Support pool.connect() event firing before all else

Fixed regression where a connection pool event specified with a keyword,
most notably ``insert=True``, would be lost when the event were set up.
This would prevent startup events that need to fire before dialect-level
events from working correctly.

The internal mechanics of the engine connection routine has been altered
such that it's now guaranteed that a user-defined event handler for the
:meth:`_pool.PoolEvents.connect` handler, when established using
``insert=True``, will allow an event handler to run that is definitely
invoked **before** any dialect-specific initialization starts up, most
notably when it does things like detect default schema name.
Previously, this would occur in most cases but not unconditionally.
A new example is added to the schema documentation illustrating how to
establish the "default schema name" within an on-connect event
(upcoming as part of I882edd5bbe06ee5b4d0a9c148854a57b2bcd4741)

Addiional changes to support setting default schema name:

The Oracle dialect now uses
``select sys_context( 'userenv', 'current_schema' ) from dual`` to get
the default schema name, rather than ``SELECT USER FROM DUAL``, to
accommodate for changes to the session-local schema name under Oracle.

Added a read/write ``.autocommit`` attribute to the DBAPI-adaptation layer
for the asyncpg dialect.   This so that when working with DBAPI-specific
schemes that need to use "autocommit" directly with the DBAPI connection,
the same ``.autocommit`` attribute which works with both psycopg2 as well
as pg8000 is available.

Fixes: #5716
Fixes: #5708
Change-Id: I7dce56b4345ffc720e25e2aaccb7e42bb29e5671

1 files changed, 13 insertions, 2 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/asyncpg.py b/lib/sqlalchemy/dialects/postgresql/asyncpg.py
index 569024790..412feda0f 100644
--- a/lib/sqlalchemy/dialects/postgresql/asyncpg.py
+++ b/lib/sqlalchemy/dialects/postgresql/asyncpg.py
@@ -491,7 +491,7 @@ class AsyncAdapt_asyncpg_connection:
     def __init__(self, dbapi, connection):
         self.dbapi = dbapi
         self._connection = connection
-        self.isolation_level = "read_committed"
+        self.isolation_level = self._isolation_setting = "read_committed"
         self.readonly = False
         self.deferrable = False
         self._transaction = None
@@ -512,10 +512,21 @@ class AsyncAdapt_asyncpg_connection:
         else:
             raise error
 
+    @property
+    def autocommit(self):
+        return self.isolation_level == "autocommit"
+
+    @autocommit.setter
+    def autocommit(self, value):
+        if value:
+            self.isolation_level = "autocommit"
+        else:
+            self.isolation_level = self._isolation_setting
+
     def set_isolation_level(self, level):
         if self._started:
             self.rollback()
-        self.isolation_level = level
+        self.isolation_level = self._isolation_setting = level
 
     async def _start_transaction(self):
         if self.isolation_level == "autocommit":