The In_thread module has functions for interaction between the Async world and other (kernel) threads. The name is to remind us to think about threads and race conditions.

All threads come from the one thread pool used for all Async-managed threads.

module Priority : module type of Core.Std.Linux_ext.Priority with type t = Core.Std.Linux_ext.Priority.t
module Helper_thread : sig .. end
type t
A Helper_thread is a thread that is dedicated to handling computations external to Async. We need them because some libraries (e.g. Sqlite3) require that certain collections of computations run in the same thread.
val create : ?priority:Priority.t -> ?name:string -> unit -> t Core.Std.Or_error.t
create ?name () creates a new helper thread. The name will be used as the thread name for any work that that is done by the thread that doesn't get its own name.

create uses a thread from Async's thread pool, reserving that thread for exclusive use by the helper thread until the helper thread is no longer used (specifically, finalized and is finished with all its work), at which point the thread is made available for general use by the pool.

val pipe_of_squeue : 'a Core.Std.Squeue.t -> 'a Async_kernel.Pipe.Reader.t
pipe_of_squeue squeue returns a pipe p and consumes the contents squeue, placing them in p. It repeatedly grabs everything from squeue, places it in p, and then waits for pushback on p.
val run : ?priority:Priority.t ->
?thread:Helper_thread.t ->
?when_finished:[ `Best | `Notify_the_scheduler | `Take_the_async_lock ] ->
?name:string -> (unit -> 'a) -> 'a Async_kernel.Deferred.t
run ?priority ?thread ?name f runs f () in another thread and returns the result as a Deferred in the Async world. If f () raises an exception (asynchronously, since it is another thread) then that exception will be raised to the monitor that called run.

Async code should not be used from within f.

If thread is not supplied, then any thread from the thread pool could be used. If you need to run routines in a specific thread (as is required by some libraries like Sqlite), you should create a helper thread and supply it to run.

If priority is supplied, the priority of the thread in the linux scheduler will be set to priority for the duration of f (), provided the thread is allowed to do so, see `man setpriority`.

If you call run several times with the same helper thread, the f () calls will run in sequence, in the order in which they are supplied to run. Each f () will complete (return or raise) before another f () starts.

For example, if you do:

      let () =
        run ~thread f1;
        run ~thread f2;
        run ~thread f3;

Then the thread will run f1 () to completion, then f2 () to completion, then f3 () to completion.

If name is supplied, the name of the thread will be set to it for the duration of the execution of f ().

when_finished describes how the helper thread behaves once f () has completed:

  • with `Take_the_lock it takes the Async lock and runs a cycle immediately
  • with `Notify_the_scheduler it just notifies the scheduler that the result is ready
  • with `Best it tries to take the lock and run a cycle, but will fallback to `Notify_the_scheduler method if the Async lock is already held by someone else. The default is `Best, and one shouldn't need to change it -- it is useful only for unit testing.
val syscall : name:string ->
(unit -> 'a) -> ('a, exn) Core.Std.Result.t Async_kernel.Deferred.t
syscall f runs f, which should be a single system call, and returns the result, handling the restarting of interrupted system calls. To avoid race conditions, the f supplied to syscall should just make a system call. That way, everything else is done holding the Async lock.
val syscall_exn : name:string -> (unit -> 'a) -> 'a Async_kernel.Deferred.t