Unrelated builders interfere with each other

[greschd]

Describe the bug

The ProcessBuilderNamespace of unrelated processes can interfere with each other, because their properties are set on the ProcessBuilderNamespace class instead of the specific instance.

Steps to reproduce

WorkChain code:

class First(WorkChain):
    @classmethod
    def define(cls, spec):
        super().define(spec)
        spec.input('a', default=lambda: orm.Int(1))
        spec.input('b')


class Second(WorkChain):
    @classmethod
    def define(cls, spec):
        super().define(spec)
        spec.input('a', default=lambda: orm.Int(2))
        spec.input('c')

Inspect builder behavior:

In [1]: from test_proc import First, Second                                                                                                                                                                                           

In [2]: builder = First.get_builder() 

In [3]: Second.get_builder() # We simply construct this builder, and do nothing with it.                                                                                                           

In [5]: builder.a() # The default for 'a' has changed to the lambda defined in `Second`.
Out[5]: <Int: uuid: fb6a1556-9f34-4d32-b7ea-cc2800ee00ef (unstored) value: 2> 

In [6]: builder.c # This should raise, since `First` doesn't have a 'c' input.

EDIT: When actually running the workchain, the default is populated correctly, and validation is also correct. I guess this slightly lowers the severity, but depending on how the user manipulates the builder it could still lead to incorrect inputs.

Your environment

• Operating system: Ubuntu-18.04 on WSL2
• Python version: 3.7.3
• aiida-core version: develop branch

Additional context

This is a follow-up from a discussion from #4419

[greschd]

@sphuber one option to resolve this would be just implementing a __getattr__ instead of the dynamically-created properties. But I think this would break auto-completion - am I right in thinking this was the reason to go with the properties in the first place?