Custom Queue(M) for Mailbox(M)

Mailboxes no longer inherit from `Channel(M)` but use a tailored
solution for agent mailboxes that's simpler and with less overall
allocations (thanks to Syn::Core).

README.md

@@ -168,14 +168,12 @@ properly reset themselves.
 
 #### Mailbox
 
-The `Earl::Mailbox(M)` module extends an agent with a `Channel(M)` along with
-methods to `#send(M)` a message to an agent and to receive them (concurrency
-safe).
+The `Earl::Mailbox(M)` module extends an agent with a queue of `M` messages
+along with methods to `#send(M)` a message to an agent and to receive them
+(concurrency safe).
 
-The module merely wraps a `Channel(M)` but proposes a standard structure for
-agents to have an incoming mailbox of messages. All agents thus behave the
-same, and we can assume that an agent that expects to receive messages has a
-`#send(M)` method.
+The goal is to provide a structured way for agents to communicate. No need to
+spawn and share channels, you directly send a message to an agent for example.
 
 An agent's mailbox will be closed when the agent is asked to stop. An agent can
 simply loop over `#receive?` until it returns `nil`, without having to check for
@@ -305,7 +303,8 @@ I.e. they include `Earl::Mailbox(M)` or `Earl::Artist(M)`. They must also
 override their `#reset` method to properly reset an agent.
 
 Note that `Earl::Pool` will replace the workers' mailbox. All workers then share
-a single `Channel(M)` for an exactly-once delivery of messages.
+a single mailbox for an exactly-once delivery of messages (only one worker will
+receive the message).
 
 For example:
 

SPEC.md

@@ -271,8 +271,8 @@ to check for `running?`.
 
 - `#mailbox=`
 
-  Direct accessor to swap the underlying `Channel(M)` object. The mailbox won't
-  be closed anymore when the agent is stopped, since the mailbox is now
+  Direct accessor to swap the underlying `Earl::Queue(M)` object. The mailbox
+  won't be closed anymore when the agent is stopped, since the mailbox is now
   considered to be shared.
 
   Despite having direct accessors to the mailbox, external agents aren't
@@ -522,7 +522,7 @@ to stop.
 
 If a pool is itself supervised by an [`Earl::Supervisor`](#earlsupervisor)
 agent, and the pool crashes, the supervisor will recycle and restart it, with
-the original channel kept open. Pending messages will be dispatched once the
+the original mailbox kept open. Pending messages will be dispatched once the
 pool workers are restarted.
 
 - `.new(capacity)`

src/logger/async_dispatcher.cr

@@ -4,7 +4,7 @@ module Earl
   module Logger
     # Alternative to `Log::AsyncDispatcher` as an `Earl::Agent` that can be
     # supervised.
-    #
+
     # :nodoc:
     class AsyncDispatcher
       include ::Log::Dispatcher
@@ -14,7 +14,7 @@ module Earl
       def initialize
         # never close the mailbox: logs sent while the program ends could raise
         # an exception because the mailbox is closed!
-        @mailbox_close_on_stop = false
+        @mailbox.close_on_stop = false
       end
 
       def finalize
@@ -39,9 +39,7 @@ module Earl
 
       def terminate : Nil
         # wait until all log messages have been processed before returning
-        queue = @mailbox.@queue.not_nil!
-
-        until queue.empty?
+        until @mailbox.empty?
           sleep(0.seconds)
         end
       end

src/mailbox.cr

@@ -1,4 +1,5 @@
 require "./agent"
+require "./queue"
 
 module Earl
   # Extends an agent with a mailbox that receives messages of type `M`.
@@ -7,37 +8,42 @@ module Earl
   # take messages from the mailbox —undocumented because they're `protected`.
   module Mailbox(M)
     macro included
-      @mailbox = Channel(M).new(10)
-      @mailbox_close_on_stop = true
+      @mailbox = Earl::Queue(M).new
     end
 
     # Replaces the mailbox. The mailbox won't be closed automatically when the
     # agent is asked to stop.
-    def mailbox=(@mailbox : Channel(M)) : Channel(M)
-      @mailbox_close_on_stop = false
+    def mailbox=(@mailbox : Queue(M)) : Queue(M)
+      @mailbox.close_on_stop = false
       @mailbox
     end
 
-    # Send a message to the `Agent`. Raises if the mailbox is closed.
+    # Sends a message to this `Agent`. Raises `ClosedError` if the mailbox is closed.
+    @[AlwaysInline]
     def send(message : M) : Nil
-      raise ClosedError.new if @mailbox.closed?
       @mailbox.send(message)
     end
 
-    # Takes a previously received message. Raises if the mailbox is closed.
+    # Takes a previously received message. Raises `ClosedError` if the mailbox is closed.
+    @[AlwaysInline]
     protected def receive : M
-      @mailbox.receive? || raise ClosedError.new
+      @mailbox.receive
     end
 
     # Takes a previously received message. Returns `nil` if the mailbox is closed.
+    @[AlwaysInline]
     protected def receive? : M?
       @mailbox.receive?
     end
 
     # :nodoc:
     def stop : Nil
-      @mailbox.close if @mailbox_close_on_stop
       super
+      @mailbox.close if @mailbox.close_on_stop?
     end
+
+    # protected def reset_mailbox : Nil
+    #   @mailbox = Queue(M).new(10)
+    # end
   end
 end

src/pool.cr

@@ -10,11 +10,11 @@ module Earl
   #
   # ### Workers
   #
-  # Crashed and unexpectedly stopped workers will be recycled and restarted,
-  # until the pool is asked to stop.
+  # Workers are permanent: crashed and unexpectedly stopped workers will be
+  # recycled and restarted until the pool itself is told to stop.
   #
   # Worker agents can return as soon as possible when asked to stop, or keep
-  # processing their mailbox until its empty.
+  # processing their mailbox until there is nothing left to do (your choice).
   #
   # ### Mailbox
   #

src/queue.cr

@@ -0,0 +1,93 @@
+require "syn/core/mutex"
+require "syn/core/condition_variable"
+require "./errors"
+
+module Earl
+  # Earl::Queue is a Channel-like object with a simpler implementation, that
+  # leverages Syn::Core structs, specifically targetted for Earl mailbox
+  # requirements.
+  #
+  # It's obviously very limited compared to Channel(T). It doesn't support
+  # `select`, only supports buffered queue (no sync channel with zero capacity),
+  # but those features aren't needed by Earl mailboxes.
+
+  # :nodoc:
+  class Queue(M)
+    property? close_on_stop : Bool = true
+    property? closed : Bool = false
+
+    def initialize(@backlog = 128)
+      {% if M == Nil %}
+        {% raise "Can't create an Earl::Mailbox(M) where M is Nil (or nilable)" %}
+      {% elsif M.union? && M.union_types.any? { |m| m == Nil } %}
+        {% raise "Can't create an Earl::Mailbox(M) where M is nilable" %}
+      {% end %}
+
+      @deque = Deque(M).new(@backlog)
+      @mutex = Syn::Core::Mutex.new
+      @readers = Syn::Core::ConditionVariable.new
+      @writers = Syn::Core::ConditionVariable.new
+    end
+
+    def send(message : M) : Nil
+      @mutex.synchronize do
+        raise ClosedError.new if @closed
+
+        until @deque.size < @backlog
+          @writers.wait(pointerof(@mutex))
+          raise ClosedError.new if @closed
+        end
+
+        @deque.push(message)
+        @readers.signal
+      end
+    end
+
+    @[AlwaysInline]
+    def receive : M
+      do_receive { raise ClosedError.new }
+    end
+
+    @[AlwaysInline]
+    def receive? : M?
+      do_receive { return nil }
+    end
+
+    private def do_receive(&) : M
+      @mutex.synchronize do
+        loop do
+          if message = @deque.shift?
+            @writers.signal
+            return message
+          end
+
+          yield if @closed
+
+          @readers.wait(pointerof(@mutex))
+        end
+      end
+    end
+
+    def empty? : Bool
+      @mutex.synchronize { @deque.empty? }
+    end
+
+    # def lazy_empty? : Bool
+    #   @deque.empty?
+    # end
+
+    def close : Nil
+      return if @closed
+
+      @mutex.synchronize do
+        @closed = true
+        @readers.broadcast
+        @writers.broadcast
+      end
+    end
+
+    # def reset : Nil
+    #   @closed = false
+    # end
+  end
+end