spin: stash stack pointer on the parent road, restore from it#966
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Resolves #965
Currently, when new road is entered, the state of the spin stack is saved on that road, and it is restored in
u3m_bailbefore longjump. However, u3m_bail is not the only way to leave a road with non-local control flow, we can also jump back all the way to the home road on a signal, e.g. SIGINT.This caused the spin stack offset to be set to zero (because apparently it was the initial value in the home road), which broke logic in
_http_spin_timer_cb, which assumes that there is always an offset of at least 4 (likely comes fromc3__rootinitialization).This PR fixes the road entry/exit logic by stashing the stack info on the road that we are about to leave, and restoring it on road promotion. That way the home road values are set on leap from the home road, so
_cm_signal_recovercorrectly restores the state of the spin stack on ^C.