Fix two GC strong-root leaks: tail event cf cycles and abandoned pipe writes

src/workerd/api/streams/internal.c++

@@ -2332,19 +2332,26 @@ jsg::Promise<void> WritableStreamInternalController::Pipe::pipeLoop(jsg::Lock& j
 
       if (handle.isArrayBuffer() || handle.isSharedArrayBuffer() || handle.isArrayBufferView() ||
           handle.isString()) {
+        // addFunctor so the Rc<State> (whose `owner` ref strongly pins the
+        // destination stream) is dropped at IoContext teardown even if the
+        // write promise never settles; a bare capture would otherwise retain
+        // both streams on the isolate heap until isolate death.
+        auto& ioContext = IoContext::current();
         return state->write(js, handle)
             .then(js,
-                [state = state.addRef()](
-                    jsg::Lock& js) mutable -> jsg::Promise<void> { return state->pipeLoop(js); },
-                [state = state.addRef()](
-                    jsg::Lock& js, jsg::Value reason) mutable -> jsg::Promise<void> {
+                ioContext.addFunctor(
+                    [state = state.addRef()](jsg::Lock& js) mutable -> jsg::Promise<void> {
+          return state->pipeLoop(js);
+        }),
+                ioContext.addFunctor([state = state.addRef()](jsg::Lock& js,
+                                         jsg::Value reason) mutable -> jsg::Promise<void> {
           if (state->isAborted() || state->isSourceReleased()) {
             return js.resolvedPromise();
           }
           auto error = jsg::JsValue(reason.getHandle(js));
           state->tryErrorParent(js, error);
           return state->pipeLoop(js);
-        });
+        }));
       }
     }
     // Undefined and null are perfectly valid values to pass through a ReadableStream,

src/workerd/api/tests/BUILD.bazel

@@ -238,6 +238,11 @@ wd_test(
     data = ["errorinfo-tail-test.js"],
 )
 
+wd_test(
+    src = "tail-cf-gc-test.wd-test",
+    data = ["tail-cf-gc-test.js"],
+)
+
 wd_test(
     src = "errorinfo-stw-test.wd-test",
     args = ["--experimental"],

src/workerd/api/tests/tail-cf-gc-test.js

@@ -0,0 +1,83 @@
+// Copyright (c) 2026 Cloudflare, Inc.
+// Licensed under the Apache 2.0 license found in the LICENSE file or at:
+//     https://opensource.org/licenses/Apache-2.0
+//
+// Regression test for a GC leak in tail events: TraceItem's fetch Request
+// held its `cf` object in an untraced strong root, and getCf() returns that
+// same mutable object. A tail handler that made `cf` reference the request
+// (or anything reaching its wrapper) closed an uncollectable JS<->C++ cycle:
+// cf root -> cf -> Request wrapper -> C++ Request -> Detail -> cf root.
+// The fix traces `cf` from Request::visitForGc so V8 can collect the cycle.
+import * as assert from 'node:assert';
+
+// Module-level state may only hold WeakRefs, or it would pin the objects
+// under test.
+const cfRefs = [];
+const reqRefs = [];
+
+export default {
+  async fetch(request, env) {
+    return new Response('ok');
+  },
+
+  tail(events) {
+    for (const event of events) {
+      const req = event.event?.request;
+      if (!req?.cf) continue;
+      // Proves the cf blob propagated through the service binding into the
+      // trace; without this the GC assertions would vacuously pass.
+      assert.strictEqual(req.cf.marker, 'tail-cf-gc');
+      // Mutate the shared cf object to reference the request, closing the
+      // would-be-uncollectable cycle through the untraced strong root.
+      req.cf.self = req;
+      cfRefs.push(new WeakRef(req.cf));
+      reqRefs.push(new WeakRef(req));
+    }
+  },
+};
+
+async function awaitGc() {
+  // Multiple GC passes with yields between them; gives the cycle collector
+  // room to reclaim and avoids the conservative stack scanner pinning the
+  // most recent allocation.
+  for (let i = 0; i < 4; i++) {
+    await scheduler.wait(0);
+    globalThis.gc();
+  }
+}
+
+export const tailRequestCfCollects = {
+  async test(ctrl, env) {
+    for (let i = 0; i < 8; i++) {
+      const res = await env.SERVICE.fetch('http://example.com/', {
+        cf: { marker: 'tail-cf-gc' },
+      });
+      assert.strictEqual(await res.text(), 'ok');
+    }
+
+    // Tail events are delivered asynchronously after each invocation; poll
+    // until they have all arrived.
+    for (let i = 0; i < 100 && cfRefs.length < 8; i++) {
+      await scheduler.wait(50);
+    }
+    assert.strictEqual(
+      cfRefs.length,
+      8,
+      `expected 8 traced requests with cf, got ${cfRefs.length}`
+    );
+
+    await awaitGc();
+    let alive = 0;
+    for (const ref of [...cfRefs, ...reqRefs]) {
+      if (ref.deref() !== undefined) alive++;
+    }
+    // Allow a couple of stragglers: the conservative stack scanner can keep
+    // the most recently touched pair rooted for an extra cycle. The leak
+    // under test would keep all of them alive.
+    assert.ok(
+      alive <= 2,
+      `expected traced request cf cycles to be collected, ` +
+        `${alive} of ${cfRefs.length + reqRefs.length} still alive`
+    );
+  },
+};

src/workerd/api/tests/tail-cf-gc-test.wd-test

@@ -0,0 +1,21 @@
+using Workerd = import "/workerd/workerd.capnp";
+
+const unitTests :Workerd.Config = (
+  v8Flags = ["--expose-gc"],
+  services = [
+    (name = "tail-cf-gc-test", worker = .tailCfGcWorker),
+  ],
+);
+
+const tailCfGcWorker :Workerd.Worker = (
+  modules = [
+    (name = "worker", esModule = embed "tail-cf-gc-test.js")
+  ],
+  bindings = [
+    (name = "SERVICE", service = "tail-cf-gc-test"),
+  ],
+  compatibilityFlags = ["nodejs_compat", "enable_weak_ref"],
+  # Self-tail: the fetch handler's trace events are delivered to the tail
+  # handler in this same module.
+  tails = ["tail-cf-gc-test"],
+);

src/workerd/api/trace.h

@@ -311,6 +311,14 @@ class TraceItem::FetchEventInfo::Request final: public jsg::Object {
     tracker.trackField("detail", detail);
   }
 
+  // getCf() hands out the same mutable cf object, so user code can make it
+  // reference this Request's wrapper; without tracing, that cycle would be
+  // uncollectable. Detail is shared only between Request instances, all of
+  // which visit it here, so the handle is re-traced on every GC cycle.
+  void visitForGc(jsg::GcVisitor& visitor) {
+    visitor.visit(detail->cf);
+  }
+
  private:
   bool redacted = true;
   kj::Own<Detail> detail;