[Zope-dev] Re: "hasattr" geddon

[Michael Dunstan]

On 12/07/2004, at 12:29 PM, Chris McDonough wrote:

> FWIW, as far as I understand the "prevent commit on conflict" behavior
> *is* currently the behavior for caught ReadConflictErrors.  The only
> time ZODB doesn't exhibit this behavior is *during the commit of a
> transaction*.  If a commit is attempted at this point and a conflict
> error is caught unintentionally, the following happens (as explained by
> Tim at http://mail.zope.org/pipermail/zodb-dev/2004-May/007404.html :
>
> - the attempt to commit fails, raising a write conflict error
>
> - the write conflict error is unintentionally caught (whether by 
> hasattr
> or a bare except or by whatever)
>
> - a new transaction begins implicitly (as if 
> "get_transaction().abort()"
> or "get_transaction().begin() were called)
>
> - ... whatever happens after this happens ..
>
> Since it's extremely unlkely for a write conflict error to be caught
> unintentionally in Zope (it would require an application to perform a
> "get_transaction().commit()" going outside the boundaries of the 
> default
> transaction manager), it's an obscure failure mode.
>
> If the transaction was not put into a committable state after a caught
> exception error, there would be no problem.  But to be fair, I don't
> think there is much of a problem right now, as very few Zope apps 
> manage
> their own transactions (except for the Catalog... gulp ;-).  But it
> would be nice if ZODB didn't do the implicit 
> "get_transaction().abort()"
> after a write conflict error and left the transaction in an
> uncommittable state instead... or at least had an option to do this.

So... it sounds like ZODB will take care of itself in the face of 
ReadConflictErrors swallowed by bare excepts (and hasattr's). (Unless 
your managing your own transactions.)

One penalty of bare excepts swallowing a ConflictError is running more 
code than is needed. The transaction is going to be thrown away 
eventually.

Is there problem with seeing bogus exceptions in code after a swallowed 
ConflictError?

I'm thinking that when zope catches any exception in a transaction it 
should check the state of the transaction to decide if should treat 
this as a ConflictError or report the exception it caught.

Consider a contrived example::

     def setFoo(self, value):
         try:
             self.foo = value
         except:
             pass
         self.fooUpdated()

Assume that the except swallows some ConflictError then an exception is 
raised from fooUpdated()
but not a ConflictError, lets say a KeyError. Sure, ZODB is tough 
enough to look after itself in this case. What about the developer 
trying to interpret the KeyError that is written to event.log?

(Hmm... I should really check that is what you could potentially see 
with a current Zope :-)

Michael