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

4 files changed, 39 insertions, 3 deletions

diff --git a/lib/sqlalchemy/dialects/oracle/base.py b/lib/sqlalchemy/dialects/oracle/base.py
index 7bdaafd82..c0f7aa5ce 100644
--- a/lib/sqlalchemy/dialects/oracle/base.py
+++ b/lib/sqlalchemy/dialects/oracle/base.py
@@ -1587,7 +1587,9 @@ class OracleDialect(default.DefaultDialect):
 
     def _get_default_schema_name(self, connection):
         return self.normalize_name(
-            connection.exec_driver_sql("SELECT USER FROM DUAL").scalar()
+            connection.exec_driver_sql(
+                "select sys_context( 'userenv', 'current_schema' ) from dual"
+            ).scalar()
         )
 
     def _resolve_synonym(
diff --git a/lib/sqlalchemy/dialects/oracle/provision.py b/lib/sqlalchemy/dialects/oracle/provision.py
index 01854fdce..d19dfc9fe 100644
--- a/lib/sqlalchemy/dialects/oracle/provision.py
+++ b/lib/sqlalchemy/dialects/oracle/provision.py
@@ -7,6 +7,7 @@ from ...testing.provision import drop_db
 from ...testing.provision import follower_url_from_main
 from ...testing.provision import log
 from ...testing.provision import run_reap_dbs
+from ...testing.provision import set_default_schema_on_connection
 from ...testing.provision import temp_table_keyword_args
 from ...testing.provision import update_db_opts
 
@@ -106,3 +107,12 @@ def _oracle_temp_table_keyword_args(cfg, eng):
         "prefixes": ["GLOBAL TEMPORARY"],
         "oracle_on_commit": "PRESERVE ROWS",
     }
+
+
+@set_default_schema_on_connection.for_db("oracle")
+def _oracle_set_default_schema_on_connection(
+    cfg, dbapi_connection, schema_name
+):
+    cursor = dbapi_connection.cursor()
+    cursor.execute("ALTER SESSION SET CURRENT_SCHEMA=%s" % schema_name)
+    cursor.close()
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":
diff --git a/lib/sqlalchemy/dialects/postgresql/provision.py b/lib/sqlalchemy/dialects/postgresql/provision.py
index 6c6dc4be6..9433ec458 100644
--- a/lib/sqlalchemy/dialects/postgresql/provision.py
+++ b/lib/sqlalchemy/dialects/postgresql/provision.py
@@ -5,6 +5,7 @@ from ... import text
 from ...testing.provision import create_db
 from ...testing.provision import drop_db
 from ...testing.provision import log
+from ...testing.provision import set_default_schema_on_connection
 from ...testing.provision import temp_table_keyword_args
 
 
@@ -64,3 +65,15 @@ def _pg_drop_db(cfg, eng, ident):
 @temp_table_keyword_args.for_db("postgresql")
 def _postgresql_temp_table_keyword_args(cfg, eng):
     return {"prefixes": ["TEMPORARY"]}
+
+
+@set_default_schema_on_connection.for_db("postgresql")
+def _postgresql_set_default_schema_on_connection(
+    cfg, dbapi_connection, schema_name
+):
+    existing_autocommit = dbapi_connection.autocommit
+    dbapi_connection.autocommit = True
+    cursor = dbapi_connection.cursor()
+    cursor.execute("SET SESSION search_path='%s'" % schema_name)
+    cursor.close()
+    dbapi_connection.autocommit = existing_autocommit