Support gzip compression of JS view assets (#1813)

lib/livebook/session.ex

@@ -285,6 +285,7 @@ defmodule Livebook.Session do
           unless non_empty_dir?(local_assets_path) do
             {:ok, archive_binary} = Runtime.read_file(runtime, archive_path)
             extract_archive!(archive_binary, local_assets_path)
+            gzip_files(local_assets_path)
           end
         end
 
@@ -2198,6 +2199,15 @@ defmodule Livebook.Session do
     end
   end
 
+  defp gzip_files(path) do
+    for path <- Path.wildcard(Path.join(path, "**")), File.regular?(path) do
+      with {:ok, content} <- File.read(path) do
+        compressed = :zlib.gzip(content)
+        File.write(path <> ".gz", compressed)
+      end
+    end
+  end
+
   defp registered_file_path(session_id, {:file, file_id}) do
     %{path: session_dir} = session_tmp_dir(session_id)
     Path.join([session_dir, "registered_files", file_id])

lib/livebook_web/controllers/session_controller.ex

@@ -117,16 +117,35 @@ defmodule LivebookWeb.SessionController do
 
     case lookup_asset(hash, asset_path) do
       {:ok, local_asset_path} ->
-        conn
-        |> put_content_type(asset_path)
-        |> cache_permanently()
-        |> send_file(200, local_asset_path)
+        conn =
+          conn
+          |> put_content_type(asset_path)
+          |> cache_permanently()
+
+        local_asset_path_gz = local_asset_path <> ".gz"
+
+        if accept_encoding?(conn, "gzip") and File.exists?(local_asset_path_gz) do
+          conn
+          |> put_resp_header("content-encoding", "gzip")
+          |> put_resp_header("vary", "Accept-Encoding")
+          |> send_file(200, local_asset_path_gz)
+        else
+          send_file(conn, 200, local_asset_path)
+        end
 
       :error ->
         send_resp(conn, 404, "Not found")
     end
   end
 
+  defp accept_encoding?(conn, encoding) do
+    encoding? = &String.contains?(&1, [encoding, "*"])
+
+    Enum.any?(get_req_header(conn, "accept-encoding"), fn accept ->
+      accept |> Plug.Conn.Utils.list() |> Enum.any?(encoding?)
+    end)
+  end
+
   defp ensure_asset?(session_id, hash, asset_path) do
     case lookup_asset(hash, asset_path) do
       {:ok, _local_asset_path} ->

test/livebook_web/controllers/session_controller_test.exs

@@ -172,7 +172,7 @@ defmodule LivebookWeb.SessionControllerTest do
     test "skips the session if assets are in cache", %{conn: conn} do
       %{notebook: notebook, hash: hash} = notebook_with_js_output()
       # Fetch the assets for the first time
-      conn = start_session_and_request_asset(conn, notebook, hash)
+      start_session_and_request_asset(conn, notebook, hash)
 
       # Use nonexistent session, so any communication would fail
       random_session_id = Livebook.Utils.random_node_aware_id()
@@ -196,13 +196,27 @@ defmodule LivebookWeb.SessionControllerTest do
     test "returns the requests asset if available in cache", %{conn: conn} do
       %{notebook: notebook, hash: hash} = notebook_with_js_output()
       # Fetch the assets for the first time
-      conn = start_session_and_request_asset(conn, notebook, hash)
+      start_session_and_request_asset(conn, notebook, hash)
 
       conn = get(conn, ~p"/public/sessions/assets/#{hash}/main.js")
 
       assert conn.status == 200
       assert "export function init(" <> _ = conn.resp_body
     end
+
+    test "supports gzip compression", %{conn: conn} do
+      %{notebook: notebook, hash: hash} = notebook_with_js_output()
+
+      start_session_and_request_asset(conn, notebook, hash)
+
+      conn =
+        conn
+        |> put_req_header("accept-encoding", "gzip")
+        |> get(~p"/public/sessions/assets/#{hash}/main.js")
+
+      assert conn.status == 200
+      assert "export function init(" <> _ = :zlib.gunzip(conn.resp_body)
+    end
   end
 
   defp start_session_and_request_asset(conn, notebook, hash) do