Implement Oracle SERIALIZABLE + real read of isolation level

There's some significant awkwardness in that we can't
read the level unless a transaction is started, which normally
does not occur unless DML is emitted.  The implementation
uses the local_transaction_id function to start a transaction.
It is not known what the performance impact of this might
have, however by default the function is called only once
on first connect and later only if the get_isolation_level()
method is used.

Fixes: #5755
Change-Id: I0453a6b0a49420826707f660931002ba2338fbf0

1 files changed, 22 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/suite/test_dialect.py b/lib/sqlalchemy/testing/suite/test_dialect.py
index 7f697b915..f860a321b 100644
--- a/lib/sqlalchemy/testing/suite/test_dialect.py
+++ b/lib/sqlalchemy/testing/suite/test_dialect.py
@@ -122,6 +122,28 @@ class IsolationLevelTest(fixtures.TestBase):
 
             eq_(conn.get_isolation_level(), existing)
 
+    def test_all_levels(self):
+        levels = requirements.get_isolation_levels(config)
+
+        all_levels = levels["supported"]
+
+        for level in set(all_levels).difference(["AUTOCOMMIT"]):
+            with config.db.connect() as conn:
+                conn.execution_options(isolation_level=level)
+
+                eq_(conn.get_isolation_level(), level)
+
+                trans = conn.begin()
+                trans.rollback()
+
+                eq_(conn.get_isolation_level(), level)
+
+            with config.db.connect() as conn:
+                eq_(
+                    conn.get_isolation_level(),
+                    levels["default"],
+                )
+
 
 class AutocommitTest(fixtures.TablesTest):