diff --git a/.gitignore b/.gitignore
index 3c1a62c..679bf0f 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@ __pycache__
 .eggs
 /dist/
 /wheelhouse/
+.DS_Store
\ No newline at end of file
diff --git a/portaudio b/portaudio
index 147dd72..7c45fed 160000
--- a/portaudio
+++ b/portaudio
@@ -1 +1 @@
-Subproject commit 147dd722548358763a8b649b3e4b41dfffbcfbb6
+Subproject commit 7c45fed9322bd86b5dd0da4547b15d9ac3b9b0ac
diff --git a/src/rtmixer.c b/src/rtmixer.c
index 9ac3133..867d659 100644
--- a/src/rtmixer.c
+++ b/src/rtmixer.c
@@ -20,17 +20,27 @@ static const struct stats EMPTY_STATS;
   }} while (false)
 #endif
 
-void remove_action(struct action** addr, const struct state* state)
+// The action is moved to the list of done actions, see flush_done_actions()
+void remove_action(struct action** addr, struct state* state)
 {
   struct action* action = *addr;
   *addr = action->next;  // Current action is removed from list
-  action->next = NULL;
-  ring_buffer_size_t written = PaUtil_WriteRingBuffer(state->result_q
-    , &action, 1);
-  if (written != 1)
+  action->next = state->done_actions;
+  state->done_actions = action;
+}
+
+// Done actions that don't fit into result_q are kept for the next block
+void flush_done_actions(struct state* state)
+{
+  while (state->done_actions)
   {
-    // TODO: Stop callback! Unrecoverable error! Log error (in "state")?
-    printf("result queue is full\n");
+    struct action* action = state->done_actions;
+    if (PaUtil_WriteRingBuffer(state->result_q, &action, 1) != 1)
+    {
+      break;  // result_q is full, try again in the next block
+    }
+    state->done_actions = action->next;
+    action->next = NULL;
   }
 }
 
@@ -128,7 +138,10 @@ int callback(const void* input, void* output, frame_t frameCount
   struct state* state = userData;
   CALLBACK_ASSERT(state);
 
-  memset(output, 0, sizeof(float) * state->output_channels * frameCount);
+  if (output)  // NULL for input-only streams
+  {
+    memset(output, 0, sizeof(float) * state->output_channels * frameCount);
+  }
 
   get_stats(frameCount, statusFlags, &(state->stats));
 
@@ -420,5 +433,6 @@ int callback(const void* input, void* output, frame_t frameCount
     }
     actionaddr = &(action->next);
   }
+  flush_done_actions(state);
   return paContinue;
 }
diff --git a/src/rtmixer.h b/src/rtmixer.h
index 9df4a4d..df7677e 100644
--- a/src/rtmixer.h
+++ b/src/rtmixer.h
@@ -55,6 +55,7 @@ struct state
   struct PaUtilRingBuffer* const action_q;  // Queue for incoming commands
   struct PaUtilRingBuffer* const result_q;  // Q for results and cmd disposal
   struct action* actions;  // Singly linked list of actions
+  struct action* done_actions;  // Finished, but not yet in result_q
   struct stats stats;
   // TODO: result_q usage?
 };
diff --git a/src/rtmixer.py b/src/rtmixer.py
index 0cdde95..b9e3d40 100644
--- a/src/rtmixer.py
+++ b/src/rtmixer.py
@@ -41,6 +41,15 @@ def __init__(self, kind, qsize=16, **kwargs):
         self._actions = {}
         self._temp_action_ptr = _ffi.new('struct action**')
 
+    def __del__(self):
+        # The C callback uses memory owned by this object, so the stream
+        # must be closed before that memory is freed.
+        try:
+            self.abort()
+            self.close()
+        except Exception:
+            pass  # e.g. __init__() failed or interpreter shutdown
+
     @property
     def actions(self):
         """The set of active "actions"."""
@@ -76,7 +85,8 @@ def cancel(self, action, time=0, allow_belated=True):
             requested_time=time,
             action=action,
         ))
-        self._enqueue(cancel_action)
+        # The callback dereferences *action*, so it must be kept alive:
+        self._enqueue(cancel_action, keep_alive=action)
         return cancel_action
 
     def fetch_and_reset_stats(self, time=0, allow_belated=True):
@@ -129,7 +139,8 @@ def _enqueue(self, action, keep_alive=None):
         self._temp_action_ptr[0] = action
         ret = self._action_q.write(self._temp_action_ptr)
         if ret != 1:
-            raise RuntimeError('Action queue is full')
+            raise RuntimeError(
+                'Action queue is full, consider increasing qsize')
         assert action not in self._actions
         self._actions[action] = keep_alive
 
@@ -149,6 +160,15 @@ class Mixer(_Base):
     except *callback* (a callback function implemented in C is used
     internally) and *dtype* (which is always ``'float32'``).
 
+    The additional keyword argument *qsize* (default: 16) sets the size
+    of the queues used to communicate with the callback.  It must be a
+    power of 2.  The callback takes new actions from its queue once per
+    audio block, so *qsize* limits how many actions can be started
+    within one block (if exceeded, a `RuntimeError` is raised), but not
+    how many actions can be active at the same time.  If many actions
+    finish at once, some of them may be removed from `actions` a few
+    blocks later.
+
     Uses default values from `sounddevice.default` (except *dtype*,
     which is always ``'float32'``).
 
@@ -217,6 +237,8 @@ class Recorder(_Base):
     except *callback* (a callback function implemented in C is used
     internally) and *dtype* (which is always ``'float32'``).
 
+    The additional keyword argument *qsize* is described in `Mixer`.
+
     Uses default values from `sounddevice.default` (except *dtype*,
     which is always ``'float32'``).
 
@@ -282,6 +304,8 @@ class MixerAndRecorder(Mixer, Recorder):
     *callback* (a callback function implemented in C is used internally)
     and *dtype* (which is always ``'float32'``).
 
+    The additional keyword argument *qsize* is described in `Mixer`.
+
     Uses default values from `sounddevice.default` (except *dtype*,
     which is always ``'float32'``).