1 files changed, 68 insertions, 0 deletions
diff --git a/testing/web-platform/tests/streams/piping/then-interception.any.js b/testing/web-platform/tests/streams/piping/then-interception.any.js
new file mode 100644
index 0000000000..543f916d94
--- /dev/null
+++ b/testing/web-platform/tests/streams/piping/then-interception.any.js
@@ -0,0 +1,68 @@
+// META: global=window,worker
+// META: script=../resources/test-utils.js
+// META: script=../resources/recording-streams.js
+'use strict';
+
+function interceptThen() {
+  const intercepted = [];
+  let callCount = 0;
+  Object.prototype.then = function(resolver) {
+    if (!this.done) {
+      intercepted.push(this.value);
+    }
+    const retval = Object.create(null);
+    retval.done = ++callCount === 3;
+    retval.value = callCount;
+    resolver(retval);
+    if (retval.done) {
+      delete Object.prototype.then;
+    }
+  }
+  return intercepted;
+}
+
+promise_test(async t => {
+  const rs = new ReadableStream({
+    start(controller) {
+      controller.enqueue('a');
+      controller.close();
+    }
+  });
+  const ws = recordingWritableStream();
+
+  const intercepted = interceptThen();
+  t.add_cleanup(() => {
+    delete Object.prototype.then;
+  });
+
+  await rs.pipeTo(ws);
+  delete Object.prototype.then;
+
+
+  assert_array_equals(intercepted, [], 'nothing should have been intercepted');
+  assert_array_equals(ws.events, ['write', 'a', 'close'], 'written chunk should be "a"');
+}, 'piping should not be observable');
+
+promise_test(async t => {
+  const rs = new ReadableStream({
+    start(controller) {
+      controller.enqueue('a');
+      controller.close();
+    }
+  });
+  const ws = recordingWritableStream();
+
+  const [ branch1, branch2 ] = rs.tee();
+
+  const intercepted = interceptThen();
+  t.add_cleanup(() => {
+    delete Object.prototype.then;
+  });
+
+  await branch1.pipeTo(ws);
+  delete Object.prototype.then;
+  branch2.cancel();
+
+  assert_array_equals(intercepted, [], 'nothing should have been intercepted');
+  assert_array_equals(ws.events, ['write', 'a', 'close'], 'written chunk should be "a"');
+}, 'tee should not be observable');