Fix in-memory queue using shared-cache

When share in-memory queue in multi-thread, the queue is not
properly synced between threads, this commit uses shared-cache
to achieve the synchronization.
fixes #31

persistqueue/sqlbase.py

@@ -2,7 +2,7 @@
 import os
 import sqlite3
 import threading
-
+import uuid
 sqlite3.enable_callback_tracebacks(True)
 
 log = logging.getLogger(__name__)
@@ -110,15 +110,24 @@ def _init(self):
         self.put_event = threading.Event()
 
     def _new_db_connection(self, path, multithreading, timeout):
-        conn = None
         if path == self._MEMORY:
-            conn = sqlite3.connect(path,
-                                   check_same_thread=not multithreading)
+            try:
+                conn = sqlite3.connect(
+                    database='file:{0}?cache=shared&mode=memory'.format(uuid.uuid1()),
+                    timeout=timeout,
+                    check_same_thread=not multithreading,
+                    uri=True)
+            except TypeError:
+                conn = sqlite3.connect(
+                    database=':memory:',
+                    timeout=timeout,
+                    check_same_thread=not multithreading)
         else:
             conn = sqlite3.connect('{}/data.db'.format(path),
                                    timeout=timeout,
                                    check_same_thread=not multithreading)
         conn.execute('PRAGMA journal_mode=WAL;')
+        conn.set_trace_callback(print)
         return conn
 
     @with_conditional_transaction

tests/test_sqlqueue.py

@@ -216,18 +216,6 @@ def test_open_close_1000(self):
     def test_open_close_single(self):
         self.skipTest('Memory based sqlite is not persistent.')
 
-    def test_multiple_consumers(self):
-        self.skipTest('Skipped due to occasional crash during '
-                      'multithreading mode.')
-
-    def test_multi_threaded_multi_producer(self):
-        self.skipTest('Skipped due to occasional crash during '
-                      'multithreading mode.')
-
-    def test_multi_threaded_parallel(self):
-        self.skipTest('Skipped due to occasional crash during '
-                      'multithreading mode.')
-
 
 class FILOSQLite3QueueTest(unittest.TestCase):
     def setUp(self):