[core] Avoid blocking in Thread<Object> constructor (#12151)

* Introduce AspiringActor, EstablishedActor

This pair of objects represents the two-phase (parent-thread /
child-thread) construction that's needed to support constructing
Thread<Object> without blocking until the child thread is up and
running.

An `AspiringActor<O>` is responsible for:
 - ownership of the actor's `Mailbox`
 - allocating the memory for (but *not* constructing) the target object `O`

Using these two pieces--the mailbox and a stable address for `O`--an
`AspiringActor<O>` can accept messages for the target object, or provide
`ActorRef<O>`s that do so, before the object has actually been
constructed by the corresponding `EstablishedActor<O>`.  (Such messages
are queued in the mailbox until after the object is constructed.)

This allows for an `AspiringActor<O>` to be created and safely used by a
thread other than the one on which the target object will (eventually)
live.

An `EstablishedActor<O>` is responsible for managing the lifetime of the
target object `O` and the open/closed state of the parent's `mailbox`.

The `O` object's lifetime is contained by that of its owning
`EstablishedActor<O>`: the `EstablishedActor` constructor executes the
`O` constructor via "placement new", constructing it at the address
provided by the parent `AspiringActor`, and the `~EstablishedActor`
destructor similarly executes the `~O` destructor (after closing the
mailbox). `EstablishedActor` should therefore live entirely on the
thread intended to own `O`.

* Remove Actor#{invoke,ask}

1 files changed, 5 insertions, 0 deletions

diff --git a/include/mbgl/actor/scheduler.hpp b/include/mbgl/actor/scheduler.hpp
index d8a26ebeab..75ead29f0a 100644
--- a/include/mbgl/actor/scheduler.hpp
+++ b/include/mbgl/actor/scheduler.hpp
@@ -31,6 +31,11 @@ class Mailbox;
 class Scheduler {
 public:
     virtual ~Scheduler() = default;
+    
+    // Used by a Mailbox when it has a message in its queue to request that it
+    // be scheduled. Specifically, the scheduler is expected to asynchronously
+    // call `receive() on the given mailbox, provided it still exists at that
+    // time.
     virtual void schedule(std::weak_ptr<Mailbox>) = 0;
 
     // Set/Get the current Scheduler for this thread