Only kill children in process group at shutdown

[minrk]

• not the whole process group (killpg), which includes kernel itself and possibly parents (closes ipyparallel downstream test broken #872)
• not children which have started their own process groups, which is the standard indicator for "leave me alone when my parent shuts down"

closes #873

[minrk]

One question here: this cleanup logic depends on psutil, but I haven't made psutil a hard dependency. So we have a choice:

1. make this cleanup an optional enhancement when psutil is available, or
2. bump psutil to a hard dependency so this always runs

[Carreau]

Previous PR made is a dependency on windows, so I think we can make it a hard dep.

[minrk]

Now requires psutil, removing conditions for handling its absence

[Carreau]

pushed 5c16fde to not make psutil necessary to build the wheel and changing what is imported where.

@echarles Is that ok with you ?

[echarles]

@echarles Is that ok with you ?

Moving _is_debugpy_available to debugger.py makes sense (related to debugpy dependency). Looking more to the psutil dependency, main branch only installs psutil for windows.

ipykernel/setup.py

Line 71 in 78c83ad

'psutil;platform_system=="Windows"',

which is needed on all platform to honor the usage_request

ipykernel/ipykernel/kernelbase.py

Lines 896 to 901 in 78c83ad

	async def usage_request(self, stream, ident, parent):
	reply_content = {
	'hostname': socket.gethostname()
	}
	if psutil is None:
	return reply_content

Any particular reason to restrict psutil to only windows?

[Carreau]

Any particular reason to restrict psutil to only windows?

Well originally it was because killpg is not avail on windows, I guess it was not added as a dep when usage_request was added, so it's an argument to make it a hard dependency.

[Carreau]

Ok, It seem we are all in agreement, merging.