Stackless does not use the interpreter head lock.

Originally reported by: Anselm Kruis (Bitbucket: akruis, GitHub: akruis)

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Affected versions: all

Stackless accesses the interpreter->tstate_head list of thread states. This list is protected by the head-lock in pystate.c. Currently stackless ignores this lock. This can cause undefined behaviour, especially during interpreter shutdown. However I have not seen this in the wild.
Nevertheless I would like to fix this issue, because the fix is straight forward.

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• Bitbucket: https://bitbucket.org/stackless-dev/stackless/issue/120

Original comment by Anselm Kruis (Bitbucket: akruis, GitHub: akruis):

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I just created pull request #26. It contains the fix for 2.7-slp.

Original comment by Anselm Kruis (Bitbucket: akruis, GitHub: akruis):

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@krisvale reviewed the pull request #26 and approved it. Many thanks.

I merged the pull request manually into 2.7-slp and 3.x-slp, because it had conflicts after the fix for #117 landed. The commit ids are:

• 2.7-slp: 1fb75d5
• 3.3-slp: fc13eecdd0ad
• 3.4-slp: 153d2d0b2aad
• default-slp: 5bff39f535ff