SQLite: make postponing transaction committing possible.

This should significantly improve performance when used to write large
amounts of messages.

This feature is enabled through the fsync configuration option.

Code refactorize around fsync.

This addresses #390 (although it doesn't necessarily fix all instances
of that problem yet).

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/390
Originally-written-by: Giel van Schijndel <me@mortis.eu>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -164,6 +164,9 @@ accounts = Test
 # at the expense of greater risk of message duplication in the event of a system
 # crash or power loss.  Default is true.  Set it to false to disable fsync.
 #
+# SQLite honors this option since v7.0.8+. However, those SQLite improvements
+# are still EXPERIMENTAL.
+#
 #fsync = true
 
 

offlineimap/folder/Base.py

@@ -70,6 +70,7 @@ class BaseFolder(object):
 
         self._sync_deletes = self.config.getdefaultboolean(
             self.repoconfname, "sync_deletes", True)
+        self._dofsync = self.config.getdefaultboolean("general", "fsync", True)
 
         # Determine if we're running static or dynamic folder filtering
         # and check filtering status.
@@ -103,6 +104,16 @@ class BaseFolder(object):
         # fails if the str is utf-8
         return self.name.decode('utf-8')
 
+    def __enter__(self):
+        """Starts a transaction. This will postpone (guaranteed) saving to disk
+        of all messages saved inside this transaction until its committed."""
+        pass
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        """Commits a transaction, all messages saved inside this transaction
+        will only now be persisted to disk."""
+        pass
+
     @property
     def accountname(self):
         """Account name as string"""
@@ -118,6 +129,9 @@ class BaseFolder(object):
         else:
             return self.repository.should_sync_folder(self.ffilter_name)
 
+    def dofsync(self):
+        return self._dofsync
+
     def suggeststhreads(self):
         """Returns True if this folder suggests using threads for actions.
 
@@ -891,6 +905,7 @@ class BaseFolder(object):
             )
             return
 
+        with self:
             for num, uid in enumerate(copylist):
                 # Bail out on CTRL-C or SIGTERM.
                 if offlineimap.accounts.Account.abort_NOW_signal.is_set():

offlineimap/folder/LocalStatusSQLite.py

@@ -92,6 +92,20 @@ class LocalStatusSQLiteFolder(BaseFolder):
             LocalStatusSQLiteFolder.locks[self.filename] = DatabaseFileLock()
         self._databaseFileLock = LocalStatusSQLiteFolder.locks[self.filename]
 
+        self._in_transactions = 0
+
+    def __enter__(self):
+        if not self.dofsync():
+            assert self.connection is not None
+            self._in_transactions += 1
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if not self.dofsync():
+            assert self._in_transactions > 0
+            self._in_transactions -= 1
+            if self._in_transactions < 1:
+                self.connection.commit()
+
     def openfiles(self):
         # Make sure sqlite is in multithreading SERIALIZE mode.
         assert sqlite.threadsafety == 1, 'Your sqlite is not multithreading safe.'
@@ -169,6 +183,7 @@ class LocalStatusSQLiteFolder(BaseFolder):
                         else:
                             self.connection.execute(sql, args)
                     success = True
+                    if not self._in_transactions:
                         self.connection.commit()
             except sqlite.OperationalError as e:
                 if e.args[0] == 'cannot commit - no transaction is active':

offlineimap/folder/Maildir.py

@@ -60,7 +60,6 @@ class MaildirFolder(BaseFolder):
     def __init__(self, root, name, sep, repository):
         self.sep = sep # needs to be set before super().__init__
         super(MaildirFolder, self).__init__(name, repository)
-        self.dofsync = self.config.getdefaultboolean("general", "fsync", True)
         self.root = root
         # check if we should use a different infosep to support Win file systems
         self.wincompatible = self.config.getdefaultboolean(
@@ -330,7 +329,7 @@ class MaildirFolder(BaseFolder):
         fd.write(content)
         # Make sure the data hits the disk.
         fd.flush()
-        if self.dofsync:
+        if self.dofsync():
             os.fsync(fd)
         fd.close()
 

offlineimap/folder/UIDMaps.py

@@ -34,7 +34,6 @@ class MappedIMAPFolder(IMAPFolder):
 
     Instance variables (self.):
       dryrun: boolean.
-      dofsync: boolean for fsync calls.
       r2l: dict mapping message uids: self.r2l[remoteuid]=localuid
       l2r: dict mapping message uids: self.r2l[localuid]=remoteuid
       #TODO: what is the difference, how are they used?
@@ -44,7 +43,6 @@ class MappedIMAPFolder(IMAPFolder):
     def __init__(self, *args, **kwargs):
         IMAPFolder.__init__(self, *args, **kwargs)
         self.dryrun = self.config.getdefaultboolean("general", "dry-run", True)
-        self.dofsync = self.config.getdefaultboolean("general", "fsync", True)
         self.maplock = Lock()
         self.diskr2l, self.diskl2r = self._loadmaps()
         self.r2l, self.l2r = None, None
@@ -114,7 +112,7 @@ class MappedIMAPFolder(IMAPFolder):
             with open(mapfilenametmp, 'wt') as mapfilefd:
                 for (key, value) in self.diskl2r.items():
                     mapfilefd.write("%d:%d\n"% (key, value))
-                if self.dofsync is True:
+                if self.dofsync():
                     fsync(mapfilefd)
             # The lock is released when the file descriptor ends.
             shutil.move(mapfilenametmp, mapfilename)