`NODE_V8_COVERAGE` inconsistent when used with workers

[AriPerkkio]

Version

v19.5.0

Platform

Darwin Aris-MacBook-Air.local 22.2.0 Darwin Kernel Version 22.2.0: Fri Nov 11 02:06:26 PST 2022; root:xnu-8792.61.2~4/RELEASE_ARM64_T8112 arm64

Subsystem

No response

What steps will reproduce the bug?

/Users/x/x/repro
├── index.mjs
└── worker.js

// worker.js
console.log("worker::NODE_V8_COVERAGE", process.env.NODE_V8_COVERAGE);

// index.mjs
import { Worker } from "worker_threads";
import { existsSync } from "fs";
import { resolve } from "path";

const NODE_V8_COVERAGE = resolve("./coverage");

const worker = new Worker("./worker.js", { env: { NODE_V8_COVERAGE } });

await new Promise((resolve, reject) => {
  worker.on("exit", resolve);
  worker.on("error", reject);
});

// Check if V8 coverage report was written to file system
console.log("existsSync(NODE_V8_COVERAGE)", existsSync(NODE_V8_COVERAGE));

$ rm -rf ./coverage # Just to be sure setup is clean
$ node index.mjs 

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

Always.

What is the expected behavior?

Worker should use options.env.NODE_V8_COVERAGE and output the coverage report there.

What do you see instead?

No coverage report is written. However worker logs process.env.NODE_V8_COVERAGE correctly.

$ rm -rf ./coverage # Just to be sure setup is clean
$ node index.mjs 

> worker::NODE_V8_COVERAGE /Users/x/y/repro/coverage
> existsSync(NODE_V8_COVERAGE) false

Worker only outputs coverage report if main thread has process.env.NODE_V8_COVERAGE set. It does not matter what value main thread has. Worker will use the one it was given in options. This is demonstrated below.

If the NODE_V8_COVERAGE is present on main thread, worker will report coverage. The worker.js is seen in ./coverage/<x>.json report.

- const worker = new Worker("./worker.js", { env: { NODE_V8_COVERAGE } });
+ process.env.NODE_V8_COVERAGE = NODE_V8_COVERAGE;
+ const worker = new Worker("./worker.js");

$ rm -rf ./coverage # Just to be sure setup is clean
$ node index.mjs 

worker::NODE_V8_COVERAGE /Users/x/y/repro/coverage
existsSync(NODE_V8_COVERAGE) true

To make things even more inconsistent, let's try having different values for NODE_V8_COVERAGE in main thread and worker.

const NODE_V8_COVERAGE = resolve("./coverage");
...
+ process.env.NODE_V8_COVERAGE = resolve("./coverage-main-thread"); // This won't be used at all, but there has to be some value in main thread's env for workers to work
const worker = new Worker("./worker.js", { env: { NODE_V8_COVERAGE } });

$ rm -rf ./coverage # Just to be sure setup is clean
$ node index.mjs 

> worker::NODE_V8_COVERAGE /Users/x/y/repro/coverage
> existsSync(NODE_V8_COVERAGE) true

$ ls
coverage    index.mjs    worker.js

The main thread's process.env.NODE_V8_COVERAGE is ignored by worker - there is no ./coverage-main-thread directory on file system.
However if main thread's process.env.NODE_V8_COVERAGE is not present, worker won't output coverage at all.

Additional information

It does not matter if worker.js runs takeCoverage():

// worker.js
console.log("worker::NODE_V8_COVERAGE", process.env.NODE_V8_COVERAGE);
+ require("v8").takeCoverage();

To summarize:

1. Worker does output V8 coverage if main thread has process.env.NODE_V8_COVERAGE set ✅
2. Worker does not output V8 coverage if main thread has no process.env.NODE_V8_COVERAGE set, and worker is given options.env.NODE_V8_COVERAGE ❌
3. Worker outputs coverage to options.env.NODE_V8_COVERAGE and not to main thread's process.env.NODE_V8_COVERAGE. If main thread's process.env.NODE_V8_COVERAGE is removed, no report is generated. So why is main threads environment variable required to be present? 😕

[bnoordhuis]

@nodejs/inspector @bcoe my hunch is that it's caused by this block of code:

node/src/node.cc

Lines 186 to 195 in 0a46107

	inspector_agent_->Start(inspector_path,
	options_->debug_options(),
	inspector_host_port(),
	is_main);
	if (options_->debug_options().inspector_enabled &&
	!inspector_agent_->IsListening()) {
	return;
	}
	profiler::StartProfilers(this);

I.e., returns before calling StartProfilers().