stream: keep async context of the pipeTo pump

lib/internal/webstreams/readablestream.js

@@ -47,6 +47,14 @@ const {
   DOMException,
 } = internalBinding('messaging');
 
+// The pipe checks the async context frame on every pump step; the
+// intrinsics reduce to a field access in optimized code, which the
+// AsyncContextFrame wrappers would put behind another inlining level.
+const {
+  getContinuationPreservedEmbedderData: currentContextFrame,
+  setContinuationPreservedEmbedderData: setContextFrame,
+} = internalBinding('async_context_frame');
+
 const {
   isArrayBufferView,
   isDataView,
@@ -1749,6 +1757,20 @@ function readableStreamPipeTo(
   let readRequest;
   let readyHook;
 
+  // The pump re-enters from the ready hook and from the chunk steps, which
+  // run in the async context of whoever cleared the backpressure or
+  // enqueued the chunk. Resume it in the context of the pipeTo() call, as
+  // the promise-based pump did.
+  const pipeContextFrame = currentContextFrame();
+  function pumpInPipeContext(prior) {
+    setContextFrame(pipeContextFrame);
+    try {
+      pump();
+    } finally {
+      setContextFrame(prior);
+    }
+  }
+
   // Ready promise rejection is handled by the destination-errored
   // watcher.
   function ignoreReadyRejection() {}
@@ -1782,6 +1804,12 @@ function readableStreamPipeTo(
   function pump() {
     if (shuttingDown) return;
 
+    const contextFrame = currentContextFrame();
+    if (contextFrame !== pipeContextFrame) {
+      pumpInPipeContext(contextFrame);
+      return;
+    }
+
     if (dest[kState].backpressure) {
       parkOnReady();
       return;

test/parallel/test-whatwg-readablestream-pipeto-async-context.js

@@ -0,0 +1,157 @@
+'use strict';
+
+// The pipeTo() pump must keep running in the async context of the
+// pipeTo() call when it resumes after backpressure or after a chunk
+// arrives, instead of taking the context of the code that cleared the
+// backpressure or enqueued the chunk. The source pulls and the writes the
+// pump issues are checked.
+
+const common = require('../common');
+const assert = require('assert');
+const { AsyncLocalStorage } = require('async_hooks');
+const { setImmediate: tick } = require('timers/promises');
+
+const als = new AsyncLocalStorage();
+
+function store() {
+  return als.getStore() ?? 'none';
+}
+
+// Settles the pending callbacks from the given context until `promise`
+// settles.
+async function settleUntil(promise, ctx, pending) {
+  let finished = false;
+  promise.then(common.mustCall(() => { finished = true; }));
+  while (!finished) {
+    await tick();
+    while (pending.length > 0) als.run(ctx, pending.shift());
+  }
+  await promise;
+}
+
+const tests = [];
+function test(name, expected, fn) {
+  tests.push({ name, expected, fn });
+}
+
+test('sink writes resolved from another context', [
+  // The first pull and write follow the source's start step.
+  'pull:creator', 'write-1:creator',
+  'pull:piper', 'write-2:piper',
+  'pull:piper', 'write-3:piper',
+  'pull:piper',
+], async (seen) => {
+  let n = 0;
+  const pending = [];
+  const rs = als.run('creator', () => new ReadableStream({
+    pull(c) {
+      seen.push(`pull:${store()}`);
+      if (++n > 3) c.close(); else c.enqueue(n);
+    },
+  }, { highWaterMark: 0 }));
+  const ws = als.run('creator', () => new WritableStream({
+    write(chunk) {
+      seen.push(`write-${chunk}:${store()}`);
+      const { promise, resolve } = Promise.withResolvers();
+      pending.push(resolve);
+      return promise;
+    },
+  }, { highWaterMark: 1 }));
+  await settleUntil(als.run('piper', () => rs.pipeTo(ws)), 'releaser', pending);
+});
+
+test('push source under backpressure', [
+  'write-a:pusher-a', 'write-b:piper', 'write-c:piper', 'write-d:piper',
+], async (seen) => {
+  let controller;
+  const pending = [];
+  const rs = als.run('creator', () => new ReadableStream({
+    start(c) { controller = c; },
+  }, { highWaterMark: 0 }));
+  const ws = als.run('creator', () => new WritableStream({
+    write(chunk) {
+      seen.push(`write-${chunk}:${store()}`);
+      const { promise, resolve } = Promise.withResolvers();
+      pending.push(resolve);
+      return promise;
+    },
+  }, { highWaterMark: 1 }));
+  const piped = als.run('piper', () => rs.pipeTo(ws));
+  await tick();
+  for (const chunk of 'abcd')
+    als.run(`pusher-${chunk}`, () => controller.enqueue(chunk));
+  als.run('closer', () => controller.close());
+  await settleUntil(piped, 'releaser', pending);
+});
+
+test('pipeThrough() with pulls resolved from another context', [
+  'pull:piper', 'transform-1:resolver',
+  'pull:piper', 'transform-2:resolver',
+  'pull:piper', 'transform-3:resolver',
+  'pull:piper',
+], async (seen) => {
+  let n = 0;
+  const pending = [];
+  const readable = als.run('piper', () => new ReadableStream({
+    pull(c) {
+      seen.push(`pull:${store()}`);
+      const { promise, resolve } = Promise.withResolvers();
+      pending.push(() => {
+        if (++n > 3) c.close(); else c.enqueue(n);
+        resolve();
+      });
+      return promise;
+    },
+  }, { highWaterMark: 0 }).pipeThrough(new TransformStream({
+    transform(chunk, c) {
+      seen.push(`transform-${chunk}:${store()}`);
+      c.enqueue(chunk);
+    },
+  })));
+  const consumed = als.run('reader', async () => {
+    // eslint-disable-next-line no-unused-vars
+    for await (const _ of readable);
+  });
+  await settleUntil(consumed, 'resolver', pending);
+});
+
+test('pipeThrough() chain into a slow sink', [
+  'pull:creator', 't1-1:piper', 't2-1:piper', 'write-1:piper',
+  'pull:piper', 't1-2:piper', 't2-2:piper', 'write-2:piper',
+  'pull:piper', 't1-3:piper', 't2-3:piper', 'write-3:piper',
+  'pull:piper',
+], async (seen) => {
+  let n = 0;
+  function transform(name) {
+    return new TransformStream({
+      transform(chunk, c) {
+        seen.push(`${name}-${chunk}:${store()}`);
+        c.enqueue(chunk);
+      },
+    });
+  }
+  const rs = als.run('creator', () => new ReadableStream({
+    pull(c) {
+      seen.push(`pull:${store()}`);
+      if (++n > 3) c.close(); else c.enqueue(n);
+    },
+  }, { highWaterMark: 0 }));
+  const ws = als.run('creator', () => new WritableStream({
+    async write(chunk) {
+      seen.push(`write-${chunk}:${store()}`);
+      await tick();
+    },
+  }, { highWaterMark: 1 }));
+  await als.run('piper', () => rs.pipeThrough(transform('t1'))
+    .pipeThrough(transform('t2')).pipeTo(ws));
+});
+
+(async () => {
+  for (const { name, expected, fn } of tests) {
+    const seen = [];
+    await fn(seen);
+    // Only the context of each callback is checked, not how the callbacks
+    // of different streams interleave.
+    assert.deepStrictEqual(seen.sort(), expected.toSorted(), name);
+  }
+})().then(common.mustCall());