Fixed a problem with spurious wakeups.

runtime/vm/message_queue.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/message_queue.h"
 
 namespace dart {
 
 MessageQueue::~MessageQueue() {
   // Ensure that all pending messages have been released.
@@ skipping 22 matching lines @@
     tail_ = msg;
   }
 
   monitor_.Exit();
 }
 
 
 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_;
     // The following update to tail_ is not strictly needed.
     if (head_ == NULL) {
       tail_ = NULL;
     }
 #if DEBUG
     result->next_ = result;  // Make sure to trigger ASSERT in Enqueue.
 #endif  // DEBUG
@@ skipping 34 matching lines @@
   tail_ = NULL;
   while (cur != NULL) {
     PortMessage* next = cur->next_;
     delete cur;
     cur = next;
   }
 }
 
 
 }  // namespace dart