Prevent cross-thread lock errors by having the cover cache get the image on the GUI thread.

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
+import os, sys, Queue
 from threading import RLock
 
 from PyQt4.Qt import QVariant, QFileInfo, QObject, SIGNAL, QBuffer, Qt, \
@@ -296,6 +296,34 @@ class Dispatcher(QObject):
     def dispatch(self, args, kwargs):
         self.func(*args, **kwargs)
 
+class FunctionDispatcher(QObject):
+    '''
+    Convenience class to use Qt signals with arbitrary python functions.
+    By default, ensures that a function call always happens in the
+    thread this Dispatcher was created in.
+    '''
+    dispatch_signal = pyqtSignal(object, object, object)
+
+    def __init__(self, func, queued=True, parent=None):
+        QObject.__init__(self, parent)
+        self.func = func
+        typ = Qt.QueuedConnection
+        if not queued:
+            typ = Qt.AutoConnection if queued is None else Qt.DirectConnection
+        self.dispatch_signal.connect(self.dispatch, type=typ)
+
+    def __call__(self, *args, **kwargs):
+        q = Queue.Queue()
+        self.dispatch_signal.emit(q, args, kwargs)
+        return q.get()
+
+    def dispatch(self, q, args, kwargs):
+        try:
+            res = self.func(*args, **kwargs)
+        except:
+            res = None
+        q.put(res)
+
 class GetMetadata(QObject):
     '''
     Convenience class to ensure that metadata readers are used only in the

src/calibre/gui2/library/models.py

@@ -12,7 +12,7 @@ from operator import attrgetter
 from PyQt4.Qt import QAbstractTableModel, Qt, pyqtSignal, QIcon, QImage, \
         QModelIndex, QVariant, QDate
 
-from calibre.gui2 import NONE, config, UNDEFINED_QDATE
+from calibre.gui2 import NONE, config, UNDEFINED_QDATE, FunctionDispatcher
 from calibre.utils.pyparsing import ParseException
 from calibre.ebooks.metadata import fmt_sidx, authors_to_string, string_to_authors
 from calibre.ptempfile import PersistentTemporaryFile
@@ -151,7 +151,7 @@ class BooksModel(QAbstractTableModel): # {{{
         self.database_changed.emit(db)
         if self.cover_cache is not None:
             self.cover_cache.stop()
-        self.cover_cache = CoverCache(db)
+        self.cover_cache = CoverCache(db, FunctionDispatcher(self.db.cover))
         self.cover_cache.start()
         def refresh_cover(event, ids):
             if event == 'cover' and self.cover_cache is not None:

src/calibre/library/caches.py

@@ -23,10 +23,11 @@ from calibre import fit_image
 
 class CoverCache(Thread):
 
-    def __init__(self, db):
+    def __init__(self, db, cover_func):
         Thread.__init__(self)
         self.daemon = True
         self.db = db
+        self.cover_func = cover_func
         self.load_queue = Queue()
         self.keep_running = True
         self.cache = {}
@@ -37,7 +38,9 @@ class CoverCache(Thread):
         self.keep_running = False
 
     def _image_for_id(self, id_):
-        img = self.db.cover(id_, index_is_id=True, as_image=True)
+        import time
+        time.sleep(0.050) # Limit 20/second to not overwhelm the GUI
+        img = self.cover_func(id_, index_is_id=True, as_image=True)
         if img is None:
             img = QImage()
         if not img.isNull():