BugzillaAPI.setLaunchpadBugId starts a transaction

It fetches the bug from the database so it can calculate the canonical
URL for it. However, code in the externalbugtracker module is
forbidden from starting transactions.

OOPS-1566CCW1529

The OOPS above is interesting because it's caused by a weird
interaction. The definition of BugzillaAPI.setLaunchpadBugId() is as
follows:

    @ensure_no_transaction
    @needs_authentication
    def setLaunchpadBugId(self, remote_bug, launchpad_bug_id):
        ...
        launchpad_bug = getUtility(IBugSet).get(launchpad_bug_id)
        ...
        self.xmlrpc_proxy.Bug.update_see_also(...)

ensure_no_transaction() checks that there is no transaction before
calling the wrapped function. needs_authentication() calls the wrapped
function, catches errors relating to authentication and then tries to
authenticate before calling the function again.

What happens is:

 * setLaunchpadBugId() is called. There is no transaction is progress,
   so it proceeds normally.

 * The line starting "launchpad_bug = " starts a transaction.

 * The call to update_see_also() fails because authentication is
   needed. needs_authentication() catches this and calls
   _authenticate().

 * _authenticate() is wrapped with ensure_no_transaction(), which
   raises a TransactionInProgress exception.