Question

for some reason I’m experiencing the Operational Error with “can’t rollback” message when I attempt to roll back my transaction in the following context:

try:
    cursors[instance].execute("lock revision, app, timeout IN SHARE MODE")
    cursors[instance].execute("insert into app (type, active, active_revision, contents, z) values ('session', true, %s, %s, 0) returning id", (cRevision, sessionId))
    sAppId = cursors[instance].fetchone()[0]
    cursors[instance].execute("insert into revision (app_id, type) values (%s, 'active')", (sAppId,))
    cursors[instance].execute("insert into timeout (app_id, last_seen) values (%s, now())", (sAppId,))
    connections[instance].commit()
except pgdb.DatabaseError, e:
    connections[instance].rollback()
    return "{status: 'error', errno:4, errmsg: \"%s\"}"%(str(e).replace('\"', '\\"').replace('\n', '\\n').replace('\r', '\\r'))

The driver in use is PGDB.

What is fundamentally wrong here?

Answer 1

What happens if you exclude the lock statement?

This is what’s happening inside pgdb.py:

def rollback(self):
    """Roll back to the start of any pending transaction."""
    if self._cnx:
        if self._tnx:
            self._tnx = False
            try:
                self._cnx.source().execute("ROLLBACK")
            except Exception:
                raise OperationalError("can't rollback")
    else:
        raise OperationalError("connection has been closed")

So I suggest you replace your connections[instance].rollback() call with:

connections[instance]._tnx = False
connections[instance]._cnx.source().execute("ROLLBACK")

to see if that gives you a more informative error message (the except clause inside pgdb is greedy).

Also: check the Postgresql log, it will have probably logged the reason!