folder.IMAP: Improve dropped connection handling in quickchanged()

The quickchanged() function was not handling dropped connections yet. If
IMAP4.select() throws a FOLDER_RETRY error, we will now discard the
connection, reconnect and retry.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/IMAP.py

@@ -89,10 +89,22 @@ class IMAPFolder(BaseFolder):
         # An IMAP folder has definitely changed if the number of
         # messages or the UID of the last message have changed.  Otherwise
         # only flag changes could have occurred.
+        retry = True # Should we attempt another round or exit?
+        while retry:
+            retry = False
             imapobj = self.imapserver.acquireconnection()
             try:
-            # Primes untagged_responses
-            imaptype, imapdata = imapobj.select(self.getfullname(), readonly = 1, force = 1)
+                # Select folder and get number of messages
+                restype, imapdata = imapobj.select(self.getfullname(), True,
+                                                   True)
+            except OfflineImapError, e:
+                # retry on dropped connections, raise otherwise
+                self.imapserver.releaseconnection(imapobj, true)
+                if e.severity == OfflineImapError.ERROR.FOLDER_RETRY:
+                    retry = True
+                else: raise
+            finally:
+                self.imapserver.releaseconnection(imapobj)
         # 1. Some mail servers do not return an EXISTS response
         # if the folder is empty.  2. ZIMBRA servers can return
         # multiple EXISTS replies in the form 500, 1000, 1500,
@@ -102,13 +114,9 @@ class IMAPFolder(BaseFolder):
         maxmsgid = 0
         for msgid in imapdata:
             maxmsgid = max(long(msgid), maxmsgid)
-
         # Different number of messages than last time?
         if maxmsgid != statusfolder.getmessagecount():
             return True      
-
-        finally:
-            self.imapserver.releaseconnection(imapobj)
     return False
 
     def cachemessagelist(self):