Make Eio.Promise lock-free

This is slightly faster and simplifies the code.

lib_eio/core/promise.ml

@@ -1,41 +1,10 @@
-(* Note on thread-safety
-
-   Promises can be shared between domains, so everything here must be thread-safe.
-
-   Wrapping everything in a mutex would be one way to do that, but that makes reads
-   slow, and only one domain would be able to read at a time.
-
-   Instead, we use an Atomic to hold the state, plus an additional mutex for the waiters
-   while in the Unresolved state. This makes resolved promises faster (at the cost of
-   making operations on unresolved promises a bit slower). It also makes reasoning about
-   the code more fun.
-
-   We can think of atomics and mutexes as "boxes", containing values and
-   invariants. To use them, you open the box to get access to the contents,
-   then close the box afterwards, restoring the invariant. For mutexes,
-   open/close is lock/unlock. For atomics, every operation implicitly opens and
-   closes the box. Any number of callers can share a reference to the box
-   itself; the runtime ensures a box can only be opened by one user at a time.
-
-   We can hold a full reference to something (meaning no-one else has access to it
-   and we can mutate it), or a fraction (giving us read-only access but also
-   ensuring that no-one else can mutate it either). *)
-
 type 'a state =
   | Resolved of 'a
-  | Unresolved of 'a Waiters.t * Mutex.t
-  (* The Unresolved state's mutex box contains:
-     - Full access to the Waiters.
-     - Half access to the promise's state.
-     - The invariant that if the promise is resolved then the waiters list is empty. *)
+  | Unresolved of Broadcast.t
 
 type !'a promise = {
   id : Ctf.id;
-
-  state : 'a state Atomic.t;
-  (* This atomic box contains either:
-     - A non-zero share of the reference to the Resolved state.
-     - A half-share of the reference to the Unresolved state. *)
+  state : 'a state Atomic.t;    (* Note: we always switch to Resolved before broadcasting *)
 }
 
 type +!'a t
@@ -51,7 +20,7 @@ let of_public_resolver : 'a u -> 'a promise = Obj.magic
 let create_with_id id =
   let t = {
     id;
-    state = Atomic.make (Unresolved (Waiters.create (), Mutex.create ()));
+    state = Atomic.make (Unresolved (Broadcast.create ()));
   } in
   to_public_promise t, to_public_resolver t
 
@@ -68,33 +37,33 @@ let create_resolved x =
 let await t =
   let t = of_public_promise t in
   match Atomic.get t.state with
-  (* If the atomic is resolved, we take a share of that reference and return
-     the remainder to the atomic (which will still be non-zero). We can then
-     continue to know that the promise is resolved after the [Atomic.get]. *)
   | Resolved x ->
     Ctf.note_read t.id;
     x
-  | Unresolved (q, mutex) ->
-    (* We discovered that the promise was unresolved, but we can't be sure it still is,
-       since we had to return the half-share reference to the atomic. So the [get] is
-       just to get access to the mutex. *)
-    Ctf.note_try_read t.id;
-    Mutex.lock mutex;
-    (* Having opened the mutex, we have:
-       - Access to the waiters.
-       - Half access to the promise's state (so we know it can't change until we close the mutex).
-       - The mutex invariant. *)
+  | Unresolved b ->
+    Suspend.enter (fun ctx enqueue ->
+        match Broadcast.suspend b (fun () -> enqueue (Ok ())) with
+        | None -> ()  (* We got resumed immediately *)
+        | Some request ->
+          match Atomic.get t.state with
+          | Resolved _ ->
+            (* The promise was resolved as we were suspending.
+               Resume now if we haven't already done so. *)
+            if Broadcast.cancel request then enqueue (Ok ())
+          | Unresolved _ ->
+            (* We observed the promise to be still unresolved after registering a waiter.
+               Therefore any resolution must happen after we were registered and we will be notified. *)
+            Ctf.note_try_read t.id;
+            Cancel.Fiber_context.set_cancel_fn ctx (fun ex ->
+                if Broadcast.cancel request then enqueue (Error ex)
+                (* else already resumed *)
+              )
+      );
     match Atomic.get t.state with
-    | Unresolved _ ->
-      (* The promise is unresolved, and can't change while we hold the mutex.
-         It's therefore safe to add a new waiter (and let [Waiters.await] close the mutex). *)
-      Waiters.await ~mutex:(Some mutex) q t.id
-    (* Otherwise, the promise was resolved by the time we took the lock.
-       Release the lock (which is fine, as we didn't change anything). *)
     | Resolved x ->
-      Mutex.unlock mutex;
       Ctf.note_read t.id;
       x
+    | Unresolved _ -> assert false
 
 let await_exn t =
   match await t with
@@ -105,30 +74,12 @@ let resolve t v =
   let rec resolve' t v =
     match Atomic.get t.state with
     | Resolved _ -> invalid_arg "Can't resolve already-resolved promise"
-    | Unresolved (q, mutex) as prev ->
-      (* The above [get] just gets us access to the mutex;
-         By the time we get here, the promise may have become resolved. *)
-      Mutex.lock mutex;
-      (* Having opened the mutex, we have:
-         - Access to the waiters.
-         - Half access to the promise's state (so we know it can't change until we close the mutex).
-         - The mutex invariant.
-         Now we open the atomic again, getting the other half access. Together,
-         this gives us full access to the state (i.e. no-one else can be using
-         it), allowing us to change it.
-         Note: we don't actually need an atomic CAS here, just a get and a set
-         would do, but this seems simplest. *)
+    | Unresolved b as prev ->
       if Atomic.compare_and_set t.state prev (Resolved v) then (
-        (* The atomic now has half-access to the fullfilled state (which counts
-           as non-zero), and we have the other half. Now we need to restore the
-           mutex invariant by clearing the wakers. *)
         Ctf.note_resolved t.id ~ex:None;
-        Waiters.wake_all q v;
-        Mutex.unlock mutex
+        Broadcast.resume_all b
       ) else (
-        (* Otherwise, the promise was already resolved when we opened the mutex.
-           Close it without any changes and retry. *)
-        Mutex.unlock mutex;
+        (* Otherwise, the promise was already resolved. Retry (to get the error). *)
         resolve' t v
       )
   in