Add endpoint to display an artist image

lib/music_library_web/controllers/artist_controller.ex

@@ -0,0 +1,41 @@
+defmodule MusicLibraryWeb.ArtistController do
+  use MusicLibraryWeb, :controller
+
+  alias MusicLibrary.Artists
+
+  # 1 year in seconds
+  @cache_duration 60 * 60 * 24 * 365
+
+  def image(conn, %{"musicbrainz_id" => artist_id}) do
+    case Artists.get_image(artist_id) do
+      nil ->
+        not_found(conn)
+
+      %{image_data: image_data, image_data_hash: etag} ->
+        case get_req_header(conn, "if-none-match") do
+          [^etag] -> extend_cache(conn)
+          _ -> respond_with_cache(conn, image_data, etag)
+        end
+    end
+  end
+
+  defp not_found(conn) do
+    conn
+    |> put_status(:not_found)
+    |> text("Not found")
+  end
+
+  defp extend_cache(conn) do
+    conn
+    |> put_resp_header("cache-control", "public, max-age=#{@cache_duration}")
+    |> send_resp(304, "")
+  end
+
+  defp respond_with_cache(conn, cover_data, etag) do
+    conn
+    |> put_resp_content_type("image/jpeg", "utf-8")
+    |> put_resp_header("cache-control", "public, max-age=#{@cache_duration}")
+    |> put_resp_header("etag", etag)
+    |> send_resp(200, cover_data)
+  end
+end