runtime: handle timers on P's in procresize/(*pp).destroy

Updates #6239
Updates #27707

Change-Id: I52cab8bf3dc8c552463725fc1d9e4e6b12230b03
Reviewed-on: https://go-review.googlesource.com/c/go/+/171828
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/proc.go

@@ -4117,6 +4117,12 @@ func (pp *p) destroy() {
 		globrunqputhead(pp.runnext.ptr())
 		pp.runnext = 0
 	}
+	if len(pp.timers) > 0 {
+		plocal := getg().m.p.ptr()
+		// The world is stopped so we don't need to hold timersLock.
+		moveTimers(plocal, pp.timers)
+		pp.timers = nil
+	}
 	// If there's a background worker, make it runnable and put
 	// it on the global queue so it can clean itself up.
 	if gp := pp.gcBgMarkWorker.ptr(); gp != nil {

src/runtime/time.go

@@ -325,6 +325,14 @@ func timerproc(tb *timersBucket) {
 	}
 }
 
+// moveTimers moves a slice of timers to pp. The slice has been taken
+// from a different P.
+// This is currently called when the world is stopped, but it could
+// work as long as the timers for pp are locked.
+func moveTimers(pp *p, timers []*timer) {
+	throw("movetimers: not yet implemented")
+}
+
 // adjusttimers looks through the timers in the current P's heap for
 // any timers that have been modified to run earlier, and puts them in
 // the correct place in the heap.