The Thread_safe module has functions that are safe to call from threads outside Async.

All the Thread_safe.block* and Thread_safe.run* functions wake up the Async scheduler to ensure that it continues in a timely manned with whatever jobs got started. Some functions take an optional ?wakeup_scheduler:bool argument, which defaults to true. One can cause the scheduler to not be woken up by supplying ~wakeup_scheduler:false, which can reduce CPU use, but increase latency, because the scheduler may not wake up for a while to process jobs.

val am_holding_async_lock : unit -> bool
am_holding_async_lock () returns true if the currently running thread is holding the Async lock.
val deferred : unit -> 'a Async_kernel.Deferred.t * ('a -> unit)
deferred () returns (d, fill) where d is a deferred that will become determined with value v once fill v is called.

It is ok to call deferred from inside or outside Async. fill must be called from outside Async.

val run_in_async_with_optional_cycle : ?wakeup_scheduler:bool ->
(unit -> [ `Do_not_run_a_cycle | `Run_a_cycle ] * 'a) ->
('a, exn) Core.Std.Result.t
run_in_async_with_optional_cycle f acquires the Async lock and runs f () while holding the lock. Depending on the result of f, it may also run a cycle.
val run_in_async : ?wakeup_scheduler:bool -> (unit -> 'a) -> ('a, exn) Core.Std.Result.t
run_in_async f acquires the Async lock and runs f () while holding the lock. It returns the result of f () to the outside world. The scheduler is woken up to ensures the code that depends on f () is run soon enough.

run_in_async doesn't run a cycle.

run_in_async does not automatically start the Async scheduler. You still need to call Scheduler.go elsewhere in your program.

val run_in_async_exn : ?wakeup_scheduler:bool -> (unit -> 'a) -> 'a
val block_on_async : (unit -> 'a Async_kernel.Deferred.t) -> ('a, exn) Core.Std.Result.t
block_on_async f runs f () in the Async world and blocks until the result becomes determined. This function can be called from the main thread (before Async is started) or from a thread outside Async.

block_on_async will run a cycle if the deferred isn't determined, in the hope that running the cycle will cause the deferred to become determined.

block_on_async will automatically start the scheduler if it isn't already running.

val block_on_async_exn : (unit -> 'a Async_kernel.Deferred.t) -> 'a
val run_in_async_wait : (unit -> 'a Async_kernel.Deferred.t) -> ('a, exn) Core.Std.Result.t
run_in_async_wait f is like block_on_async f, except that it must be called from a thread outside Async. Upon returning from run_in_async_wait, it is guaranteed that the caller does not have the Async lock.
val run_in_async_wait_exn : (unit -> 'a Async_kernel.Deferred.t) -> 'a