[FR]: Use jest-monocart-coverage with v8 and lcov instead of c8

[Aghassi]

What is the current behavior?

The current coverage reporters are not accurate enough and lead to bad line assignments and incorrect coverage values.

Describe the feature

The current coverage behavior is actually kind of buggy and I've spent a month debugging it so I figured I'd ask for this to be added. On our end, we've patched our way out of the coverage collection because it just doesn't do what we want and it has been cumbersome to get right.

How I fixed this on my end:

1. I needed to add flags around

   rules_jest/jest/private/jest_config_template.mjs

   Lines 152 to 165 in 940a9fe

   	let coverageFile = path.basename(process.env.COVERAGE_OUTPUT_FILE);
   	let coverageDirectory = path.dirname(process.env.COVERAGE_OUTPUT_FILE);
   	if (process.env.SPLIT_COVERAGE_POST_PROCESSING == "1") {
   	// in split coverage post processing mode bazel assumes that the COVERAGE_OUTPUT_FILE
   	// will be created by lcov_merger which runs as a separate action with everything in
   	// COVERAGE_DIR provided as inputs. so we'll just create the final coverage at
   	// `COVERAGE_DIR/coverage.dat` which then later moved by merger.sh to final location.
   	coverageDirectory = process.env.COVERAGE_DIR;
   	coverageFile = "coverage.dat";
   	}
   	config.coverageDirectory = coverageDirectory;
   	config.coverageReporters = ["text", ["lcovonly", { file: coverageFile }]];

   that let's me eject from this so I can set these variables myself. That may not be necessary for this feature. The main thing is not to set coverageReporters if one is provided. I also removed

   rules_jest/jest/private/jest_config_template.mjs

   Lines 174 to 179 in 940a9fe

   	config.collectCoverageFrom = readFileSync(process.env.COVERAGE_MANIFEST)
   	.toString("utf8")
   	.split("\n")
   	.filter((f) => f != "")
   	.map((f) => path.relative(jestConfigDir, f));
   	}

   as it never worked right and messed up split coverage in our case.
2. Set coverageReporters to coverageReporters: ['none'],
3. Add the following to reporters

    [
     'jest-monocart-coverage',
      {
        name: 'Coverage Report',
        outputDir: `${process.env.COVERAGE_DIR}/jest_mcr_temp`,
        reports: [['v8'], ['lcovonly']],
        sourceFilter: '**/*.{ts,tsx}',
      },
    ],

4. Create a globalTeardown script with the following

import fs from 'fs'
import path from 'path'

const outputDir = `${process.env.COVERAGE_DIR}/jest_mcr_temp`

export default async function globalTeardown() {
  // If bazel coverage is being run, otherwise don't do this
  if (process.env.COVERAGE_DIR) {
    /**
     * generate the lcov report by merging all the existing
     * lcov info files from the coverage dir generated
     * by each test
     */
    const filePath = path.join(outputDir, 'lcov.info')

    /**
     * We nest in a dir because otherwise we will gather
     * json files that bazel itself generates and we wish to avoid
     * gathering.
     */
    if (!fs.existsSync(path.dirname(filePath))) {
      fs.mkdirSync(path.dirname(filePath), { recursive: true })
    }
    // If the lcov file doesn't exist, make one otherwise
    // coverage fails. This applies to non standard file types like
    // html, json, etc.
    if (!fs.existsSync(filePath)) {
      fs.writeFileSync(filePath, '', {
        encoding: 'utf8',
      })
    }
    // Following is taken from https://github.com/aspect-build/rules_jest/blob/940a9fe3c319e1351352e2ffa889e05db790343e/jest/private/jest_config_template.mjs#L151C1-L162
    let coverageFile = path.basename(process.env.COVERAGE_OUTPUT_FILE as string)
    let coverageDirectory = path.dirname(
      process.env.COVERAGE_OUTPUT_FILE as string
    )
    if (process.env.SPLIT_COVERAGE_POST_PROCESSING === '1') {
      // in split coverage post processing mode bazel assumes that the COVERAGE_OUTPUT_FILE
      // will be created by lcov_merger which runs as a separate action with everything in
      // COVERAGE_DIR provided as inputs. so we'll just create the final coverage at
      // `COVERAGE_DIR/split_coverage.dat` which then later moved by merger.sh to final location.
      coverageDirectory = process.env.COVERAGE_DIR as string
      coverageFile = 'coverage.dat'
    }
    // end of rules_js code

    // Write coverage data to a file that Bazel knows how to handle
    await fs.promises.copyFile(
      path.join(outputDir, 'lcov.info'),
      path.join(coverageDirectory, coverageFile)
    )
  }
}

5. add the teardown to the config

  globalTeardown: path.resolve(__dirname, 'globalTeardown.js'),