Improve callback scheduling.

Collect all timeouts to be run before running any of them; this
prevents starvation when a slow callback reschedules itself.

Call time() again before setting poll_timeout to avoid scheduling
anomalies with slow callbacks.

Closes #947.

diff --git a/tornado/ioloop.py b/tornado/ioloop.py
index ca309232d17ab872ec19cc5c7dd6cadbb3dfdedc..bd5ff6fb4d3df4c40d8fd49f10825564427c387c 100644 (file)
--- a/tornado/ioloop.py
+++ b/tornado/ioloop.py
@@ -686,19 +686,16 @@ class PollIOLoop(IOLoop):
 
         try:
             while True:
-                poll_timeout = _POLL_TIMEOUT
-
                 # Prevent IO event starvation by delaying new callbacks
                 # to the next iteration of the event loop.
                 with self._callback_lock:
                     callbacks = self._callbacks
                     self._callbacks = []
-                for callback in callbacks:
-                    self._run_callback(callback)
-                # Closures may be holding on to a lot of memory, so allow
-                # them to be freed before we go into our poll wait.
-                callbacks = callback = None
 
+                # Add any timeouts that have come due to the callback list.
+                # Do not run anything until we have determined which ones
+                # are ready, so timeouts that call add_timeout cannot
+                # schedule anything in this iteration.
                 if self._timeouts:
                     now = self.time()
                     while self._timeouts:
@@ -708,11 +705,9 @@ class PollIOLoop(IOLoop):
                             self._cancellations -= 1
                         elif self._timeouts[0].deadline <= now:
                             timeout = heapq.heappop(self._timeouts)
-                            self._run_callback(timeout.callback)
+                            callbacks.append(timeout.callback)
                             del timeout
                         else:
-                            seconds = self._timeouts[0].deadline - now
-                            poll_timeout = min(seconds, poll_timeout)
                             break
                     if (self._cancellations > 512
                             and self._cancellations > (len(self._timeouts) >> 1)):
@@ -723,10 +718,25 @@ class PollIOLoop(IOLoop):
                                           if x.callback is not None]
                         heapq.heapify(self._timeouts)
 
+                for callback in callbacks:
+                    self._run_callback(callback)
+                # Closures may be holding on to a lot of memory, so allow
+                # them to be freed before we go into our poll wait.
+                callbacks = callback = None
+
                 if self._callbacks:
                     # If any callbacks or timeouts called add_callback,
                     # we don't want to wait in poll() before we run them.
                     poll_timeout = 0.0
+                elif self._timeouts:
+                    # If there are any timeouts, schedule the first one.
+                    # Use self.time() instead of 'now' to account for time
+                    # spent running callbacks.
+                    poll_timeout = self._timeouts[0].deadline - self.time()
+                    poll_timeout = max(0, min(poll_timeout, _POLL_TIMEOUT))
+                else:
+                    # No timeouts and no callbacks, so use the default.
+                    poll_timeout = _POLL_TIMEOUT
 
                 if not self._running:
                     break