runtime: don't issue cgocheck error for timer bucket source pointer

The cgo checker was issuing an error with cgocheck=2 when a timer
bucket was stored in a pollDesc. The pollDesc values are allocated
using persistentalloc, so they are not in the Go heap. The code is OK
since timer bucket pointers point into a global array, and as such are
never garbage collected or moved.

Mark timersBucket notinheap to avoid the problem. timersBucket values
only occur in the global timers array.

Fixes #23435

Change-Id: I835f31caafd54cdacc692db5989de63bb49e7697
Reviewed-on: https://go-review.googlesource.com/87637
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: ianlancetaylor

misc/cgo/errors/ptr_test.go

@@ -349,6 +349,14 @@ var ptrTests = []ptrTest{
 		body:    `var wg sync.WaitGroup; wg.Add(100); for i := 0; i < 100; i++ { go func(i int) { for j := 0; j < 100; j++ { C.f(); runtime.GOMAXPROCS(i) }; wg.Done() }(i) }; wg.Wait()`,
 		fail:    false,
 	},
+	{
+		// Test poller deadline with cgocheck=2.  Issue #23435.
+		name:    "deadline",
+		c:       `#define US 10`,
+		imports: []string{"os", "time"},
+		body:    `r, _, _ := os.Pipe(); r.SetDeadline(time.Now().Add(C.US * time.Microsecond))`,
+		fail:    false,
+	},
 }
 
 func TestPointerChecks(t *testing.T) {

src/runtime/time.go

@@ -59,6 +59,7 @@ func (t *timer) assignBucket() *timersBucket {
 	return t.tb
 }
 
+//go:notinheap
 type timersBucket struct {
 	lock         mutex
 	gp           *g