

src/runtime/time.go

@@ -636,6 +636,9 @@ func (t *timer) modify(when, period int64, f func(arg any, seq uintptr, delay in
 		}
 		if t.state&timerHeaped == 0 && when <= bubble.now {
 			systemstack(func() {
+				if !async && t.isChan {
+					unlock(&t.sendLock)
+				}
 				t.unlockAndRun(bubble.now, bubble)
 			})
 			return pending