Create LocalStatus or LocalStatusSQLite folders

Depending on the configuration we use the plain text or the new
experimental sqlite backend for the LocalStatus cache. Make plain text
the default status backend but allow people to configure
status_backend=sqlite in their [Account ...] section.

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

Changelog.draft.rst

@@ -17,6 +17,9 @@ New Features
   readonly = True. When e.g. using offlineimap for backup purposes you
   can thus make sure that no changes in your backup trickle back into
   the main IMAP server.
+* optional: experimental SQLite-based backend for the LocalStatus
+  cache. Plain text remains the default. Enable by setting
+  status_backend=sqlite in the [Account ...] section
 
 Changes
 -------
@@ -41,11 +44,3 @@ Pending for the next major release
 
 * UIs get shorter and nicer names. (API changing)
 * Implement IDLE feature. (delayed until next major release)
-
-
-Stalled
-=======
-
-* Learn Sqlite support.
-    Stalled: it would need to learn the ability to choose between the current
-    format and SQL to help testing the long term.

offlineimap.conf

@@ -197,6 +197,23 @@ remoterepository = RemoteExample
 # You can also specify parameters to the commands
 # presynchook = imapfilter -c someotherconfig.lua
 
+# OfflineImap caches the state of the synchronisation to e.g. be able to
+# determine if a mail has been deleted on one side or added on the
+# other.
+#
+# The default and historical backend is 'plain' which writes out the
+# state in plain text files. On Repositories with large numbers of
+# mails, the performance might not be optimal, as we write out the
+# complete file for each change.  Another new backend 'sqlite' is
+# available which stores the status in sqlite databases. BE AWARE THIS
+# IS EXPERIMENTAL STUFF.
+#
+# If you switch the backend, you may want to delete the old cache
+# directory in ~/.offlineimap/Account-<account>/LocalStatus manually
+# once you are sure that things work.
+#
+#status_backend = plain
+
 # If you have a limited amount of bandwidth available you can exclude larger
 # messages (e.g. those with large attachments etc).  If you do this it
 # will appear to offlineimap that these messages do not exist at all.  They

offlineimap/repository/LocalStatus.py

@@ -18,7 +18,8 @@
 
 from Base import BaseRepository
 from offlineimap import folder
-import offlineimap.folder.LocalStatus
+from offlineimap.folder.LocalStatus import LocalStatusFolder
+from offlineimap.folder.LocalStatusSQLite import LocalStatusSQLiteFolder
 import os
 import re
 
@@ -26,9 +27,25 @@ class LocalStatusRepository(BaseRepository):
     def __init__(self, reposname, account):
         BaseRepository.__init__(self, reposname, account)
         self.directory = os.path.join(account.getaccountmeta(), 'LocalStatus')
+
+        #statusbackend can be 'plain' or 'sqlite'
+        backend = self.account.getconf('status_backend', 'plain')
+        if backend == 'sqlite':
+            self._backend = 'sqlite'
+            self.LocalStatusFolderClass = LocalStatusSQLiteFolder
+            self.directory += '-sqlite'
+        elif backend == 'plain':
+            self._backend = 'plain'
+            self.LocalStatusFolderClass = LocalStatusFolder
+        else:
+            raise SyntaxWarning("Unknown status_backend '%s' for account '%s'" \
+                                % (backend, account.name))
+
         if not os.path.exists(self.directory):
             os.mkdir(self.directory, 0700)
-        self.folders = None
+
+        # self._folders is a list of LocalStatusFolders()
+        self._folders = None
 
     def getsep(self):
         return '.'
@@ -40,16 +57,22 @@ class LocalStatusRepository(BaseRepository):
         return os.path.join(self.directory, foldername)
 
     def makefolder(self, foldername):
-        # "touch" the file, truncating it.
+        """Create a LocalStatus Folder
+
+        Empty Folder for plain backend. NoOp for sqlite backend as those
+        are created on demand."""
+        # Invalidate the cache.
+        self._folders = None
+        if self._backend == 'sqlite':
+            return
+
         filename = self.getfolderfilename(foldername)
         file = open(filename + ".tmp", "wt")
         file.write(offlineimap.folder.LocalStatus.magicline + '\n')
-        file.flush()
-        os.fsync(file.fileno())
         file.close()
         os.rename(filename + ".tmp", filename)
         # Invalidate the cache.
-        self.folders = None
+        self._folders = None
 
     def getfolder(self, foldername):
         """Return the Folder() object for a foldername"""
@@ -72,4 +95,3 @@ class LocalStatusRepository(BaseRepository):
         """Forgets the cached list of folders, if any.  Useful to run
         after a sync run."""
         self._folders = None
-