From 2449435053b8eb4ab9d68ce76cddb1994cee74af Mon Sep 17 00:00:00 2001
From: marcopiraccini <marco.piraccini@gmail.com>
Date: Sat, 28 Feb 2026 14:09:57 +0100
Subject: [PATCH 1/2] stream: fix TransformStream race on cancel with pending
 write

Signed-off-by: marcopiraccini <marco.piraccini@gmail.com>
---
 lib/internal/webstreams/transformstream.js    |  8 ++-
 ...hatwg-transformstream-cancel-write-race.js | 54 +++++++++++++++++++
 2 files changed, 60 insertions(+), 2 deletions(-)
 create mode 100644 test/parallel/test-whatwg-transformstream-cancel-write-race.js

diff --git a/lib/internal/webstreams/transformstream.js b/lib/internal/webstreams/transformstream.js
index 371b597dc681d6..fa5e5803c159de 100644
--- a/lib/internal/webstreams/transformstream.js
+++ b/lib/internal/webstreams/transformstream.js
@@ -72,7 +72,6 @@ const {
 const assert = require('internal/assert');
 
 const kSkipThrow = Symbol('kSkipThrow');
-
 const getNonWritablePropertyDescriptor = (value) => {
   return {
     __proto__: null,
@@ -524,7 +523,12 @@ function transformStreamDefaultControllerError(controller, error) {
 
 async function transformStreamDefaultControllerPerformTransform(controller, chunk) {
   try {
-    return await controller[kState].transformAlgorithm(chunk, controller);
+    const transformAlgorithm = controller[kState].transformAlgorithm;
+    if (typeof transformAlgorithm !== 'function') {
+      // Algorithms were cleared by a concurrent cancel/abort/close.
+      return;
+    }
+    return await transformAlgorithm(chunk, controller);
   } catch (error) {
     transformStreamError(controller[kState].stream, error);
     throw error;
diff --git a/test/parallel/test-whatwg-transformstream-cancel-write-race.js b/test/parallel/test-whatwg-transformstream-cancel-write-race.js
new file mode 100644
index 00000000000000..eafd9aaacf99ed
--- /dev/null
+++ b/test/parallel/test-whatwg-transformstream-cancel-write-race.js
@@ -0,0 +1,54 @@
+'use strict';
+
+const common = require('../common');
+const { TransformStream } = require('stream/web');
+const { setTimeout } = require('timers/promises');
+
+// Test for https://github.com/nodejs/node/issues/62036
+// A late write racing with reader.cancel() should not throw an internal "transformAlgorithm is not a function" TypeError.
+
+async function test() {
+  const stream = new TransformStream({
+    transform(chunk, controller) {
+      controller.enqueue(chunk);
+    },
+  });
+
+  await setTimeout(0);
+
+  const reader = stream.readable.getReader();
+  const writer = stream.writable.getWriter();
+
+  // Release backpressure.
+  const pendingRead = reader.read();
+
+  // Simulate client disconnect / shutdown.
+  const pendingCancel = reader.cancel(new Error('client disconnected'));
+
+  // Late write racing with cancel.
+  const pendingLateWrite = writer.write('late-write');
+
+  const results = await Promise.allSettled([
+    pendingRead,
+    pendingCancel,
+    pendingLateWrite,
+  ]);
+
+  // pendingRead should fulfill (with done:true or a value).
+  // pendingCancel should fulfill.
+  // pendingLateWrite may reject, but must NOT reject with an internal
+  // TypeError about transformAlgorithm not being a function.
+  for (const result of results) {
+    if (result.status === 'rejected') {
+      const err = result.reason;
+      if (err instanceof TypeError &&
+          /transformAlgorithm is not a function/.test(err.message)) {
+        throw new Error(
+          'Internal implementation error leaked: ' + err.message
+        );
+      }
+    }
+  }
+}
+
+test().then(common.mustCall());

From 398e70014ef69e06e60870985b876c983f956d27 Mon Sep 17 00:00:00 2001
From: marcopiraccini <marco.piraccini@gmail.com>
Date: Sat, 28 Feb 2026 14:23:30 +0100
Subject: [PATCH 2/2] linting fixup

Signed-off-by: marcopiraccini <marco.piraccini@gmail.com>
---
 test/parallel/test-whatwg-transformstream-cancel-write-race.js | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/test/parallel/test-whatwg-transformstream-cancel-write-race.js b/test/parallel/test-whatwg-transformstream-cancel-write-race.js
index eafd9aaacf99ed..49dcbf8fbfcdc5 100644
--- a/test/parallel/test-whatwg-transformstream-cancel-write-race.js
+++ b/test/parallel/test-whatwg-transformstream-cancel-write-race.js
@@ -5,7 +5,8 @@ const { TransformStream } = require('stream/web');
 const { setTimeout } = require('timers/promises');
 
 // Test for https://github.com/nodejs/node/issues/62036
-// A late write racing with reader.cancel() should not throw an internal "transformAlgorithm is not a function" TypeError.
+// A late write racing with reader.cancel() should not throw an
+// internal "transformAlgorithm is not a function" TypeError.
 
 async function test() {
   const stream = new TransformStream({