From 28f4c68534912687d27401b1f2307165fa01ae5e Mon Sep 17 00:00:00 2001
From: Alessio Attilio <attilio.alessio@protonmail.com>
Date: Sun, 26 Jul 2026 20:59:35 +0200
Subject: [PATCH] stream: prevent enqueue after cancel in Readable.toWeb()

---
 lib/internal/webstreams/adapters.js           |  1 +
 ...webstreams-adapters-cancel-backpressure.js | 52 +++++++++++++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 test/parallel/test-webstreams-adapters-cancel-backpressure.js

diff --git a/lib/internal/webstreams/adapters.js b/lib/internal/webstreams/adapters.js
index 42e1221bb30765..edec59b3fe704c 100644
--- a/lib/internal/webstreams/adapters.js
+++ b/lib/internal/webstreams/adapters.js
@@ -541,6 +541,7 @@ function newReadableStreamFromStreamReadable(streamReadable, options = kEmptyObj
     streamReadable.pause();
 
     streamReadable.on('data', function onData(chunk) {
+      if (wasCanceled) return;
       // Copy the Buffer to detach it from the pool.
       if (Buffer.isBuffer(chunk) && !objectMode)
         chunk = new Uint8Array(chunk);
diff --git a/test/parallel/test-webstreams-adapters-cancel-backpressure.js b/test/parallel/test-webstreams-adapters-cancel-backpressure.js
new file mode 100644
index 00000000000000..25e2b82e3f17fc
--- /dev/null
+++ b/test/parallel/test-webstreams-adapters-cancel-backpressure.js
@@ -0,0 +1,52 @@
+'use strict';
+const common = require('../common');
+const { PassThrough, Readable, pipeline } = require('node:stream');
+const { WritableStream } = require('node:stream/web');
+const { setTimeout } = require('node:timers/promises');
+
+// Regression test for https://github.com/nodejs/node/issues/64529
+// Readable.toWeb() uncaughtException when the stream is canceled during backpressure resume
+
+process.on('uncaughtException', (err) => {
+  console.error('Uncaught exception!', err);
+  process.exit(1);
+});
+
+async function run() {
+  for (let i = 0; i < 50; i++) {
+    const src = new Readable({
+      read() {
+        this.push(Buffer.alloc(16 * 1024, 1));
+        if ((this.bytes = (this.bytes || 0) + 16384) > 512 * 1024) this.push(null);
+      },
+    });
+    const pt = new PassThrough({ highWaterMark: 16384 });
+    pipeline(src, pt, () => {});
+    const web = Readable.toWeb(pt);
+
+    const ac = new AbortController();
+    const writer = new WritableStream(
+      {
+        async write() {
+          await setTimeout(1);
+        }
+      },
+      { highWaterMark: 1 },
+    );
+
+    // Disconnect randomly to catch the exact tick window
+    setTimeout(i % 10).then(() => ac.abort()).then(common.mustCall());
+
+    try {
+      await web.pipeTo(writer, { signal: ac.signal });
+    } catch {
+      // Ignore abort errors
+    }
+
+    await new Promise((r) => setImmediate(r));
+  }
+}
+
+run().then(common.mustCall(() => {
+  process.exit(0);
+}));