Ensure only one thread can run the update function for mobileread.

src/calibre/gui2/store/mobileread_plugin.py

@@ -10,6 +10,7 @@ import difflib
 import heapq
 import time
 from contextlib import closing
+from threading import RLock
 
 from lxml import html
 
@@ -25,6 +26,7 @@ class MobileReadStore(StorePlugin):
     
     def genesis(self):
         self.config = DynamicConfig('store_' + self.name)
+        self.rlock = RLock()
     
     def open(self, parent=None, detail_item=None, external=False):
         url = 'http://www.mobileread.com/'
@@ -60,6 +62,7 @@ class MobileReadStore(StorePlugin):
             yield s
 
     def update_book_list(self, timeout=10):
+        with self.rlock:
             url = 'http://www.mobileread.com/forums/ebooks.php?do=getlist&type=html'
             
             last_download = self.config.get(self.name + '_last_download', None)