Harden public asset endpoint against invalid payloads

Closes #143

lib/music_library/assets/image.ex

@@ -22,14 +22,17 @@ defmodule MusicLibrary.Assets.Image do
       [:music_library, :assets, :image, :resize],
       %{},
       fn ->
-        {:ok, thumb} = Operation.thumbnail_buffer(cover_data, size)
-        result = Image.write_to_buffer(thumb, extension(format))
-        {result, %{}}
+        with {:ok, thumb} <- Operation.thumbnail_buffer(cover_data, size),
+             {:ok, _binary} = result <- Image.write_to_buffer(thumb, extension(format)) do
+          {result, %{}}
+        else
+          {:error, _reason} = error -> {error, %{}}
+        end
       end
     )
   end
 
-  @spec convert(binary(), String.t(), String.t()) :: {:ok, binary()}
+  @spec convert(binary(), String.t(), String.t()) :: {:ok, binary()} | {:error, term()}
   def convert(cover_data, data_format, target_format) do
     if data_format == target_format do
       {:ok, cover_data}
@@ -38,9 +41,12 @@ defmodule MusicLibrary.Assets.Image do
         [:music_library, :assets, :image, :convert],
         %{},
         fn ->
-          {:ok, image} = Image.new_from_buffer(cover_data)
-          result = Image.write_to_buffer(image, extension(target_format))
-          {result, %{}}
+          with {:ok, image} <- Image.new_from_buffer(cover_data),
+               {:ok, _binary} = result <- Image.write_to_buffer(image, extension(target_format)) do
+            {result, %{}}
+          else
+            {:error, _reason} = error -> {error, %{}}
+          end
         end
       )
     end

lib/music_library/assets/transform.ex

@@ -25,6 +25,30 @@ defmodule MusicLibrary.Assets.Transform do
     |> Base.url_encode64(padding: false)
   end
 
+  @doc """
+  Decodes a Base64-encoded JSON payload into a transform struct.
+
+  Returns `{:error, :invalid_payload}` if the payload is not valid Base64 or JSON.
+
+    iex> alias MusicLibrary.Assets.Transform
+    iex> payload = "eyJoYXNoIjoiYWJjMTIzIiwid2lkdGgiOjMwMH0"
+    iex> Transform.decode(payload)
+    {:ok, %Transform{hash: "abc123", width: 300}}
+
+    iex> alias MusicLibrary.Assets.Transform
+    iex> Transform.decode("!!!invalid")
+    {:error, :invalid_payload}
+  """
+  @spec decode(payload()) :: {:ok, t()} | {:error, :invalid_payload}
+  def decode(payload) do
+    with {:ok, decoded} <- Base.url_decode64(payload, padding: false),
+         {:ok, params} when is_map(params) <- JSON.decode(decoded) do
+      {:ok, struct!(__MODULE__, %{hash: params["hash"], width: params["width"]})}
+    else
+      _ -> {:error, :invalid_payload}
+    end
+  end
+
   @doc """
     iex> alias MusicLibrary.Assets.Transform
     iex> payload = "eyJoYXNoIjoiYWJjMTIzIiwid2lkdGgiOjMwMH0"