Use one queue object in FunctionDispatch.

Theory: the producer (Qt GUI cover function) exists only once per instance of FunctionDispatcher. This follows from the fact that the dispatcher instance is created on the recipient thread. The consumer (the cover cache) could in theory be multiple threads (but it isn't). Because the items produced by the producer are not equivalent, we need to ensure that the order of items put in the queue by the producer is equal to the order of the requests. To guarantee this order, regardless of the number of consumer threads, we ensure that only one request to the producer can be outstanding.

src/calibre/gui2/__init__.py

@@ -1,7 +1,7 @@
 __license__   = 'GPL v3'
 __copyright__ = '2008, Kovid Goyal <kovid at kovidgoyal.net>'
 """ The GUI """
-import os, sys, Queue
+import os, sys, Queue, threading
 from threading import RLock
 
 from PyQt4.Qt import QVariant, QFileInfo, QObject, SIGNAL, QBuffer, Qt, \
@@ -311,11 +311,14 @@ def __init__(self, func, queued=True, parent=None):
         if not queued:
             typ = Qt.AutoConnection if queued is None else Qt.DirectConnection
         self.dispatch_signal.connect(self.dispatch, type=typ)
+        self.q = Queue.Queue()
+        self.lock = threading.Lock()
 
     def __call__(self, *args, **kwargs):
-        q = Queue.Queue()
-        self.dispatch_signal.emit(q, args, kwargs)
-        return q.get()
+        with self.lock:
+            self.dispatch_signal.emit(self.q, args, kwargs)
+            res = self.q.get()
+        return res
 
     def dispatch(self, q, args, kwargs):
         try: