We were hitting a very peculiar deadlock bug that was reproduceable but

depended on the operations being performed and the state of the garbage

collector. The deadlock occurred due to Django's Signal.connect()

triggering a RelationCounterField weak reference to drop while

iterating through signal handlers, which resulted in a callback in

RelationCounterField that then attempted to disconnect signals. Since

Signal.connect() already grabbed a lock, Signal.disconnect() was

blocked waiting on that lock, causing the process (whether unit tests or

web server) to hang.
In order to get around this, the state tracking has been changed a bit,

in a way that actually cleans up the code and brings some consistency

and order to how unsaved instances work. Previously, unsaved model

instances (those with a primary key of None) required listening both for

a save (to begin tracking state) and a weak reference drop (to clean up

signal handlers), which required the use of non-weak signal callbacks,

preventing Signal from doing its own cleanup.
In the new design, InstanceState has been updated to be used for both

saved and unsaved instances. It now owns the signal handlers, and

connects to them using weak references, so that Signal can handle

disconnecting (in a way that avoids deadlocks) when the state is

no longer referenced. An InstanceState that represents an unsaved

instance will listen for when the instance is saved, remove the existing

references to itself, and register new InstanceStates for the saved

reference.
InstanceState also listens for when the weak reference is cleared for

both saved and unsaved instances, so that the resets always happen when

needed. It also no longer listens to pre_delete for unsaved instances,

reducing the number of signal connections.
This all avoids the problem of nested locks in Signal, creates

consistency in how all instances are tracked, and further reduces the

chances of state or signal connection leaks.

Set up a reproduceable unit test run in Review Board that consistently

eventually ended up in a deadlock (due to the garbage collector deciding

to run at some point). After this change, I could not reproduce any

deadlocks in the test runs.
All unit tests for Djblets and Review Board pass.