node:child_process: expose _forkChild IPC bootstrap helper #2717

Summary

Node exposes child_process._forkChild(fd, serializationMode) as an enumerable function on the node:child_process namespace. Perry's child-process surface now includes fork() and IPC-related work, but _forkChild itself is absent from the manifest, generated declarations, and native namespace callable list.

This is an internal/legacy export, but it is part of Node's observable Object.keys(require("node:child_process")) surface and is used by Node's child-process IPC bootstrap path to install process.send() / channel handling in forked children.

Expected Node behavior

With Node v25.9.0:

const cp = require("node:child_process");

console.log(Object.keys(cp)); // includes "_forkChild"
console.log(Object.prototype.propertyIsEnumerable.call(cp, "_forkChild")); // true
console.log(typeof cp._forkChild); // "function"
console.log(cp._forkChild.length); // 2
console.log(cp._forkChild.name); // "_forkChild"

Calling the helper participates in Node's IPC child bootstrap and expects an IPC fd plus serialization mode; invalid direct calls throw from that bootstrap path rather than resolving to undefined.

Current Perry evidence

On origin/main:

• crates/perry-api-manifest/src/entries.rs registers child_process.exec, execSync, execFile, execFileSync, spawn, spawnSync, fork, ChildProcess, and the Node-undefined Stream compatibility property, but no _forkChild entry.
• crates/perry-runtime/src/object/native_module.rs::is_native_module_callable_export() lists the same child-process callable exports but omits ("child_process", "_forkChild").
• Source search across crates, docs, test-files, and test-parity finds no _forkChild / forkChild implementation, declaration, or fixture.
• Existing child-process work such as merged PR feat(node:child_process): async spawn reactor, fork()+IPC, exec error/encoding parity (#1933–#1938) #1955 covers async spawn, fork() + IPC, and exec parity, while open node:child_process: honor spawn and exec option semantics beyond cwd/env/shell #2555 is about spawn/exec option semantics; neither tracks this namespace helper export.

I did not use a current-tree target/release/perry binary for this check; this is Node-probe plus current source evidence.

Suggested test surface

Add a small shape fixture under test-parity/node-suite/child_process/:

import * as cp from "node:child_process";

console.log("_forkChild enumerable:", Object.prototype.propertyIsEnumerable.call(cp, "_forkChild"));
console.log("_forkChild type:", typeof cp._forkChild);
console.log("_forkChild length:", cp._forkChild?.length);
console.log("_forkChild name:", cp._forkChild?.name);

If a minimal implementation is added before full IPC bootstrap behavior, the fixture can stay shape-only and a follow-up can cover valid fd/serialization behavior.

Scope / non-goals

• Scope: expose the enumerable _forkChild function shape on the child_process namespace, generated declarations/reference docs, and captured-property reads.
• Scope: eventually route valid calls into the child IPC bootstrap path if Perry needs parity for Node forked-child startup.
• Non-goal: changing child_process.fork() lifecycle behavior or spawn/exec option semantics (node:child_process: honor spawn and exec option semantics beyond cwd/env/shell #2555).
• Non-goal: reintroducing child_process.Stream; current source comments correctly note Node returns undefined for that name in current Node.