Raise exception if `Process.play()` called on closed process

[chrisjsewell]

As of 7004bd9, a paused workchain will hang if it is closed then played. This manifested in an aiida-core test (aiidateam/aiida-core#4751)

Perhaps as a "safety measure" Process.play() should raise an exception if it is called on a closed process, and maybe other methods?

[sphuber]

Just want to mention here that so far it has been a common use case to rely on the behavior that playing a non-paused process should be simply a no-op and not raise. There are some bugs in aiida-core that are yet unsolved that can leave the state between node and actual process to be out of sync. Playing all processes can often coax them back into sync. If incorrect plays are going to start raising this might have undesired side effects here

[chrisjsewell]

hey thanks @sphuber can you review aiidateam/aiida-core#4751 quickly?

[chrisjsewell]

Just want to mention here that so far it has been a common use case to rely on the behavior that playing a non-paused process should be simply a no-op and not raise.

but are these non-paused processes already closed?

If incorrect plays are going to start raising this might have undesired side effects here

yep we could always make it no-op, although this may make it harder to understand why playing a process is not working