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, 5 insertions, 2 deletions

diff --git a/lib/sqlalchemy/pool/base.py b/lib/sqlalchemy/pool/base.py
index d67f32442..ac487452c 100644
--- a/lib/sqlalchemy/pool/base.py
+++ b/lib/sqlalchemy/pool/base.py
@@ -132,7 +132,7 @@ class _ConnDialect:
     def do_close(self, dbapi_connection: DBAPIConnection) -> None:
         dbapi_connection.close()
 
-    def do_ping(self, dbapi_connection: DBAPIConnection) -> bool:
+    def _do_ping_w_event(self, dbapi_connection: DBAPIConnection) -> bool:
         raise NotImplementedError(
             "The ping feature requires that a dialect is "
             "passed to the connection pool."
@@ -1266,6 +1266,7 @@ class _ConnectionFairy(PoolProxiedConnection):
         threadconns: Optional[threading.local] = None,
         fairy: Optional[_ConnectionFairy] = None,
     ) -> _ConnectionFairy:
+
         if not fairy:
             fairy = _ConnectionRecord.checkout(pool)
 
@@ -1304,7 +1305,9 @@ class _ConnectionFairy(PoolProxiedConnection):
                                 "Pool pre-ping on connection %s",
                                 fairy.dbapi_connection,
                             )
-                        result = pool._dialect.do_ping(fairy.dbapi_connection)
+                        result = pool._dialect._do_ping_w_event(
+                            fairy.dbapi_connection
+                        )
                         if not result:
                             if fairy._echo:
                                 pool.logger.debug(