DataError when underlying type changes

[psycoteam]

Originally submitted by: Chad Whitacre (whit537)

psycopg2 caches the attributes of composite types, so that when the type is altered in the database, the next attempt to cast that type in psycopg2 results in a DataError. This seems to introduce a race condition between modifying the database and restarting processes accessing the database.

More information:

gratipay/gratipay.com#1583

[dvarrazzo]

Caching of the attribute is necessary but it's local to the scope you have registered the composite.

Avoid using register_composite with global=True: use it on the connection or on the cursor, and dispose the connection or the cursor after the data type is changed. Alternatively just call register_composite again.

[psycoteam]

Originally submitted by: Chad Whitacre

Thanks @dvarrazzo. Scoping our register_composite calls to the connection or cursor would narrow the race condition, but wouldn't eliminate it. I'm not sure we can eliminate it, though, so what would you think of raising a subclass of DataError in CompositeCaster.parse instead of DataError itself? That way our application could catch that (without swallowing other, unrelated DataErrors) and then retry register_composite or take other measures. Would this be acceptable to you?

[dvarrazzo]

Hello Chad,

the use case for such subclass is fairly restricted, and I think it would be more the hassle to document it than its general usefulness, not to mention that people is usually unhappy to get a new feature in rel. 2.5.2.

So my suggestion is you make your own subclass of the adapter, which should be something as easy as (untested):

class CompositeChanged(DataError):
    pass

class OurCompositeCaster(CompositeCaster):
    def parse(self, s, curs):
        try:
            return super(OurCompositeCaster, self).parse(s, curs)
        except DataError, e:
            raise CompositeChanged(str(e))

this should do everything you need.

[psycoteam]

Originally submitted by: Chad Whitacre

Sounds sensible. Thanks Daniele! :-)