#19 libextl: document EXTL_SAFE/'restricted mode'

[Arnout Engelen]

When executing lua code in 'restricted mode' calling C API functions not annotated with EXTL_SAFE produces a 'Attempt to call an unsafe function "...." in restricted mode' warning.

We should document when lua code runs in 'restricted mode' and when it is acceptable to annotate a C function with EXTL_SAFE.

[Arnout Engelen]

ioncore_current seems like a good candidate for EXTL_SAFE, but I'm reluctant to add it without properly understanding what the issue is here.

[Philipp Hartwig]

I think a function has to be considered unsafe when it can alter a list that any of our foo_i functions could iterate over. For example when calling mplex:managed_i(fn), the function fn must not alter the list of managed regions of mplex because it could thereby violate assumptions that the code of WMPlex.managed_i relies on.

[Philipp Hartwig]

I've added some documentation in libextl/3387b2203cbb25818958a4b14c39cdc444dd325e and notion-doc/180422475c5f8cd64cbb3e804024d2dabd97976f

Any comments or suggestions? :)