`struct.pack` can be None sometimes, spits Tracebacks when it is about execnet HOT 4 CLOSED

Original comment by @hpk42

If a global is None, it means that the interpreter is shutting down. Part of this procedure sets all module globals to None (to break reference cycles). I guess we can try to do pack = struct.unpack on the class level and then access it via self.pack or so. When @RonnyPfannschmidt refactored this code he probably didn't guard against this behaviour because there are no explicit tests that the core input/output works when setting globals to None.

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Original comment by @alfredodeza

It seems the more I try to fix this the more globals I find that need to be changed.

After making Message a class attribute I got len used in to_io to have issues there as well.

Would it be OK to catch TypeError and not complain in that final try/except block?

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Original comment by @hpk42

it would prevent the shutdown sequence to run but i guess we could go with it for the time being. In this case the "if Message is not None" is a better check than catching type error i think. And make a comment that it's because of interpreter shutdown that it can be None.

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Original comment by @hpk42

add changelog for an earlier change:

fix issue22 -- during interpreter shutdown don't throw
an exception when we can't send a termination sequence
anymore as we are about to die anyway.

→ <<cset 45113a98f6be>>

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