Avoid inspector stack capture in RPC GC finalizers

Skip V8 stack collection for RPC disposal warnings emitted during GC.
Add an inspector-enabled regression covering optimized string flattening and explicit disposal.

Author: ruifigueira

src/workerd/api/tests/BUILD.bazel

@@ -186,6 +186,15 @@ wd_test(
     ],
 )
 
+wd_test(
+    src = "rpc-stub-gc-warning-inspector-test.wd-test",
+    args = ["--experimental"],
+    data = [
+        "rpc-stub-gc-warning-inspector-test.js",
+        "rpc-stub-gc-warning-inspector-trigger.js",
+    ],
+)
+
 # Test to validate timing semantics for JSRPC streaming responses.
 # This test verifies that Return events occur when the handler returns,
 # NOT when the stream is fully consumed.

src/workerd/api/tests/rpc-stub-gc-warning-inspector-test.js

@@ -0,0 +1,60 @@
+// 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
+import assert from 'node:assert';
+
+const WARNING_PREFIXES = [
+  'An RPC stub was not disposed properly',
+  'An RPC result was not disposed properly',
+];
+const TIMEOUT_MS = 5000;
+const POLL_MS = 10;
+
+let fetchOutcomes = 0;
+const warnings = [];
+
+export default {
+  tailStream(onset) {
+    const isFetch = onset.event.info?.type === 'fetch';
+    return (event) => {
+      if (
+        event.event.type === 'log' &&
+        event.event.level === 'warn' &&
+        WARNING_PREFIXES.some((prefix) =>
+          event.event.message?.[0]?.startsWith(prefix)
+        )
+      ) {
+        warnings.push(event.event.message[0]);
+      }
+      if (isFetch && event.event.type === 'outcome') {
+        fetchOutcomes++;
+      }
+    };
+  },
+};
+
+async function waitForFetchOutcome(count) {
+  const deadline = Date.now() + TIMEOUT_MS;
+  while (fetchOutcomes < count && Date.now() < deadline) {
+    await scheduler.wait(POLL_MS);
+  }
+  assert.strictEqual(
+    fetchOutcomes,
+    count,
+    `fetch outcome ${count} was not tailed`
+  );
+}
+
+export const test = {
+  async test(ctrl, env) {
+    let response = await env.TRIGGER.fetch('http://example.com/?dispose');
+    assert.strictEqual(await response.text(), 'disposed');
+    await waitForFetchOutcome(1);
+    assert.deepStrictEqual(warnings, []);
+
+    response = await env.TRIGGER.fetch('http://example.com/');
+    assert.match(await response.text(), /^leaked: /);
+    await waitForFetchOutcome(2);
+    assert.strictEqual(warnings.length, 1);
+  },
+};

src/workerd/api/tests/rpc-stub-gc-warning-inspector-test.wd-test

@@ -0,0 +1,38 @@
+using Workerd = import "/workerd/workerd.capnp";
+
+# The test entrypoint is the streaming tail worker: it drives the trigger worker and asserts on
+# the leaked-stub warnings that reach it. The trigger runs with the inspector enabled so that the
+# warning path also attempts to capture a JS stack trace for the DevTools console.
+const unitTests :Workerd.Config = (
+  services = [
+    ( name = "trigger",
+      worker = (
+        modules = [
+          (name = "worker", esModule = embed "rpc-stub-gc-warning-inspector-trigger.js")
+        ],
+        compatibilityFlags = [
+          "nodejs_compat",
+          "enable_nodejs_inspector_local_dev",
+          "experimental",
+        ],
+        bindings = [
+          (name = "MyService", service = (
+            name = "trigger",
+            entrypoint = "MyService")),
+        ],
+        streamingTails = ["rpc-stub-gc-warning-inspector-test"],
+      )
+    ),
+    ( name = "rpc-stub-gc-warning-inspector-test",
+      worker = (
+        modules = [
+          (name = "worker", esModule = embed "rpc-stub-gc-warning-inspector-test.js")
+        ],
+        compatibilityFlags = ["nodejs_compat", "experimental"],
+        bindings = [
+          (name = "TRIGGER", service = "trigger"),
+        ],
+      ),
+    ),
+  ],
+);

src/workerd/api/tests/rpc-stub-gc-warning-inspector-trigger.js

@@ -0,0 +1,70 @@
+// 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
+import { RpcTarget, WorkerEntrypoint } from 'cloudflare:workers';
+
+class Counter extends RpcTarget {
+  increment() {
+    return 1;
+  }
+}
+
+export class MyService extends WorkerEntrypoint {
+  getCounter() {
+    return new Counter();
+  }
+}
+
+const chunk = 'x'.repeat(4096);
+
+function buildConsString() {
+  let result = chunk;
+  for (let i = 0; i < 16; i++) result += chunk;
+  return result;
+}
+
+// charCodeAt() flattens the ConsString. Once V8 optimizes this function, an allocation failure
+// can start a minor GC from a runtime call whose optimized frame has no deoptimization metadata.
+// If a leaked-stub finalizer runs in that GC and tries to capture a JS stack trace for the
+// inspector, V8 aborts the process.
+function scan(str) {
+  let hash = 0;
+  for (let i = 0; i < str.length; i += 4093) {
+    hash = (hash * 31 + str.charCodeAt(i)) | 0;
+  }
+  return hash;
+}
+
+export default {
+  async fetch(request, env) {
+    const dispose = new URL(request.url).searchParams.has('dispose');
+
+    if (dispose) {
+      let stub = await env.MyService.getCounter();
+      stub[Symbol.dispose]();
+      stub = null;
+      gc();
+      gc();
+      return new Response('disposed');
+    }
+
+    let hash = 0;
+    for (let i = 0; i < 30000; i++) hash = (hash + scan(buildConsString())) | 0;
+
+    let stubs = [];
+    for (let i = 0; i < 32; i++) {
+      const stub = await env.MyService.getCounter();
+      await stub.increment();
+      stubs.push(stub);
+    }
+    stubs = null;
+
+    // Allocation-driven collections in scan() exercise the original crash. Explicit collections
+    // ensure that the warning is emitted even when heap sizing does not trigger one in this loop.
+    for (let i = 0; i < 20000; i++) hash = (hash + scan(buildConsString())) | 0;
+    gc();
+    gc();
+
+    return new Response(`leaked: ${hash}`);
+  },
+};

src/workerd/api/worker-rpc.c++

@@ -849,7 +849,8 @@ JsRpcStub::~JsRpcStub() noexcept(false) {
           "let the other side know that you are no longer using them. You cannot rely on "
           "the garbage collector for this because it may take arbitrarily long before actually "
           "collecting unreachable objects. As a shortcut, calling dispose() on the result of "
-          "an RPC call disposes all stubs within it."_kj);
+          "an RPC call disposes all stubs within it."_kj,
+          CaptureInspectorStackTrace::NO);
     }
   }
 
@@ -885,7 +886,8 @@ RpcStubDisposalGroup::~RpcStubDisposalGroup() noexcept(false) {
             "An RPC result was not disposed properly. One of the RPC calls you made expects you "
             "to call dispose() on the return value, but you didn't do so. You cannot rely on "
             "the garbage collector for this because it may take arbitrarily long before actually "
-            "collecting unreachable objects."_kj);
+            "collecting unreachable objects."_kj,
+            CaptureInspectorStackTrace::NO);
       }
     }
   } else {

src/workerd/io/io-context.c++

@@ -420,12 +420,14 @@ bool IoContext::hasWarningHandler() {
       ::kj::_::Debug::shouldLog(::kj::LogSeverity::INFO);
 }
 
-void IoContext::logWarning(kj::StringPtr description) {
-  KJ_REQUIRE_NONNULL(currentLock).logWarning(description);
+void IoContext::logWarning(
+    kj::StringPtr description, CaptureInspectorStackTrace captureStackTrace) {
+  KJ_REQUIRE_NONNULL(currentLock).logWarning(description, captureStackTrace);
 }
 
-void IoContext::logWarningOnce(kj::StringPtr description) {
-  KJ_REQUIRE_NONNULL(currentLock).logWarningOnce(description);
+void IoContext::logWarningOnce(
+    kj::StringPtr description, CaptureInspectorStackTrace captureStackTrace) {
+  KJ_REQUIRE_NONNULL(currentLock).logWarningOnce(description, captureStackTrace);
 }
 
 void IoContext::logErrorOnce(kj::StringPtr description) {

src/workerd/io/io-context.h

@@ -391,11 +391,13 @@ class IoContext final: public kj::Refcounted, private kj::TaskSet::ErrorHandler
 
   // Log a warning. Emits to the Chrome DevTools inspector (if connected), stderr, and to the
   // streaming tail worker tracer (if active).
-  void logWarning(kj::StringPtr description);
+  void logWarning(kj::StringPtr description,
+      CaptureInspectorStackTrace captureStackTrace = CaptureInspectorStackTrace::YES);
 
   // Log a warning, deduplicating so that each unique message is only logged once for the lifetime
   // of an isolate. Emits to the same destinations as logWarning().
-  void logWarningOnce(kj::StringPtr description);
+  void logWarningOnce(kj::StringPtr description,
+      CaptureInspectorStackTrace captureStackTrace = CaptureInspectorStackTrace::YES);
 
   // Log an internal error message. Deduplicates log messages such that a single unique message will
   // only be logged once for the lifetime of an isolate.

src/workerd/io/worker.c++

@@ -2532,14 +2532,16 @@ bool Worker::Lock::isInspectorEnabled() {
   return worker.script->isolate->impl->inspector != kj::none;
 }
 
-void Worker::Lock::logWarning(kj::StringPtr description) {
+void Worker::Lock::logWarning(
+    kj::StringPtr description, CaptureInspectorStackTrace captureStackTrace) {
   // const_cast OK because we are a lock on this isolate.
-  const_cast<Isolate&>(worker.getIsolate()).logWarning(description, *this);
+  const_cast<Isolate&>(worker.getIsolate()).logWarning(description, *this, captureStackTrace);
 }
 
-void Worker::Lock::logWarningOnce(kj::StringPtr description) {
+void Worker::Lock::logWarningOnce(
+    kj::StringPtr description, CaptureInspectorStackTrace captureStackTrace) {
   // const_cast OK because we are a lock on this isolate.
-  const_cast<Isolate&>(worker.getIsolate()).logWarningOnce(description, *this);
+  const_cast<Isolate&>(worker.getIsolate()).logWarningOnce(description, *this, captureStackTrace);
 }
 
 void Worker::Lock::logErrorOnce(kj::StringPtr description) {
@@ -3572,10 +3574,11 @@ void Worker::Isolate::disconnectInspector() {
   impl->inspectorClient->resetChannel();
 }
 
-void Worker::Isolate::logWarning(kj::StringPtr description, Lock& lock) {
+void Worker::Isolate::logWarning(
+    kj::StringPtr description, Lock& lock, CaptureInspectorStackTrace captureStackTrace) {
   if (impl->inspector != kj::none) {
     JSG_WITHIN_CONTEXT_SCOPE(lock, lock.getContext(), [&](jsg::Lock& js) {
-      logMessage(js, static_cast<uint16_t>(cdp::LogType::WARNING), description);
+      logMessage(js, static_cast<uint16_t>(cdp::LogType::WARNING), description, captureStackTrace);
     });
   }
 
@@ -3608,9 +3611,10 @@ void Worker::Isolate::logWarning(kj::StringPtr description, Lock& lock) {
   }
 }
 
-void Worker::Isolate::logWarningOnce(kj::StringPtr description, Lock& lock) {
+void Worker::Isolate::logWarningOnce(
+    kj::StringPtr description, Lock& lock, CaptureInspectorStackTrace captureStackTrace) {
   impl->warningOnceDescriptions.findOrCreate(description, [&] {
-    logWarning(description, lock);
+    logWarning(description, lock, captureStackTrace);
     return kj::str(description);
   });
 }
@@ -3622,7 +3626,10 @@ void Worker::Isolate::logErrorOnce(kj::StringPtr description) {
   });
 }
 
-void Worker::Isolate::logMessage(jsg::Lock& js, uint16_t type, kj::StringPtr description) {
+void Worker::Isolate::logMessage(jsg::Lock& js,
+    uint16_t type,
+    kj::StringPtr description,
+    CaptureInspectorStackTrace captureStackTrace) {
   if (impl->inspector != kj::none) {
     // We want to log a warning to the devtools console, as if `console.warn()` were called.
     // However, the only public interface to call the real `console.warn()` is via JavaScript,
@@ -3653,7 +3660,9 @@ void Worker::Isolate::logMessage(jsg::Lock& js, uint16_t type, kj::StringPtr des
       params.initArgs(1)[0].initString().setValue(description);
       params.setExecutionContextId(v8_inspector::V8ContextInfo::executionContextId(js.v8Context()));
       params.setTimestamp(impl->inspectorClient->currentTimeMS());
-      stackTraceToCDP(js, params.initStackTrace());
+      if (captureStackTrace) {
+        stackTraceToCDP(js, params.initStackTrace());
+      }
 
       auto notification = getCdpJsonCodec().encode(event);
       KJ_IF_SOME(i, currentInspectorSession) {

src/workerd/io/worker.h

@@ -42,6 +42,8 @@ namespace workerd {
 
 WD_STRONG_BOOL(StructuredLogging);
 WD_STRONG_BOOL(ProcessStdioPrefixed);
+// Inspector stack capture enters V8 and is unsafe from GC finalizers.
+WD_STRONG_BOOL(CaptureInspectorStackTrace);
 
 namespace api {
 class DurableObjectState;
@@ -461,11 +463,15 @@ class Worker::Isolate: public kj::AtomicRefcounted {
       kj::WebSocket& webSocket) const;
 
   // Log a warning to the inspector if attached, and log an INFO severity message.
-  void logWarning(kj::StringPtr description, Worker::Lock& lock);
+  void logWarning(kj::StringPtr description,
+      Worker::Lock& lock,
+      CaptureInspectorStackTrace captureStackTrace = CaptureInspectorStackTrace::YES);
 
   // logWarningOnce() only logs the warning if it has not already been logged for this
   // worker instance.
-  void logWarningOnce(kj::StringPtr description, Worker::Lock& lock);
+  void logWarningOnce(kj::StringPtr description,
+      Worker::Lock& lock,
+      CaptureInspectorStackTrace captureStackTrace = CaptureInspectorStackTrace::YES);
 
   // Log an ERROR severity message, if it has not already been logged for this worker instance.
   void logErrorOnce(kj::StringPtr description);
@@ -570,7 +576,10 @@ class Worker::Isolate: public kj::AtomicRefcounted {
 
   // Log a message as if with console.{log,warn,error,etc}. `type` must be one of the cdp::LogType
   // enum, which unfortunately we cannot forward-declare, ugh.
-  void logMessage(jsg::Lock& js, uint16_t type, kj::StringPtr description);
+  void logMessage(jsg::Lock& js,
+      uint16_t type,
+      kj::StringPtr description,
+      CaptureInspectorStackTrace captureStackTrace = CaptureInspectorStackTrace::YES);
 
   class SubrequestClient;
   class ResponseStreamWrapper;
@@ -728,8 +737,10 @@ class Worker::Lock {
   v8::Local<v8::Context> getContext();
 
   bool isInspectorEnabled();
-  void logWarning(kj::StringPtr description);
-  void logWarningOnce(kj::StringPtr description);
+  void logWarning(kj::StringPtr description,
+      CaptureInspectorStackTrace captureStackTrace = CaptureInspectorStackTrace::YES);
+  void logWarningOnce(kj::StringPtr description,
+      CaptureInspectorStackTrace captureStackTrace = CaptureInspectorStackTrace::YES);
 
   void logErrorOnce(kj::StringPtr description);