mysql: do not retry a failing query forever

Due to a bug in MariaDB Connector/C 2.3.2, some tests like testBasicStore and
test_max_allowed_packet were retrying the same failing query indefinitely.

neo/storage/database/mysqldb.py

@@ -120,6 +120,10 @@ class MySQLDatabaseManager(DatabaseManager):
         if LOG_QUERIES:
             logging.debug('querying %s...',
                 getPrintableQuery(query.split('\n', 1)[0][:70]))
+        # Try 3 times at most. When it fails too often for the same
+        # query then the disconnection is likely caused by this query.
+        # We don't want to enter into an infinite loop.
+        retry = 2
         while 1:
             conn = self.conn
             try:
@@ -133,11 +137,13 @@ class MySQLDatabaseManager(DatabaseManager):
                 break
             except OperationalError as m:
                 code, m = m.args
-                if self._active or SERVER_GONE_ERROR != code != SERVER_LOST:
+                if self._active or SERVER_GONE_ERROR != code != SERVER_LOST \
+                   or not retry:
                     raise DatabaseFailure('MySQL error %d: %s\nQuery: %s'
                         % (code, m, getPrintableQuery(query[:1000])))
                 logging.info('the MySQL server is gone; reconnecting')
                 self._connect()
+                retry -= 1
         r = query.split(None, 1)[0]
         if r in ("INSERT", "REPLACE", "DELETE", "UPDATE"):
             self._active = 1

neo/tests/threaded/test.py

@@ -64,6 +64,8 @@ class Test(NEOThreadedTest):
     def testBasicStore(self, cluster):
         if 1:
             storage = cluster.getZODBStorage()
+            storage.sync()
+            storage.app.max_reconnection_to_master = 0
             data_info = {}
             compressible = 'x' * 20
             compressed = compress(compressible)