From 9b6a5f7377b43d76167a5e00d6d007de0890047b Mon Sep 17 00:00:00 2001
From: Buck Doyle <b@chromatin.ca>
Date: Tue, 20 Jan 2026 20:43:55 -0600
Subject: [PATCH 1/2] Add rate-limiter mail delivery strategy

---
 config/prod.exs                             |  4 +
 lib/prison_rideshare/application.ex         |  1 +
 lib/prison_rideshare/mailer_rate_limiter.ex | 82 +++++++++++++++++++++
 3 files changed, 87 insertions(+)
 create mode 100644 lib/prison_rideshare/mailer_rate_limiter.ex

diff --git a/config/prod.exs b/config/prod.exs
index ca15fad..cca10d4 100644
--- a/config/prod.exs
+++ b/config/prod.exs
@@ -15,4 +15,8 @@ config :prison_rideshare, PrisonRideshare.Repo,
   pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
   ssl: true
 
+config :prison_rideshare, PrisonRideshare.Mailer,
+  deliver_later_strategy: PrisonRideshare.MailerRateLimiter,
+  rate_limit_ms: String.to_integer(System.get_env("MAILER_RATE_LIMIT_MS") || "30000")
+
 config :prison_rideshare, gas_price_endpoint: System.get_env("GAS_PRICE_ENDPOINT")
diff --git a/lib/prison_rideshare/application.ex b/lib/prison_rideshare/application.ex
index 2438a96..cbb96b8 100644
--- a/lib/prison_rideshare/application.ex
+++ b/lib/prison_rideshare/application.ex
@@ -10,6 +10,7 @@ defmodule PrisonRideshare.Application do
       {Phoenix.PubSub, name: PrisonRideshare.PubSub},
       # Start the Ecto repository
       PrisonRideshare.Repo,
+      PrisonRideshare.MailerRateLimiter,
       # Start the endpoint when the application starts
       PrisonRideshareWeb.Endpoint,
       # Start your own worker by calling: PrisonRideshare.Worker.start_link(arg1, arg2, arg3)
diff --git a/lib/prison_rideshare/mailer_rate_limiter.ex b/lib/prison_rideshare/mailer_rate_limiter.ex
new file mode 100644
index 0000000..ca9276c
--- /dev/null
+++ b/lib/prison_rideshare/mailer_rate_limiter.ex
@@ -0,0 +1,82 @@
+defmodule PrisonRideshare.MailerRateLimiter do
+  @behaviour Bamboo.DeliverLaterStrategy
+  use GenServer
+
+  require Logger
+
+  @default_interval_ms 30000
+
+  def start_link(opts \\ []) do
+    GenServer.start_link(__MODULE__, opts, name: __MODULE__)
+  end
+
+  @impl Bamboo.DeliverLaterStrategy
+  def deliver_later(adapter, email, config) do
+    GenServer.cast(__MODULE__, {:enqueue, adapter, email, config})
+  end
+
+  @impl GenServer
+  def init(_opts) do
+    {:ok,
+     %{
+       queue: :queue.new(),
+       interval_ms: delivery_interval_ms(),
+       in_flight: false
+     }}
+  end
+
+  @impl GenServer
+  def handle_cast({:enqueue, adapter, email, config}, state) do
+    state = %{state | queue: :queue.in({adapter, email, config}, state.queue)}
+    {:noreply, maybe_schedule(state)}
+  end
+
+  @impl GenServer
+  def handle_info(:deliver_next, state) do
+    case :queue.out(state.queue) do
+      {{:value, {adapter, email, config}}, queue} ->
+        deliver(adapter, email, config)
+        state = %{state | queue: queue}
+
+        if :queue.is_empty(queue) do
+          {:noreply, %{state | in_flight: false}}
+        else
+          Process.send_after(self(), :deliver_next, state.interval_ms)
+          {:noreply, state}
+        end
+
+      {:empty, _queue} ->
+        {:noreply, %{state | in_flight: false}}
+    end
+  end
+
+  defp maybe_schedule(%{in_flight: true} = state), do: state
+
+  defp maybe_schedule(state) do
+    send(self(), :deliver_next)
+    %{state | in_flight: true}
+  end
+
+  defp deliver(adapter, email, config) do
+    try do
+      case adapter.deliver(email, config) do
+        {:error, error} ->
+          Logger.error("Email delivery failed: #{inspect(error)}")
+
+        _ ->
+          :ok
+      end
+    rescue
+      exception ->
+        Logger.error(Exception.format(:error, exception, __STACKTRACE__))
+    catch
+      kind, reason ->
+        Logger.error(Exception.format(kind, reason, __STACKTRACE__))
+    end
+  end
+
+  defp delivery_interval_ms do
+    config = Application.get_env(:prison_rideshare, PrisonRideshare.Mailer, [])
+    config[:rate_limit_ms] || @default_interval_ms
+  end
+end

From e0e63473d4a07368cad97780e9561784125877cf Mon Sep 17 00:00:00 2001
From: Buck Doyle <b@chromatin.ca>
Date: Sat, 21 Mar 2026 09:39:31 -0700
Subject: [PATCH 2/2] Add logging about queueing

---
 lib/prison_rideshare/mailer_rate_limiter.ex | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/prison_rideshare/mailer_rate_limiter.ex b/lib/prison_rideshare/mailer_rate_limiter.ex
index ca9276c..a9010f7 100644
--- a/lib/prison_rideshare/mailer_rate_limiter.ex
+++ b/lib/prison_rideshare/mailer_rate_limiter.ex
@@ -28,6 +28,8 @@ defmodule PrisonRideshare.MailerRateLimiter do
   @impl GenServer
   def handle_cast({:enqueue, adapter, email, config}, state) do
     state = %{state | queue: :queue.in({adapter, email, config}, state.queue)}
+    queue_size = :queue.len(state.queue)
+    Logger.info("Mail queued: to=#{inspect(email.to)} subject=#{inspect(email.subject)} queue_size=#{queue_size}")
     {:noreply, maybe_schedule(state)}
   end
 
@@ -35,12 +37,16 @@ defmodule PrisonRideshare.MailerRateLimiter do
   def handle_info(:deliver_next, state) do
     case :queue.out(state.queue) do
       {{:value, {adapter, email, config}}, queue} ->
+        remaining = :queue.len(queue)
+        Logger.info("Mail sending: to=#{inspect(email.to)} subject=#{inspect(email.subject)} remaining=#{remaining}")
         deliver(adapter, email, config)
         state = %{state | queue: queue}
 
         if :queue.is_empty(queue) do
+          Logger.info("Mail queue drained")
           {:noreply, %{state | in_flight: false}}
         else
+          Logger.info("Mail queue scheduling next delivery in #{state.interval_ms}ms remaining=#{remaining}")
           Process.send_after(self(), :deliver_next, state.interval_ms)
           {:noreply, state}
         end
@@ -64,7 +70,7 @@ defmodule PrisonRideshare.MailerRateLimiter do
           Logger.error("Email delivery failed: #{inspect(error)}")
 
         _ ->
-          :ok
+          Logger.info("Mail sent: to=#{inspect(email.to)} subject=#{inspect(email.subject)}")
       end
     rescue
       exception ->