Fibers
Fibers are the primitive of choice to represent concurrency. Every time you need
to increase the concurrency level, just spawn a fiber. Fibers are
cooperative and only
transfer control to other fibers in well-defined points (sync primitives, IO
functions and any suspending function such as this_fiber.yield()). These
points are also used by the interruption API.
No two fibers from the same Lua VM run in parallel (even when the underlying VM’s thread pool has threads available).
Functions
spawn(f: function) → fiber
Spawns a new fiber to run f. Post semantics are used, so the current fiber
(the one calling spawn()) continues to run until it reaches a suspension
point.
join(self: fiber)
Read pthread_join().
Returns the values returned by the fiber’s start function. If that fiber exits with an error, that error is re-raised here (and fiber is considered joined).
this_fiber.yield()
Reschedule current fiber to be executed in the next round so other ready fibers have a chance to run now. You usually don’t need to call this function as any suspending function already do that.
this_fiber.{forbid,allow}_suspend()
this_fiber.forbid_suspend()
this_fiber.allow_suspend()A call to forbid_suspend() will put the fiber in the state of
suspension-disallowed and any attempt to suspend the fiber while it is in this
state will raise an error.
forbid_suspend() may be called multiple times. A matching number of calls to
allow_suspend() will put the fiber out of the suspension-disallowed
state. You must not call allow_suspend() if there was no prior call to
forbid_suspend().
These functions aren’t generally useful and they would have no purpose in
preemptive multitasking. However a cooperative multitasking environment offers
opportunities to avoid some round-trips to sync primitives. These opportunities
shouldn’t really be used and the programmer should just rely on the classical
sync primitives. However I can’t tame every wild programmer out there so there
is this mechanism to at least document the code in mechanisms similar to
assert() statements from native languages.
They’re only useful if there are comprehensive test cases. Still, the use of these functions may make the code more readable. And some tools may be developed to understand these blocks and do some simple static analysis.