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WIP: Persist coroutines yielded from hook #30

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evg-zhabotinsky commented Jan 11, 2019

Inspired by wanting preemptive multitasking in OpenComputers. See my other repo for explanation of changes to debug.sethook() that let Lua hooks yield. After posting to OC forum, it turned out that Eris cannot persist coroutines yielded from hooks (and even has a special error message for them), so here's my attempt to fix that.

The main problem seems to be that normal yields never happen from Lua code, there's no opcode for that. At the same time, hook yields look like such opcode was the last one, there's no enclosing C call. I assumed that this case should be treated the same way as a Lua stack frame, since it is one, but thread->ci->func no longer points to the closure and has to be restorestack()ed. I've additionally stored the value of restored ci->func for unpersisting, since restorestack() relies on ci->extra, which will only be unpersisted after the loop. This breaks serialization format compatibility, but is fine for testing, and a workaround can be implemented later. Actually, that only happens when persisting hook-yielded coroutines, so there's nothing to break.

This PR is a WIP, intended to be cleaned up and squashed when it's ready. The code currently seems to work, and I've added a simple test for it. However, I suspect more thorough testing is required, and I have no idea what kind of tests to make. I'm not very familiar with Lua and/or Eris internals. Need help producing more tests!

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