[WIP] Add hooks necessary for Chrome coverage

[wagenet]

This adds hooks necessary for gathering coverage data directly from Chrome. It's a bit rough right now and untested but it does work. I'd love to see if we can actually get this to full first-class support.

To make use of this, I have this additional code in my own app:

/* eslint-disable no-undef */
const chromeRemoteInterface = require('chrome-remote-interface');
const fs = require('node:fs');

const { mergeProcessCovs } = require('@bcoe/v8-coverage');

function buildContext({ coverageDir }) {
	const remoteInterfaces = {};

	if (!fs.existsSync(coverageDir)) {
		fs.mkdirSync(coverageDir);
	}

	return {
		remoteInterfaces,

		config: {
			// this here will be the browser runner
			async browser_on_dev_tools_ready() {
				const match = this.devToolsUrl.match(/ws:\/\/(.+):(\d+)\//);
				if (match) {
					const [_, host, port] = match;
					const remoteInterface = await chromeRemoteInterface({ host, port });
					remoteInterfaces[this.launcherId] = remoteInterface;

					const { Profiler } = remoteInterface;
					await Profiler.enable();
					await Profiler.startPreciseCoverage({ callCount: true, detailed: true });
				}
			},

			// this here will be the browser runner
			async browser_on_before_finish() {
				const remoteInterface = remoteInterfaces[this.launcherId];
				if (remoteInterface) {
					// Clear it out so we don't do it twice
					remoteInterfaces[this.launcherId] = null;

					const { Profiler } = remoteInterface;

					const results = await Profiler.takePreciseCoverage();

					const coveragePath = `${coverageDir}/coverage-${this.launcherId}.json`;
					fs.writeFileSync(coveragePath, JSON.stringify(results, null, 2));

					remoteInterface.close();
				}
			},

			// this here will be the internal reporter
			after_reporter_finish() {
				let coverage;

				const pastCoveragePath = `${coverageDir}/past-coverage.json`;
				if (fs.existsSync(pastCoveragePath)) {
					const coverageData = fs.readFileSync(pastCoveragePath);
					coverage = JSON.parse(coverageData);
				}

				for (const id of Object.keys(remoteInterfaces)) {
					const coverageData = fs.readFileSync(`${coverageDir}/coverage-${id}.json`);
					const coverageJson = JSON.parse(coverageData);
					if (coverage) {
						coverage = mergeProcessCovs([coverage, coverageJson]);
					} else {
						coverage = coverageJson;
					}
				}

				const jsonString = JSON.stringify(coverage, null, 2);

				console.log(`Merged ${Object.keys(remoteInterfaces).length} files.`);
				console.log(`Total size: ${jsonString.length}`);
				console.log(`Writing coverage to ${coverageDir}/coverage.json.`);
				fs.writeFileSync(`${coverageDir}/coverage.json`, jsonString, 'utf8');
			},
		},
	};
}

module.exports = { buildContext };

Then in my testem.js:

const { buildContext } = require('testem-helpers');

const coverageContext = buildContext({ coverageDir: path.join(__dirname, 'coverage') });

module.exports = {
	// other config
	..coverageContext.config,
};

[NullVoxPopuli]

would love to see this landed <3 🎉
looks interesting 👀