Because cloudpickle tries to pickle a function's globals, when it
pickled an attrs instance, it would try to pickle the `__repr__` method
and its globals, which included a `threading.local`. This broke
cloudpickle for all attrs classes unless they explicitly specified
`repr=False`. Modules, however, are pickled by reference, not by value,
so moving the repr into a different module means we can put `_compat`
into the function's globals and not worry about direct references.
Includes a test to ensure that attrs and cloudpickle remain compatible.
Also adds an explanation of the reason we even *have* that global
thread-local variable. It wasn't completely obvious to a reader why
the thread-local was needed to track reference cycles in `__repr__`
calls, and the test did not previously contain a cycle that touched
a non-attrs value. This change adds a comment explaining the need
and tests a cycle that contains non-attrs values.
Fixes:
- https://github.com/python-attrs/attrs/issues/458
- https://github.com/cloudpipe/cloudpickle/issues/320
554d6f293d