fix(cache): add periodic TTL sweeper to bound ETS growth

lib/microwaveprop/cache.ex

@@ -11,6 +11,7 @@ defmodule Microwaveprop.Cache do
   use GenServer
 
   @table :microwaveprop_cache
+  @sweep_interval_ms to_timeout(minute: 1)
 
   @spec start_link(keyword()) :: GenServer.on_start()
   def start_link(opts) do
@@ -56,6 +57,25 @@ defmodule Microwaveprop.Cache do
     :ok
   end
 
+  @doc """
+  Delete every entry whose TTL has elapsed. Returns the number of rows
+  removed.
+
+  Because TTL enforcement in `fetch_or_store/3` is lazy, keys minted
+  dynamically (e.g. per-URL HRRR idx entries) would otherwise accumulate
+  forever. A supervised timer calls this on an interval; tests can call
+  it directly to avoid waiting on the timer.
+
+  Uses `:ets.select_delete/2` so the sweep happens in a single BIF call
+  without blocking concurrent readers or writers.
+  """
+  @spec sweep() :: non_neg_integer()
+  def sweep do
+    now = System.monotonic_time(:millisecond)
+    match_spec = [{{:_, :_, :"$1"}, [{:"=<", :"$1", now}], [true]}]
+    :ets.select_delete(@table, match_spec)
+  end
+
   @impl true
   def init(_opts) do
     _ =
@@ -67,6 +87,18 @@ defmodule Microwaveprop.Cache do
         write_concurrency: true
       ])
 
+    schedule_sweep()
     {:ok, %{}}
   end
+
+  @impl true
+  def handle_info(:sweep, state) do
+    _ = sweep()
+    schedule_sweep()
+    {:noreply, state}
+  end
+
+  defp schedule_sweep do
+    Process.send_after(self(), :sweep, @sweep_interval_ms)
+  end
 end

test/microwaveprop/cache_test.exs

@@ -43,4 +43,36 @@ defmodule Microwaveprop.CacheTest do
       assert Cache.fetch_or_store(:k, 1_000, fn -> "fresh" end) == "fresh"
     end
   end
+
+  describe "sweep/0" do
+    test "removes expired entries" do
+      Cache.put(:expired, "old", 1)
+      Process.sleep(5)
+
+      assert Cache.sweep() >= 1
+      assert :ets.lookup(:microwaveprop_cache, :expired) == []
+    end
+
+    test "leaves unexpired entries in place" do
+      Cache.put(:fresh, "new", 60_000)
+
+      _ = Cache.sweep()
+
+      assert [{:fresh, "new", _}] = :ets.lookup(:microwaveprop_cache, :fresh)
+    end
+
+    test "handles a mix of expired and unexpired entries" do
+      Cache.put(:expired_a, 1, 1)
+      Cache.put(:expired_b, 2, 1)
+      Cache.put(:fresh, 3, 60_000)
+      Process.sleep(5)
+
+      deleted = Cache.sweep()
+      assert deleted >= 2
+
+      assert :ets.lookup(:microwaveprop_cache, :expired_a) == []
+      assert :ets.lookup(:microwaveprop_cache, :expired_b) == []
+      assert [{:fresh, 3, _}] = :ets.lookup(:microwaveprop_cache, :fresh)
+    end
+  end
 end