[MERGE] Make cmd_branch check the destination before doing any I/O on the source.

[Martin Pool]

On 25 Jul 2006, John Arbash Meinel <john at arbash-meinel.com> wrote:
> Michael Ellerman wrote:
> > On Thu, 2006-07-06 at 23:24 -0500, John Arbash Meinel wrote:
> 
> ...
> 
> > 
> >> I wasn't thinking to completely scrap it, just cleanup the mkdir if you
> >> fail the rest.
> >> (Just like our locking lock A, then B, but if B fails, unlock A)
> > 

> > OK, I've got this, which works, but triggers my "never catch Exception"
> > handler. Any ideas on how to do it better?

The other possibility is 

  completed = False
  try:
    do_stuff()
    completed = True 
  finally:
    if not completed:
      unwind()

But, given that Python's bare raise really does let the exception
continue as if it was not caught, I don't think this is needed.

> In general, I agree with not catching Exception, or not having a blank
> 'except:'. But it turns out that:
> try:
>   foo()
> except:
>   raise
> 
> Actually returns the traceback from the 'foo' exception, it doesn't
> reset it to start from where the 'raise' occurs.
> So I think it is fine.

I agree that bare except paired with bare raise is acceptable.  The code
should re-raise by default unless it specifically knows what to do with
the exception.

> That will raise the original exception delete_tree() fails, though you
> lose the traceback. And if delete_tree() succeeds, it raises the
> original exception with the original traceback.

Well, in Python you have to lose one.  I think emitting a warning for
the less-important one is the best compromise.

-- 
Martin