ensure event handlers called for all do_pingticket_5648

The support for pool ping listeners to receive exception events via the
:meth:`.ConnectionEvents.handle_error` event added in 2.0.0b1 for
:ticket:`5648` failed to take into account dialect-specific ping routines
such as that of MySQL and PostgreSQL. The dialect feature has been reworked
so that all dialects participate within event handling.   Additionally,
a new boolean element :attr:`.ExceptionContext.is_pre_ping` is added
which identifies if this operation is occurring within the pre-ping
operation.

For this release, third party dialects which implement a custom
:meth:`_engine.Dialect.do_ping` method can opt in to the newly improved
behavior by having their method no longer catch exceptions or check
exceptions for "is_disconnect", instead just propagating all exceptions
outwards. Checking the exception for "is_disconnect" is now done by an
enclosing method on the default dialect, which ensures that the event hook
is invoked for all exception scenarios before testing the exception as a
"disconnect" exception. If an existing ``do_ping()`` method continues to
catch exceptions and check "is_disconnect", it will continue to work as it
did previously, but ``handle_error`` hooks will not have access to the
exception if it isn't propagated outwards.

Fixes: #5648
Change-Id: I6535d5cb389e1a761aad8c37cfeb332c548b876d

1 files changed, 2 insertions, 9 deletions

diff --git a/lib/sqlalchemy/dialects/mysql/mysqldb.py b/lib/sqlalchemy/dialects/mysql/mysqldb.py
index 5c9d11a53..0868401d4 100644
--- a/lib/sqlalchemy/dialects/mysql/mysqldb.py
+++ b/lib/sqlalchemy/dialects/mysql/mysqldb.py
@@ -168,15 +168,8 @@ class MySQLDialect_mysqldb(MySQLDialect):
         return on_connect
 
     def do_ping(self, dbapi_connection):
-        try:
-            dbapi_connection.ping(False)
-        except self.dbapi.Error as err:
-            if self.is_disconnect(err, dbapi_connection, None):
-                return False
-            else:
-                raise
-        else:
-            return True
+        dbapi_connection.ping(False)
+        return True
 
     def do_executemany(self, cursor, statement, parameters, context=None):
         rowcount = cursor.executemany(statement, parameters)