refactor: Rely on the Instance to notify puppeteers

Previously, we were using a Registry with duplicated keys, but they
are not very error-friendly. In particular, the documentation states:

> For example, if a process crashes, its keys are automatically
> removed from the registry but the change may not propagate
> immediately.

In lkn, we rely on keys rather than PID to identify and reach
processes. The Instance already remembers its registered Puppeteers
key. As a consequence, we can rely on the Instance to dispatch
notification, we do not need the Registry.

3 files changed, 11 insertions, 20 deletions

diff --git a/lib/lkn/core.ex b/lib/lkn/core.ex
index 2719c4a..0033ecd 100644
--- a/lib/lkn/core.ex
+++ b/lib/lkn/core.ex
@@ -52,11 +52,6 @@ defmodule Lkn.Core do
       {:via, Registry, {Lkn.Core.Registry, {:engine, instance_key, sys}}}
     end
 
-    @spec notify_group(Instance.k) :: t
-    def notify_group(instance_key) do
-      {:engine, instance_key, :notifier}
-    end
-
     @spec instance(Instance.k) :: t
     def instance(instance_key) do
       {:via, Registry, {Lkn.Core.Registry, {:instance, instance_key}}}
@@ -94,7 +89,6 @@ defmodule Lkn.Core do
   def init(_) do
     children = [
       supervisor(Registry, [:unique, Lkn.Core.Registry], id: :core_registry),
-      supervisor(Registry, [:duplicate, Lkn.Core.Notifier], id: :core_notifier),
       supervisor(Lkn.Core.Pool.Supervisor, [])
     ]
 
diff --git a/lib/lkn/core/instance.ex b/lib/lkn/core/instance.ex
index 40d5f9a..6174998 100644
--- a/lib/lkn/core/instance.ex
+++ b/lib/lkn/core/instance.ex
@@ -274,8 +274,6 @@ defmodule Lkn.Core.Instance do
   @spec unregister_puppeteer(k, Puppeteer.k) :: :ok
   def unregister_puppeteer(instance_key, puppeteer_key) do
     GenServer.cast(Name.instance(instance_key), {:unregister_puppeteer, puppeteer_key})
-
-    Registry.unregister(Lkn.Core.Notifier, Name.notify_group(instance_key))
   end
 
   def handle_call(:lock, _from, state) do
@@ -284,7 +282,7 @@ defmodule Lkn.Core.Instance do
   def handle_call({:register_puppeteer, puppeteer_key, puppeteer_module}, _from, state) do
     if !State.closed?(state) do
       # we remember this new puppeteer
-      s2 = State.register_puppeteer(state, puppeteer_key, puppeteer_module)
+      state = State.register_puppeteer(state, puppeteer_key, puppeteer_module)
 
       # we compute a digest of the map and each puppets
       map = Lkn.Core.Entity.digest(state.map_key)
@@ -299,7 +297,7 @@ defmodule Lkn.Core.Instance do
         puppets
       )
 
-      {:reply, true, s2}
+      {:reply, true, state}
     else
       {:reply, false, state}
     end
@@ -380,12 +378,15 @@ defmodule Lkn.Core.Instance do
   def handle_cast({:unregister_puppeteer, puppeteer_key}, state) do
     {:noreply, State.unregister_puppeteer(state, puppeteer_key)}
   end
+  def handle_cast({:notify_group, notif}, state) do
+    Enum.map(state.puppeteers, fn {key, _} ->
+      notif.(key)
+    end)
+
+    {:noreply, state}
+  end
 
   def notify_puppeteers(instance_key, notif) do
-    Registry.dispatch(Lkn.Core.Notifier, Name.notify_group(instance_key), fn entries ->
-      for {_, key} <- entries do
-        notif.(key)
-      end
-    end)
+    GenServer.cast(Name.instance(instance_key), {:notify_group, notif})
   end
 end
diff --git a/lib/lkn/core/pool.ex b/lib/lkn/core/pool.ex
index aabef1d..686f18a 100644
--- a/lib/lkn/core/pool.ex
+++ b/lib/lkn/core/pool.ex
@@ -176,11 +176,7 @@ defmodule Lkn.Core.Pool do
   """
   @spec register_puppeteer(Lkn.Core.Map.k, Lkn.Core.Puppeteer.k, Lkn.Core.Puppeteer.m) :: Lkn.Core.Instance.k
   def register_puppeteer(map_key, puppeteer_key, puppeteer_module) do
-    instance_key = GenServer.call(Name.pool(map_key), {:register, puppeteer_key, puppeteer_module})
-
-    Registry.register(Lkn.Core.Notifier, Name.notify_group(instance_key), puppeteer_key)
-
-    instance_key
+    GenServer.call(Name.pool(map_key), {:register, puppeteer_key, puppeteer_module})
   end
 
   @doc """