Describe the bug

plumpy uses the contextvars module to enable access to the process itself (via Process.current()) at any point in time during a task run by the process.

https://github.com/aiidateam/plumpy/blob/33f4e9f029e0573f168237b8db030de537453878/plumpy/processes.py#L516-L534

When used in combination with asyncio's call_soon, call_later or call_at methods, each individual function execution actually gets their own copy of this context.
This means that as long as a handle to these scheduled executions remains in memory, the copy of the 'process stack' context var (and thus the process itself) remain in memory [1].

One example of this happening is the Transport future opened by aiida-core for file transfers - it is passed around to several classes inside aiida-core and even outside (e.g. paramiko).

The problem is: if any of these places forgets to shed the reference to this handle, not only does the transport itself remain in memory but also the Process that was responsible for creating it (and everything it references).

One can work around this issue by explicitly passing an empty context to call_soon, call_later or call_at where the context is not needed, but I would like to point out that this behaviour may be unexpected to users of plumpy and can be very difficult to debug.

Your environment

• aiida-core Current develop

@muhrin

[1] Not sure whether memory consumption during execution should be a concern as well - if it's a shallow copy, it should be ok.