stream: narrow pipeTo guard to writer release check

The shuttingDown guard added by the previous commit is too broad: it
also skips an already-read chunk while the destination is still
writable. The WHATWG Streams spec requires pipeTo to write such chunks
during shutdown.

Check writer[kState].stream instead. It becomes undefined only after
finalize() releases the writer, which is precisely the assertion case.

Add a regression test that aborts after enqueue() and verifies that the
already-read chunk is written.

Signed-off-by: Qingyu Wang <wangqingyu.c0l1n@bytedance.com>
Assisted-by: Codex

Author: colinaaa

lib/internal/webstreams/readablestream.js

@@ -1760,7 +1760,7 @@ function readableStreamPipeTo(
   function forwardChunk() {
     const chunk = pendingChunk;
     pendingChunk = undefined;
-    if (shuttingDown) return;
+    if (writer[kState].stream === undefined) return;
     writableStreamDefaultWriterWriteWithRequest(writer, chunk, writeTracker);
     pump();
   }

test/parallel/test-webstreams-pipeto-writer-released-race.js

@@ -31,3 +31,33 @@ const { ReadableStream, WritableStream } = require('stream/web');
     sourceController.enqueue('chunk');
   }));
 }
+
+{
+  const ac = new AbortController();
+  let sourceController;
+  const chunks = [];
+
+  const source = new ReadableStream({
+    start(controller) {
+      sourceController = controller;
+    },
+  }, { highWaterMark: 0 });
+
+  const dest = new WritableStream({
+    write: common.mustCall((chunk) => {
+      chunks.push(chunk);
+    }),
+  }, { highWaterMark: 1 });
+
+  assert.rejects(
+    source.pipeTo(dest, { signal: ac.signal }),
+    { name: 'AbortError' },
+  ).then(common.mustCall(() => {
+    assert.deepStrictEqual(chunks, ['chunk']);
+  }));
+
+  setImmediate(common.mustCall(() => {
+    sourceController.enqueue('chunk');
+    ac.abort();
+  }));
+}