MessagePort.postMessage with custom iterator as transfer list causes Assertion `(index) < (length())' failed.

[KhafraDev]

Version

v21.0.0-pre (I assume every version w/ structuredClone)

Platform

Linux DESKTOP-L4O1H93 5.10.102.1-microsoft-standard-WSL2 #1 SMP Wed Mar 2 00:30:59 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

No response

What steps will reproduce the bug?

structuredClone({}, {
 transfer: {
 *[Symbol.iterator]() {}
 }
})

or

new MessageChannel().port1.postMessage({}, {
 transfer: {
 *[Symbol.iterator]() {}
 }
})

How often does it reproduce? Is there a required condition?

No response

What is the expected behavior? Why is that the expected behavior?

this should work and return an empty object

What do you see instead?

./node[4717]: ../../src/util.h:392:T& node::MaybeStackBuffer<T, kStackStorageSize>::operator[](size_t) [with T = v8::Local<v8::Value> long unsigned int kStackStorageSize = 8; size_t = long unsigned int]: Assertion `(index) < (length())' failed.
 1: 0x561c3fcd9e94 node::Abort() [./node]
 2: 0x561c3fcd9f28 [./node]
 3: 0x561c3fd3310c [./node]
 4: 0x561c3fd35a4e node::worker::MessagePort::PostMessage(v8::FunctionCallbackInfo<v8::Value> const&) [./node]
 5: 0x561c3ffb4c00 v8::internal::FunctionCallbackArguments::Call(v8::internal::CallHandlerInfo) [./node]
 6: 0x561c3ffb54f9 [./node]
 7: 0x561c3ffb59e5 v8::internal::Builtin_HandleApiCall(int, unsigned long*, v8::internal::Isolate*) [./node]
 8: 0x561c40a14df6 [./node]
Aborted

Additional information

The webidl states that a sequence is basically anything with a Symbol.iterator, not exclusive to arrays. https://webidl.spec.whatwg.org/#es-sequence

[targos]

Would it be spec-compliant to just convert the value to an array with Array.from before passing it to the C++ side? It would also take care of potential errors thrown during iteration.

[KhafraDev]

not entirely, but it's better than nothing. For example, something that isn't null and doesn't have a Symbol.iterator won't throw with Array.from, but spec-wise is expected to (Array.from(3) // []).

[KhafraDev]

someone could use

node/lib/internal/crypto/webidl.js

Line 324 in 952cf0d

function createSequenceConverter(converter) {

which would be spec compliant, but afaik MessagePort is entirely native so I'm not sure how that would work.

or maybe using

node/src/node_messaging.cc

Line 947 in 952cf0d

static Maybe<bool> ReadIterable(Environment* env,

would work also, it looks compliant, and probably the reason that Workers don't throw when transferring the same object.