prevent synchronization of identical folders from multiple threads

Use a mutex to lock the sync transaction, uniquely identified by the remote
folder name.

Serializing the syncs prevents any IDLE thread from syncing the same folder of
a concurrent autorefresh thread. Othewise, we could download new messages
twice, or read "one message past the last one" which in some cases creates a
duplicated messages.  Also prevents IDLE threads from reading new messages
before the autorefresh thread finishes synchronization.

Github-ref: #421
Originally-submitted-by: Michael Hohmuth <hohmuth@sax.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/accounts.py

@@ -15,7 +15,7 @@
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 
 from subprocess import Popen, PIPE
-from threading import Event
+from threading import Event, Lock
 import os
 import time
 from sys import exc_info
@@ -29,6 +29,9 @@ from offlineimap.ui import getglobalui
 from offlineimap.threadutil import InstanceLimitedThread
 
 FOLDER_NAMESPACE = 'LIMITED_FOLDER_'
+# Key: account name, Value: Dict of Key: remotefolder name, Value: lock.
+SYNC_MUTEXES = {}
+SYNC_MUTEXES_LOCK = Lock()
 
 try:
     import fcntl
@@ -430,11 +433,35 @@ class SyncableAccount(Account):
             self.ui.error(e, exc_info()[2], msg="Calling hook")
 
 
+#XXX: This function should likely be refactored. This should not be passed the
+# account instance.
 def syncfolder(account, remotefolder, quick):
     """Synchronizes given remote folder for the specified account.
 
-    Filtered folders on the remote side will not invoke this function. However,
-    this might be called in a concurrently."""
+    Filtered folders on the remote side will not invoke this function.
+
+    When called in concurrently for the same localfolder, syncs are
+    serialized."""
+
+    def acquire_mutex():
+        account_name = account.getname()
+        localfolder_name = localfolder.getfullname()
+
+        with SYNC_MUTEXES_LOCK:
+            if SYNC_MUTEXES.get(account_name) is None:
+                SYNC_MUTEXES[account_name] = {}
+            # The localfolder full name is good to uniquely identify the sync
+            # transaction.
+            if SYNC_MUTEXES[account_name].get(localfolder_name) is None:
+                #XXX: This lock could be an external file lock so we can remove
+                # the lock at the account level.
+                SYNC_MUTEXES[account_name][localfolder_name] = Lock()
+
+        # Acquire the lock.
+        SYNC_MUTEXES[account_name][localfolder_name].acquire()
+
+    def release_mutex():
+        SYNC_MUTEXES[account.getname()][localfolder.getfullname()].release()
 
     def check_uid_validity():
         # If either the local or the status folder has messages and
@@ -528,6 +555,9 @@ def syncfolder(account, remotefolder, quick):
         # Load local folder.
         localfolder = account.get_local_folder(remotefolder)
 
+        # Acquire the mutex to start syncing.
+        acquire_mutex()
+
         # Add the folder to the mbnames mailboxes.
         mbnames.add(account.name, localrepos.getlocalroot(),
             localfolder.getname())
@@ -616,3 +646,5 @@ def syncfolder(account, remotefolder, quick):
             if folder in locals():
                 locals()[folder].dropmessagelistcache()
         statusfolder.closefiles()
+        # Release the mutex of this sync transaction.
+        release_mutex()