Fixed a problem with spurious wakeups.

runtime/vm/message_queue.cc

Index: runtime/vm/message_queue.cc
diff --git a/runtime/vm/message_queue.cc b/runtime/vm/message_queue.cc
index 2f67a2153141ab93978c8373030ee6c24fedeb84..a4b7b6ae37bcb64192d0fe8234ed4ac31206cdcf 100644
--- a/runtime/vm/message_queue.cc
+++ b/runtime/vm/message_queue.cc
@@ -40,9 +40,13 @@ void MessageQueue::Enqueue(PortMessage* msg) {
 PortMessage* MessageQueue::Dequeue(int64_t millis) {
   MonitorLocker ml(&monitor_);
   PortMessage* result = head_;
-  if (result == NULL) {
+  // Wait may wake up prematurely, so we need to loop until result != NULL

[siva, 2011/10/21 21:18:33]

// Wait for message to be enqueued or timeout.

+  while (result == NULL) {
     ml.Wait(millis);
     result = head_;
+    // If there's a time-out the caller must check for NULL anyway.
+    if (0 < millis)
+      break;

[siva, 2011/10/21 21:18:33]

This is not correct, if a spurious wake up happens before the specified time you
are just waking up and returning as
though it timed out.

The right way to do this:
while (result == NULL) {
  WaitResult result = ml.Wait(millis);
  if (result == kTimedOut) {
    return NULL;
  }
  result = head_;
}

[jrgfogh, 2011/10/24 09:20:42]

I know. Neither way is strictly correct. My code has the problem you mention.
Your fix may result in a live-lock, since it restarts the wait with the same
timeout each time. The correct solution would be to decrease millis by the
elapsed time. sgjesse and I discussed it and decided that a "correct" solution
would be needlessly complicated, since the caller would have to check for NULL
anyway if 0 != millis.

   }
   if (result != NULL) {
     head_ = result->next_;