Check for and cleanup dirty zope.sqlalchemy state at end of request

`zope.sqlalchemy.register` adds a set of event listeners to a session
which trigger population of an internal `id(session) => state` map
whenever a session becomes dirty. This map is usually cleared when the
DB transaction is committed as a result of the transaction manager being
committed or aborted at the end of a web request by pyramid_tm.

If however `request.session` is accessed outside of pyramid_tm's tween
and DB changes are made, the event listeners will fire and references to
the session's ID will be left in the internal _SESSION_STATE map when
the request ends.

This commit adds a check for a non-empty map at the end of a request and
logs a warning, to help track down causes of DB writes that are causing
the issue, it then cleans up the dirty state.

Unfortunately there are no public APIs for deregistering a session [1], so
we clear zope.sqlalchemy's private internal map directly.

[1] zopefoundation/zope.sqlalchemy#9

h/db/__init__.py

@@ -115,6 +115,19 @@ def close_the_sqlalchemy_session(request):
             })
         session.close()
 
+        # zope.sqlalchemy maintains an internal `id(session) => state` map for
+        # each active DB session which is registered with it.
+        #
+        # Ensure that this is empty at the end of the request as otherwise it
+        # can cause request <=> DB transaction integration to break in future
+        # requests if a future `Session` object reuses the same ID.
+        dm = zope.sqlalchemy.datamanager
+        if len(dm._SESSION_STATE) > 0:
+            log.warn('request ended with non-empty zope.sqlalchemy session', extra={
+                'state': dm._SESSION_STATE
+            })
+            dm._SESSION_STATE = {}
+
     return session