sqlite: close the database when no more threads need access

It's required to have more than one connection to the database for the
maxconnections configuration option to work with threads. However,
connection.close() is closing all the connections. Only close the connection
when no more thread need it.

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

offlineimap/folder/LocalStatusSQLite.py

@@ -41,6 +41,9 @@ 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
+    threads_open_lock = Lock()
 
     def __init__(self, name, repository):
         self.sep = '.' # Needs to be set before super.__init__()
@@ -62,34 +65,39 @@ class LocalStatusSQLiteFolder(BaseFolder):
         self.connection = None
 
     def openfiles(self):
-        # Try to establish connection, no need for threadsafety in __init__.
-        try:
-            self.connection = sqlite.connect(self.filename, check_same_thread=False)
-        except sqlite.OperationalError as e:
-            # Operation had failed.
-            six.reraise(UserWarning,
-                        UserWarning(
-                            "cannot open database file '%s': %s.\nYou might "
-                            "want to check the rights to that file and if it "
-                            "cleanly opens with the 'sqlite<3>' command."%
-                            (self.filename, e)),
-                        exc_info()[2])
+        # Protect the creation/upgrade of database accross threads.
+        with LocalStatusSQLiteFolder.threads_open_lock:
+            # Try to establish connection, no need for threadsafety in __init__.
 
-        # Make sure sqlite is in multithreading SERIALIZE mode.
-        assert sqlite.threadsafety == 1, 'Your sqlite is not multithreading safe.'
+            try:
+                self.connection = sqlite.connect(self.filename,
+                                                 check_same_thread=False)
+                LocalStatusSQLiteFolder.threads_open_count += 1
+            except sqlite.OperationalError as e:
+                # Operation had failed.
+                six.reraise(UserWarning,
+                            UserWarning(
+                                "cannot open database file '%s': %s.\nYou might"
+                                " want to check the rights to that file and if "
+                                "it cleanly opens with the 'sqlite<3>' command"%
+                                (self.filename, e)),
+                            exc_info()[2])
 
-        # Test if db version is current enough and if db is readable.
-        try:
-            cursor = self.connection.execute(
-                "SELECT value from metadata WHERE key='db_version'")
-        except sqlite.DatabaseError:
-            # db file missing or corrupt, recreate it.
-            self.__create_db()
-        else:
-            # Fetch db version and upgrade if needed.
-            version = int(cursor.fetchone()[0])
-            if version < LocalStatusSQLiteFolder.cur_version:
-                self.__upgrade_db(version)
+            # 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(
+                    "SELECT value from metadata WHERE key='db_version'")
+            except sqlite.DatabaseError:
+                # db file missing or corrupt, recreate it.
+                self.__create_db()
+            else:
+                # Fetch db version and upgrade if needed.
+                version = int(cursor.fetchone()[0])
+                if version < LocalStatusSQLiteFolder.cur_version:
+                    self.__upgrade_db(version)
 
     def purge(self):
         """Remove any pre-existing database."""
@@ -159,8 +167,8 @@ class LocalStatusSQLiteFolder(BaseFolder):
     def __upgrade_db(self, from_ver):
         """Upgrade the sqlite format from version 'from_ver' to current"""
 
-        if hasattr(self, 'connection'):
-            self.connection.close() #close old connections first
+        if self.connection is not None:
+            self.connection.close() # Close old connections first.
         self.connection = sqlite.connect(self.filename,
                                          check_same_thread=False)
 
@@ -185,8 +193,8 @@ class LocalStatusSQLiteFolder(BaseFolder):
 
         self.connection must point to the opened and valid SQlite
         database connection."""
-        self.ui._msg('Creating new Local Status db for %s:%s' \
-                         % (self.repository, self))
+        self.ui._msg('Creating new Local Status db for %s:%s'%
+                     (self.repository, self))
         self.connection.executescript("""
         CREATE TABLE metadata (key VARCHAR(50) PRIMARY KEY, value VARCHAR(128));
         INSERT INTO metadata VALUES('db_version', '2');
@@ -229,10 +237,13 @@ class LocalStatusSQLiteFolder(BaseFolder):
             self.messagelist[uid]['mtime'] = row[2]
 
     def closefiles(self):
-        try:
-            self.connection.close()
-        except:
-            pass
+        with LocalStatusSQLiteFolder.threads_open_lock:
+            LocalStatusSQLiteFolder.threads_open_count -= 1
+            if self.threads_open_count < 1:
+                try:
+                    self.connection.close()
+                except:
+                    pass
 
     # Interface from LocalStatusFolder
     def save(self):