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

2 files changed, 13 insertions, 25 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/_psycopg_common.py b/lib/sqlalchemy/dialects/postgresql/_psycopg_common.py
index d9ddefd38..739cbc5a9 100644
--- a/lib/sqlalchemy/dialects/postgresql/_psycopg_common.py
+++ b/lib/sqlalchemy/dialects/postgresql/_psycopg_common.py
@@ -178,20 +178,15 @@ class _PGDialect_common_psycopg(PGDialect):
     def do_ping(self, dbapi_connection):
         cursor = None
         before_autocommit = dbapi_connection.autocommit
+
+        if not before_autocommit:
+            dbapi_connection.autocommit = True
+        cursor = dbapi_connection.cursor()
         try:
-            if not before_autocommit:
-                self._do_autocommit(dbapi_connection, True)
-            cursor = dbapi_connection.cursor()
-            try:
-                cursor.execute(self._dialect_specific_select_one)
-            finally:
-                cursor.close()
-                if not before_autocommit and not dbapi_connection.closed:
-                    self._do_autocommit(dbapi_connection, before_autocommit)
-        except self.dbapi.Error as err:
-            if self.is_disconnect(err, dbapi_connection, cursor):
-                return False
-            else:
-                raise
-        else:
-            return True
+            cursor.execute(self._dialect_specific_select_one)
+        finally:
+            cursor.close()
+            if not before_autocommit and not dbapi_connection.closed:
+                dbapi_connection.autocommit = before_autocommit
+
+        return True
diff --git a/lib/sqlalchemy/dialects/postgresql/asyncpg.py b/lib/sqlalchemy/dialects/postgresql/asyncpg.py
index e00584503..2acc5fea3 100644
--- a/lib/sqlalchemy/dialects/postgresql/asyncpg.py
+++ b/lib/sqlalchemy/dialects/postgresql/asyncpg.py
@@ -992,15 +992,8 @@ class PGDialect_asyncpg(PGDialect):
         return ([], opts)
 
     def do_ping(self, dbapi_connection):
-        try:
-            dbapi_connection.ping()
-        except self.dbapi.Error as err:
-            if self.is_disconnect(err, dbapi_connection, None):
-                return False
-            else:
-                raise
-        else:
-            return True
+        dbapi_connection.ping()
+        return True
 
     @classmethod
     def get_pool_class(cls, url):