mbnames: don't duplicate entries in autorefresh mode

mbnames is initialized and written once in the run from OfflineImap.__sync().

However, the in-memory instance is fed with data at sync time for each folder
and the intermediate files are written as soon as all the folders are synced for
the account. All of this is done inside the SyncableAccount.__sync() method
while the syncrunner is looping on this.

This means that we duplicate entries for mbnames in each loop (most likely when
autorefresh is enabled).

It is wrong to have duplicates in mbnames for each account. Ignore duplicates
when adding data in the mbnames intermediate files.

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/467
Reported-and-tested-by: Shin Kojima <shin@kojima.org>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/accounts.py

@@ -309,6 +309,9 @@ class SyncableAccount(Account):
             remotefolder.getvisiblename().
             replace(self.remoterepos.getsep(), self.localrepos.getsep()))
 
+
+    # The syncrunner will loop on this method. This means it is called more than
+    # once during the run.
     def __sync(self):
         """Synchronize the account once, then return.
 

offlineimap/mbnames.py

@@ -32,6 +32,7 @@ _mbLock = Lock()
 _mbnames = None
 
 
+# Called at sync time for each folder.
 def add(accountname, folder_root, foldername):
     global _mbnames
     if _mbnames.is_enabled() is not True:
@@ -41,12 +42,14 @@ def add(accountname, folder_root, foldername):
         _mbnames.addAccountFolder(accountname, folder_root, foldername)
 
 
+# Called once.
 def init(conf, ui, dry_run):
     global _mbnames
     if _mbnames is None:
         _mbnames = _Mbnames(conf, ui, dry_run)
 
 
+# Called once.
 def prune(accounts):
     global _mbnames
     if _mbnames.is_enabled() is True:
@@ -55,6 +58,7 @@ def prune(accounts):
         _mbnames.pruneAll()
 
 
+# Called once.
 def write():
     """Write the mbnames file."""
 
@@ -66,6 +70,7 @@ def write():
         _mbnames.write()
 
 
+# Called as soon as all the folders are synced for the account.
 def writeIntermediateFile(accountname):
     """Write intermediate mbnames file."""
 
@@ -93,6 +98,7 @@ class _IntermediateMbnames(object):
         self._dryrun = dry_run
 
     def add(self, foldername):
+        if foldername not in self._foldernames:
             self._foldernames.append(foldername)
 
     def get_folder_root(self):