Are the functions `coroutine.close()` and `lua_resetthread()` provided
only to allow to-be-closed variables to be closed immediately (as
opposed to when the threads are garbage collected)?
Yes.
I ask because when looking at the sources, it is not clear to me that
those functions are called during garbage collection.
They are not.
Supposed I declared a to-be-closed variable in a coroutine, yielded
the coroutine, but never intended to resume it. Would the to-be-closed
variable in the coroutine ever be closed without explicitly calling
`coroutine.close()`?
No. The manual is explicit about that:
If a coroutine yields and is never resumed again, some variables
may never go out of scope, and therefore they will never be closed.
(These variables are the ones created inside the coroutine and in
scope at the point where the coroutine yielded.) Similarly, if a
coroutine ends with an error, it does not unwind its stack, so it does
not close any variable. In both cases, you can either use finalizers
or call @Lid{coroutine.close} to close the variables. However, if
the coroutine was created through @Lid{coroutine.wrap}, then its
corresponding function will close the coroutine in case of errors.
