This fixes the transient failure in testExceptionInSubAbortSub() and
is presumed to be correct, too.  The old code had the comment:
    # This should never fail
but did not to guarantee that it never failed.  I'm assuming that the
write fix is to log the error and ignore it, because that's what the
call to tpc_abort() immediately above does.

XXX commit() is a really long method.  It would be nice to break it up
into easier-to-understand chunks.