postgres connection hstore support can disappear in low memory situations

[Ronan Klyne]

Present in 1.8 and master. Probably versions between but I haven't checked them all

In order to correctly handle hstore data on new connections Django registers a signal handler to the connection_created signal.

This signal is created as a weakref and can be garbage collected, causing all future connections to misbehave when receiving hstore data from Postgres.

This problem has also been seen and fixed in django-hstore - ​https://github.com/djangonauts/django-hstore/pull/128

[Ronan Klyne]

Pull request created - ​https://github.com/django/django/pull/9557

[Tim Graham]

I don't know much about how garbage collection works in Python, but if this is indeed the correct solution, then perhaps some clarifications to ​Django's documentation would be useful. It says,"Django stores signal handlers as weak references by default. Thus, if your receiver is a local function, it may be garbage collected." but my understanding is that the receiver here (register_type_handlers) isn't a local function.

[Ronan Klyne]

I honestly don't know enough about the GC to explain this properly in the docs but I have seen the problem happen in django-hstore. I was reading the Django code and saw the same pattern so I pushed the same fix as a PR here. At the very least it seemed like a more explicit description of the desired effect - I don't think we would ever want this signal handler to be garbage collected.

Django itself is using a module level function and django-hstore uses a module level singleton callable class. That's close enough that I would imagine that it's vulnerable to whatever peculiarity was breaking django-hstore, though I can't explain why the garbage collector would collect it.

When this problem occurs it manifests as an hstore field being set to a string value on the Model - the dictionary deserialisation is never triggered.

Maybe you just want to archive this until that situation arises in the wild?

[Tim Graham]

Sure, I think we should wait to fix a problem until we see it, so we can confirm the change is needed and that it fixes the issue. Thanks.