WorkerController shouldnt block on get_result after cancel

Summary:
Right now, calling WorkerController.get_result after WorkerController.cancel
will block. Thus it is a leaky abstraction. So please please don't do that.

Thankfully, MultiThreadedCall doesn't do that.

This just cleans up this leaky abstraction (which makes the code easier
to read, for those trying to understand workerController.ml and worker.ml).

(In particular, while I was working on it, I found it very surprising that
cancellation worked at all without blocking).

Reviewed By: dabek

Differential Revision: D7537683

fbshipit-source-id: c9425d84fa5393e2a4ade04a10e70a7e26450d79

hphp/hack/src/procs/workerController.ml

@@ -145,6 +145,7 @@ and ('a, 'b) delayed = 'a * 'b worker_handle
 and 'b worker_handle =
   | Processing of 'b slave
   | Cached of 'b
+  | Canceled
   | Failed of exn
 
 (** The Controller's slave has a Worker. The Worker is itself a single process
@@ -153,6 +154,7 @@ and 'b worker_handle =
 and 'a slave = {
 
   worker: worker;      (* The associated worker *)
+
   slave_pid: int; (* The actual slave pid *)
 
   (* The file descriptor we might pass to select in order to
@@ -405,6 +407,7 @@ let get_result d =
   match snd !d with
   | Cached x -> x
   | Failed exn -> raise exn
+  | Canceled -> raise End_of_file
   | Processing s ->
     with_worker_exn d s begin fun () ->
       let res = s.result () in
@@ -443,7 +446,7 @@ let select ds additional_fds =
   List.fold_right
     ~f:(fun d acc ->
       match snd !d with
-      | Cached _ | Failed _ ->
+      | Cached _ | Canceled | Failed _ ->
           { acc with readys = d :: acc.readys }
       | Processing s when List.mem ready_fds s.infd ->
           { acc with readys = d :: acc.readys }
@@ -456,6 +459,7 @@ let get_worker h =
   match snd !h with
   | Processing {worker; _} -> worker
   | Cached _
+  | Canceled
   | Failed _ -> invalid_arg "Worker.get_worker"
 
 let get_job h = fst !h
@@ -478,7 +482,8 @@ let wait_for_cancel d =
   | Processing s ->
     with_worker_exn d s begin fun () ->
       s.wait_for_cancel ();
-      s.worker.busy <- false
+      s.worker.busy <- false;
+      d := fst !d, Canceled
     end
   | _ -> ()