globals: expose MessageChannel, MessagePort, and BroadcastChannel #2597

Summary

Current Node exposes the worker messaging constructors MessageChannel, MessagePort, and BroadcastChannel as globals. Perry tracks the node:worker_threads module gap in #2507, but the global aliases are a separate observable surface: browser-compatible packages often feature-detect or instantiate these constructors through globalThis without importing node:worker_threads.

Today Perry's native globalThis constructor population omits all three names, so globalThis.MessageChannel, globalThis.MessagePort, and globalThis.BroadcastChannel are not Node-compatible even if the module implementation later lands.

Expected Node behavior

Node documents these as global classes in the Globals API:

• BroadcastChannel is a global class added in v18.0.0.
• MessageChannel is a global class added in v15.0.0.
• MessagePort is a global class added in v15.0.0.

A local Node v25.9.0 probe shows the globals are also identical to the node:worker_threads exports:

BroadcastChannel function function true
MessageChannel function function true
MessagePort function function true
channel ports true true function function
broadcast proto close,constructor,name,onmessage,onmessageerror,postMessage,ref,unref

Probe:

const wt = require("node:worker_threads");
for (const k of ["BroadcastChannel", "MessageChannel", "MessagePort"]) {
  console.log(k, typeof globalThis[k], typeof wt[k], globalThis[k] === wt[k]);
}
const ch = new MessageChannel();
console.log("channel ports", ch.port1 instanceof MessagePort, ch.port2 instanceof MessagePort, typeof ch.port1.postMessage, typeof ch.port1.close);
const bc = new BroadcastChannel("probe");
console.log("broadcast proto", Object.getOwnPropertyNames(BroadcastChannel.prototype).sort().join(","));
bc.close(); ch.port1.close(); ch.port2.close();

Current Perry evidence

Source inspection against origin/main:

• crates/perry-runtime/src/object/global_this.rs::GLOBAL_THIS_BUILTIN_CONSTRUCTORS prepopulates many global constructor names (TextEncoderStream, URL, AbortController, Blob, Request, Response, etc.) but does not include MessageChannel, MessagePort, or BroadcastChannel.
• crates/perry-codegen/src/expr/helpers.rs::is_global_this_builtin_name() mirrors the global constructor routing list and also omits all three names.
• A source grep for MessageChannel, MessagePort, and BroadcastChannel finds only the partial worker_threads import bookkeeping and parity probes; there is no native global constructor routing for these names.
• docs/runtime-parity.md lists MessageChannel, MessagePort, and BroadcastChannel under Web / Global APIs as covered, so the docs currently overstate the native global surface.
• test-files/test_parity_worker_threads.ts probes the module exports, but there is no focused parity test for the global aliases or module/global identity.

I did not run a Perry binary because this checkout does not have target/release/perry; this issue is based on committed origin/main source, current Node docs, and a local Node v25.9.0 probe.

Suggested test surface

Add parity tests that cover both direct global access and module identity:

const wt = require("node:worker_threads");
console.log(typeof globalThis.MessageChannel === "function");
console.log(typeof globalThis.MessagePort === "function");
console.log(typeof globalThis.BroadcastChannel === "function");
console.log(globalThis.MessageChannel === wt.MessageChannel);
console.log(globalThis.MessagePort === wt.MessagePort);
console.log(globalThis.BroadcastChannel === wt.BroadcastChannel);

const ch = new globalThis.MessageChannel();
console.log(ch.port1 instanceof globalThis.MessagePort);
console.log(ch.port2 instanceof globalThis.MessagePort);
ch.port1.close();
ch.port2.close();

For BroadcastChannel, also check constructor/prototype shape and close, postMessage, ref, unref, onmessage, and onmessageerror once #2507 provides the underlying implementation.

Scope / non-goals

This issue is scoped to global exposure, constructor identity, and minimal constructor/prototype shape. It should coordinate with #2507 for real message delivery, worker lifecycle, transfer semantics, and cross-thread behavior. A first fix can make the globals resolve to the same constructor objects as node:worker_threads once those constructors exist.

Duplicate search

Checked existing issues and PRs with:

• EventSource BroadcastChannel MessageChannel MessagePort global
• globalThis.MessageChannel BroadcastChannel MessagePort

No issue or PR covered the global alias/identity gap. #2507 covers the node:worker_threads object/channel implementation and should remain the underlying behavior tracker, but it does not explicitly track these global aliases.