Don't link on_connect to first_connect event handler

Adjusted the dialect initialization process such that the
:meth:`_engine.Dialect.on_connect` is not called a second time on the first
connection.   The hook is called first, then the
:meth:`_engine.Dialect.initialize` is called if that connection is the
first for that dialect, then no more events are called.   This eliminates
the two calls to the "on_connect" function which can produce very difficult
debugging situations.

Fixes: #5497
Change-Id: Icefc2e884e30ee7b4ac84b99dc54bf992a6085e3

1 files changed, 12 insertions, 4 deletions

diff --git a/lib/sqlalchemy/engine/create.py b/lib/sqlalchemy/engine/create.py
index 985a12fa0..dc895ee15 100644
--- a/lib/sqlalchemy/engine/create.py
+++ b/lib/sqlalchemy/engine/create.py
@@ -627,9 +627,9 @@ def create_engine(url, **kwargs):
                 )
                 if conn is None:
                     return
+
                 do_on_connect(conn)
 
-            event.listen(pool, "first_connect", on_connect)
             event.listen(pool, "connect", on_connect)
 
         def first_connect(dbapi_connection, connection_record):
@@ -640,9 +640,17 @@ def create_engine(url, **kwargs):
             dialect.initialize(c)
             dialect.do_rollback(c.connection)
 
-        event.listen(
-            pool, "first_connect", first_connect, _once_unless_exception=True
-        )
+        if do_on_connect:
+            event.listen(
+                pool, "connect", first_connect, _once_unless_exception=True
+            )
+        else:
+            event.listen(
+                pool,
+                "first_connect",
+                first_connect,
+                _once_unless_exception=True,
+            )
 
     dialect_cls.engine_created(engine)
     if entrypoint is not dialect_cls: