sqlite: properly serialize operations on the databases

1. There is one database per folder and sqlite requires to serialize the
writings. Instead of locking at LocalStatusSQLiteFolder instance level,
introduce a new DatabaseFileLock object which is shared across threads. This
fixes the concurrent writes issues that some users might experience by
duplications or flags restored to the previous state.

2. Close the database only when we are sure no other threads will use the
connection on a *per-file* basis. Previous fix 677afb8d8f is wrong
because the same lock is shared for all the database files.

Github-fix: https://github.com/OfflineIMAP/offlineimap/issues/350
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/accounts.py

@@ -425,7 +425,8 @@ class SyncableAccount(Account):
 def syncfolder(account, remotefolder, quick):
     """Synchronizes given remote folder for the specified account.
 
-    Filtered folders on the remote side will not invoke this function."""
+    Filtered folders on the remote side will not invoke this function. However,
+    this might be called in a concurrently."""
 
     def check_uid_validity(localfolder, remotefolder, statusfolder):
         # If either the local or the status folder has messages and
@@ -489,8 +490,7 @@ def syncfolder(account, remotefolder, quick):
         might not correspond. But, if we're cloning a folder into a new one,
         [min_uid, ...] does correspond to [1, ...].
 
-        This is just for IMAP-IMAP. For Maildir-IMAP, use maxage instead.
+        This is just for IMAP-IMAP. For Maildir-IMAP, use maxage instead."""
-        """
 
         new.cachemessagelist()
         min_uid = partial.retrieve_min_uid()

offlineimap/folder/LocalStatusSQLite.py

@@ -16,13 +16,39 @@
 #    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 
 import os
-import six
 import sqlite3 as sqlite
 from sys import exc_info
 from threading import Lock
 
+import six
+
 from .Base import BaseFolder
 
+class DatabaseFileLock(object):
+    """Lock at database file level."""
+
+    def __init__(self):
+        self._lock = Lock()
+        self._counter = 0
+
+    def __enter__(self):
+        self._lock.acquire()
+
+    def __exit__(self, typ, value, tb):
+        self._lock.release()
+
+    def registerNewUser(self):
+        self._counter += 1
+
+    def removeOneUser(self):
+        self._counter -= 1
+
+    def getLock(self):
+        return self._lock
+
+    def shouldClose(self):
+        return self._counter < 1
+
 
 class LocalStatusSQLiteFolder(BaseFolder):
     """LocalStatus backend implemented with an SQLite database
@@ -42,11 +68,10 @@ class LocalStatusSQLiteFolder(BaseFolder):
     # Current version of our db format.
     cur_version = 2
     # Keep track on how many threads need access to the database.
-    threads_open_count = 0
+    locks = {} # Key: filename, value: DatabaseFileLock instance.
-    threads_open_lock = Lock()
 
     def __init__(self, name, repository):
-        self.sep = '.' # Needs to be set before super.__init__()
+        self.sep = '.' # Needs to be set before super().__init__().
         super(LocalStatusSQLiteFolder, self).__init__(name, repository)
         self.root = repository.root
         self.filename = os.path.join(self.getroot(), self.getfolderbasename())
@@ -60,19 +85,24 @@ class LocalStatusSQLiteFolder(BaseFolder):
             raise UserWarning("SQLite database path '%s' is not a directory."%
                 dirname)
 
+        self.connection = None
         # This lock protects against concurrent writes in same connection.
         self._dblock = Lock()
-        self.connection = None
 
     def openfiles(self):
-        # Protect the creation/upgrade of database accross threads.
+        # Make sure sqlite is in multithreading SERIALIZE mode.
-        with LocalStatusSQLiteFolder.threads_open_lock:
+        assert sqlite.threadsafety == 1, 'Your sqlite is not multithreading safe.'
-            # Try to establish connection, no need for threadsafety in __init__.
 
+        # Protect the creation/upgrade of database accross threads.
+        if self.filename not in LocalStatusSQLiteFolder.locks:
+            LocalStatusSQLiteFolder.locks[self.filename] = DatabaseFileLock()
+        databaseFileLock = LocalStatusSQLiteFolder.locks[self.filename]
+        with databaseFileLock.getLock():
+            # Try to establish connection, no need for threadsafety in __init__.
             try:
                 self.connection = sqlite.connect(self.filename,
                                                  check_same_thread=False)
-                LocalStatusSQLiteFolder.threads_open_count += 1
+                databaseFileLock.registerNewUser()
             except sqlite.OperationalError as e:
                 # Operation had failed.
                 six.reraise(UserWarning,
@@ -83,9 +113,6 @@ class LocalStatusSQLiteFolder(BaseFolder):
                                 (self.filename, e)),
                             exc_info()[2])
 
-            # Make sure sqlite is in multithreading SERIALIZE mode.
-            assert sqlite.threadsafety == 1, 'Your sqlite is not multithreading safe.'
-
             # Test if db version is current enough and if db is readable.
             try:
                 cursor = self.connection.execute(
@@ -118,30 +145,30 @@ class LocalStatusSQLiteFolder(BaseFolder):
     def isnewfolder(self):
         return self._newfolder
 
-    def __sql_write(self, sql, vars=None, executemany=False):
+    def __sql_write(self, sql, args=None, executemany=False):
         """Execute some SQL, retrying if the db was locked.
 
         :param sql: the SQL string passed to execute()
-        :param vars: the variable values to `sql`. E.g. (1,2) or {uid:1,
+        :param args: the variable values to `sql`. E.g. (1,2) or {uid:1,
             flags:'T'}. See sqlite docs for possibilities.
         :param executemany: bool indicating whether we want to
             perform conn.executemany() or conn.execute().
-        :returns: the Cursor() or raises an Exception"""
+        :returns: None or raises an Exception."""
 
         success = False
         while not success:
-            self._dblock.acquire()
             try:
-                if vars is None:
+                with LocalStatusSQLiteFolder.locks[self.filename].getLock():
+                    if args is None:
                         if executemany:
-                        cursor = self.connection.executemany(sql)
+                            self.connection.executemany(sql)
                         else:
-                        cursor = self.connection.execute(sql)
+                            self.connection.execute(sql)
                     else:
                         if executemany:
-                        cursor = self.connection.executemany(sql, vars)
+                            self.connection.executemany(sql, args)
                         else:
-                        cursor = self.connection.execute(sql, vars)
+                            self.connection.execute(sql, args)
                     success = True
                     self.connection.commit()
             except sqlite.OperationalError as e:
@@ -152,9 +179,6 @@ class LocalStatusSQLiteFolder(BaseFolder):
                     success = False
                 else:
                     raise
-            finally:
-                self._dblock.release()
-        return cursor
 
     def __upgrade_db(self, from_ver):
         """Upgrade the sqlite format from version 'from_ver' to current"""
@@ -229,9 +253,10 @@ class LocalStatusSQLiteFolder(BaseFolder):
             self.messagelist[uid]['mtime'] = row[2]
 
     def closefiles(self):
-        with LocalStatusSQLiteFolder.threads_open_lock:
+        databaseFileLock = LocalStatusSQLiteFolder.locks[self.filename]
-            LocalStatusSQLiteFolder.threads_open_count -= 1
+        with databaseFileLock.getLock():
-            if self.threads_open_count < 1:
+            databaseFileLock.removeOneUser()
+            if databaseFileLock.shouldClose():
                 try:
                     self.connection.close()
                 except:

offlineimap/repository/LocalStatus.py

@@ -95,6 +95,7 @@ class LocalStatusRepository(BaseRepository):
         # data. This might happen if the user removed the folder (maildir) and
         # it is re-created afterwards.
         folder.purge()
+        folder.openfiles()
         folder.save()
         folder.closefiles()