Start documenting the threading model

webrtc/modules/pacing/paced_sender_unittest.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 616 matching lines @@
                                sequence_number++, second_capture_time_ms, 250,
                                false);
   }
 
   // Expect everything to be queued.
   EXPECT_EQ(second_capture_time_ms - capture_time_ms,
             send_bucket_->QueueInMs());
 
   for (int i = 0; i < 10; ++i) {
     clock_.AdvanceTimeMilliseconds(5);
-    EXPECT_EQ(0, send_bucket_->TimeUntilNextProcess());
+    // TimeUntilNextProcess must not return 0 when paused.  If it does,
+    // we risk running a busy loop, so ideally it should return a large value.
+    EXPECT_LT(1000, send_bucket_->TimeUntilNextProcess());
     send_bucket_->Process();
   }
 
   // Expect high prio packets to come out first followed by normal
   // prio packets and low prio packets (all in capture order).
   {
     ::testing::InSequence sequence;
     EXPECT_CALL(callback_,
                 TimeToSendPacket(ssrc_high_priority, _, capture_time_ms, _, _))
         .Times(packets_to_send_per_interval)
@@ skipping 420 matching lines @@
       .WillRepeatedly(Return(0));
   send_bucket_->Process();
   EXPECT_EQ(1, send_bucket_->TimeUntilNextProcess());
   clock_.AdvanceTimeMilliseconds(1);
   send_bucket_->Process();
   EXPECT_EQ(5, send_bucket_->TimeUntilNextProcess());
 }
 
 }  // namespace test
 }  // namespace webrtc