Question about pretxnchangegroup

[Doug Simon]

As per the instructions at http://www.selenic.com/mercurial/wiki/index.cgi/EnsureCommitPolicy 
,
I want to use the pretxnchangegroup hook to verify that pushes to a  
shared repo do not break
the build. The problem is that the test requires accessing the state  
that would be pushed.
As far as I understand it, the working directory in the destination  
repo is not updated
before my pretxnchangegroup hook executes which means I have  
incorporate an explicit
'hg update' in my hook. Assuming that this is correct, should I run  
another 'hg update'
at the end of my hook in the case that the test fails? Otherwise, the  
working directory
would now reflects changeset(s) that no longer exist, right?