LibWeb: Close acquired writer in AO readable_stream_pipe_to()

Also adds a test to prove that the WritableStream's close callback is
called.

Tests/LibWeb/Text/expected/Streams/ReadableStream-pipeTo-Writer-close-callback-called.txt

@@ -0,0 +1 @@
+Writer has been closed.

Tests/LibWeb/Text/input/Streams/ReadableStream-pipeTo-Writer-close-callback-called.html

@@ -0,0 +1,20 @@
+<script src="../include.js"></script>
+<script>
+ asyncTest(done => {
+ const writableStream = new WritableStream({
+ close() {
+ println("Writer has been closed.");
+ }
+ }
+ );
+ const stream = new ReadableStream({
+ pull(controller) {
+ controller.close();
+ }
+ });
+
+ stream.pipeTo(writableStream).then(() => {
+ done();
+ });
+ });
+</script>

Userland/Libraries/LibWeb/Streams/AbstractOperations.cpp

@@ -314,11 +314,17 @@ JS::NonnullGCPtr<WebIDL::Promise> readable_stream_pipe_to(ReadableStream& source
  WebIDL::resolve_promise(realm, promise, JS::js_undefined());
  };
 
- auto success_steps = [promise, &realm](ByteBuffer) {
+ auto success_steps = [promise, &realm, writer](ByteBuffer) {
+ // Make sure we close the acquired writer.
+ WebIDL::resolve_promise(realm, writable_stream_default_writer_close(*writer), JS::js_undefined());
+
  WebIDL::resolve_promise(realm, promise, JS::js_undefined());
  };
 
- auto failure_steps = [promise, &realm](JS::Value error) {
+ auto failure_steps = [promise, &realm, writer](JS::Value error) {
+ // Make sure we close the acquired writer.
+ WebIDL::resolve_promise(realm, writable_stream_default_writer_close(*writer), JS::js_undefined());
+
  WebIDL::reject_promise(realm, promise, error);
  };