Gracefully degrade on v$transaction not readable

Fixed regression which occured due to [ticket:5755] which implemented
isolation level support for Oracle.   It has been reported that many Oracle
accounts don't actually have permission to query the ``v$transaction``
view so this feature has been altered to gracefully fallback when it fails
upon database connect, where the dialect will assume "READ COMMITTED" is
the default isolation level as was the case prior to SQLAlchemy 1.3.21.
However, explicit use of the :meth:`_engine.Connection.get_isolation_level`
method must now necessarily raise an exception, as Oracle databases with
this restriction explicitly disallow the user from reading the current
isolation level.

Fixes: #5784
Change-Id: Iefc82928744f3c944c18ae8000eb3c9e52e523bc

1 files changed, 20 insertions, 0 deletions

diff --git a/lib/sqlalchemy/engine/interfaces.py b/lib/sqlalchemy/engine/interfaces.py
index fddbc501a..b6f7bc49a 100644
--- a/lib/sqlalchemy/engine/interfaces.py
+++ b/lib/sqlalchemy/engine/interfaces.py
@@ -876,6 +876,26 @@ class Dialect(object):
 
         raise NotImplementedError()
 
+    def get_default_isolation_level(self, dbapi_conn):
+        """Given a DBAPI connection, return its isolation level, or
+        a default isolation level if one cannot be retrieved.
+
+        This method may only raise NotImplementedError and
+        **must not raise any other exception**, as it is used implicitly upon
+        first connect.
+
+        The method **must return a value** for a dialect that supports
+        isolation level settings, as this level is what will be reverted
+        towards when a per-connection isolation level change is made.
+
+        The method defaults to using the :meth:`.Dialect.get_isolation_level`
+        method unless overridden by a dialect.
+
+        .. versionadded:: 1.3.22
+
+        """
+        raise NotImplementedError()
+
     @classmethod
     def get_dialect_cls(cls, url):
         """Given a URL, return the :class:`.Dialect` that will be used.