stream: fix pipeTo to defer writes per WHATWG spec

mcollina · aduh95 · commit ea2df2a16f18 · 2026-02-28T12:07:00.000+01:00
The WHATWG Streams spec requires that pipeTo's chunk handling must queue a microtask before calling the write algorithm. This ensures that enqueue() does not synchronously trigger writes. Previously, PipeToReadableStreamReadRequest[kChunk] would synchronously call writableStreamDefaultWriterWrite(), which violated the spec and caused the WPT test "enqueue() must not synchronously call write algorithm" to fail. Fix by wrapping the write operation in queueMicrotask(), which defers it to the next microtask as required by the spec. Refs: whatwg/streams#1243 PR-URL: #61800 Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: Mattias Buelens <mattias@buelens.com>
diff --git a/lib/internal/webstreams/readablestream.js b/lib/internal/webstreams/readablestream.js
@@ -1662,9 +1662,14 @@ class PipeToReadableStreamReadRequest {
   }
 
   [kChunk](chunk) {
-    this.state.currentWrite = writableStreamDefaultWriterWrite(this.writer, chunk);
-    setPromiseHandled(this.state.currentWrite);
-    this.promise.resolve(false);
+    // Per spec, pipeTo must queue a microtask for the write to avoid
+    // synchronous write during enqueue(). See WHATWG Streams spec
+    // "ReadableStreamPipeTo" step 15's "chunk steps".
+    queueMicrotask(() => {
+      this.state.currentWrite = writableStreamDefaultWriterWrite(this.writer, chunk);
+      setPromiseHandled(this.state.currentWrite);
+      this.promise.resolve(false);
+    });
   }
 
   [kClose]() {
diff --git a/test/wpt/status/streams.json b/test/wpt/status/streams.json
@@ -2,14 +2,6 @@
   "idlharness-shadowrealm.window.js": {
     "skip": "ShadowRealm support is not enabled"
   },
-  "piping/general-addition.any.js": {
-    "fail": {
-      "note": "Blocked on https://github.com/whatwg/streams/issues/1243",
-      "expected": [
-        "enqueue() must not synchronously call write algorithm"
-      ]
-    }
-  },
   "queuing-strategies-size-function-per-global.window.js": {
     "skip": "Browser-specific test"
   },
