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, 14 insertions, 13 deletions

diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index f8126fa30..3e4e6fb9a 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -669,16 +669,11 @@ class DefaultDialect(Dialect):
     def _dialect_specific_select_one(self):
         return str(expression.select(1).compile(dialect=self))
 
-    def do_ping(self, dbapi_connection: DBAPIConnection) -> bool:
-        cursor = None
+    def _do_ping_w_event(self, dbapi_connection: DBAPIConnection) -> bool:
         try:
-            cursor = dbapi_connection.cursor()
-            try:
-                cursor.execute(self._dialect_specific_select_one)
-            finally:
-                cursor.close()
+            return self.do_ping(dbapi_connection)
         except self.loaded_dbapi.Error as err:
-            is_disconnect = self.is_disconnect(err, dbapi_connection, cursor)
+            is_disconnect = self.is_disconnect(err, dbapi_connection, None)
 
             if self._has_events:
                 try:
@@ -687,19 +682,25 @@ class DefaultDialect(Dialect):
                         self,
                         is_disconnect=is_disconnect,
                         invalidate_pool_on_disconnect=False,
+                        is_pre_ping=True,
                     )
                 except exc.StatementError as new_err:
                     is_disconnect = new_err.connection_invalidated
 
-                # other exceptions modified by the event handler will be
-                # thrown
-
             if is_disconnect:
                 return False
             else:
                 raise
-        else:
-            return True
+
+    def do_ping(self, dbapi_connection: DBAPIConnection) -> bool:
+        cursor = None
+
+        cursor = dbapi_connection.cursor()
+        try:
+            cursor.execute(self._dialect_specific_select_one)
+        finally:
+            cursor.close()
+        return True
 
     def create_xid(self):
         """Create a random two-phase transaction ID.