almost working coverage

package.json

@@ -14,6 +14,8 @@
   },
   "dependencies": {
     "@jest/expect": "^29.0.1",
+    "@jest/transform": "^29.0.1",
+    "collect-v8-coverage": "^1.0.1",
     "jest-circus": "^29.0.1",
     "jest-each": "^29.0.1",
     "jest-mock": "^29.0.1",

src/index.js

@@ -1,16 +1,20 @@
 import { Piscina } from "piscina";
 import supportsColor from "supports-color";
 import { MessageChannel } from "worker_threads";
+import { shouldInstrument } from "@jest/transform";
+import { fileURLToPath } from "url";
 
 /** @typedef {import("@jest/test-result").Test} Test */
 
 export default class LightRunner {
   // TODO: Use real private fields when we drop support for Node.js v12
   _config;
   _piscina;
+  _testContext;
 
-  constructor(config) {
+  constructor(config, testContext) {
     this._config = config;
+    this._testContext = testContext;
 
     // Jest's logic to decide when to spawn workers and when to run in the
     // main thread is quite complex:
@@ -36,6 +40,36 @@ export default class LightRunner {
     });
   }
 
+  filterCoverage(result, projectConfig) {
+    if (!result.v8Coverage) {
+      return result;
+    }
+
+    const coverageOptions = {
+      changedFiles: this._testContext.changedFiles,
+      collectCoverage: true,
+      collectCoverageFrom: this._config.collectCoverageFrom,
+      collectCoverageOnlyFrom: this._config.collectCoverageOnlyFrom,
+      coverageProvider: this._config.coverageProvider,
+      sourcesRelatedToTestsInChangedFiles:
+        this._testContext.sourcesRelatedToTestsInChangedFiles,
+    };
+
+    return {
+      ...result,
+      v8Coverage: result.v8Coverage
+        .filter(res => res.url.startsWith("file://"))
+        .map(res => ({ ...res, url: fileURLToPath(res.url) }))
+        .filter(
+          ({ url }) =>
+            // TODO: will this work on windows? It might be better if `shouldInstrument` deals with it anyways
+            url.startsWith(projectConfig.rootDir) &&
+            shouldInstrument(url, coverageOptions, projectConfig)
+        )
+        .map(result => ({ result })),
+    };
+  }
+
   /**
    * @param {Array<Test>} tests
    * @param {*} watcher
@@ -44,7 +78,12 @@ export default class LightRunner {
    * @param {*} onFailure
    */
   runTests(tests, watcher, onStart, onResult, onFailure) {
-    const { updateSnapshot, testNamePattern } = this._config;
+    const {
+      updateSnapshot,
+      testNamePattern,
+      collectCoverage,
+      coverageProvider,
+    } = this._config;
 
     return Promise.all(
       tests.map(test => {
@@ -54,11 +93,21 @@ export default class LightRunner {
 
         return this._piscina
           .run(
-            { test, updateSnapshot, testNamePattern, port: mc.port1 },
+            {
+              test,
+              updateSnapshot,
+              testNamePattern,
+              port: mc.port1,
+              collectV8Coverage: collectCoverage && coverageProvider === "v8",
+            },
             { transferList: [mc.port1] }
           )
           .then(
-            result => void onResult(test, result),
+            result =>
+              void onResult(
+                test,
+                this.filterCoverage(result, test.context.config)
+              ),
             error => void onFailure(test, error)
           );
       })

src/worker-runner.js

@@ -6,6 +6,7 @@ import { expect } from "expect";
 import * as circus from "jest-circus";
 import { inspect } from "util";
 import { isWorkerThread } from "piscina";
+import { CoverageInstrumenter } from "collect-v8-coverage";
 
 /** @typedef {{ failures: number, passes: number, pending: number, start: number, end: number }} Stats */
 /** @typedef {{ ancestors: string[], title: string, duration: number, errors: Error[], skipped: boolean }} InternalTestResult */
@@ -54,6 +55,7 @@ export default async function run({
   updateSnapshot,
   testNamePattern,
   port,
+  collectV8Coverage,
 }) {
   const projectSnapshotSerializers = await initialSetup(test.context.config);
 
@@ -67,6 +69,13 @@ export default async function run({
   /** @type {Array<InternalTestResult>} */
   const results = [];
 
+  let instrumenter = null;
+  if (collectV8Coverage) {
+    instrumenter = new CoverageInstrumenter();
+
+    await instrumenter.startInstrumenting();
+  }
+
   const { tests, hasFocusedTests } = await loadTests(test.path);
 
   const snapshotResolver = await snapshot.buildSnapshotResolver(
@@ -86,13 +95,18 @@ export default async function run({
   await runTestBlock(tests, hasFocusedTests, testNamePatternRE, results, stats);
   stats.end = performance.now();
 
+  let v8Coverage = undefined;
+  if (instrumenter) {
+    v8Coverage = await instrumenter.stopInstrumenting();
+  }
+
   snapshotState.save();
 
   // Restore the project-level serializers, so that serializers
   // installed by one test file don't leak to other files.
   arrayReplace(snapshot.getSerializers(), projectSnapshotSerializers);
 
-  return toTestResult(stats, results, test);
+  return toTestResult(stats, results, test, v8Coverage);
 }
 
 async function loadTests(testFile) {
@@ -236,9 +250,10 @@ function callAsync(fn) {
  * @param {Stats} stats
  * @param {Array<InternalTestResult>} tests
  * @param {import("@jest/test-result").Test} testInput
+ * @param {import("@jest/test-result").V8CoverageResult} v8Coverage
  * @returns {import("@jest/test-result").TestResult}
  */
-function toTestResult(stats, tests, { path, context }) {
+function toTestResult(stats, tests, { path, context }, v8Coverage) {
   const { start, end } = stats;
   const runtime = end - start;
 
@@ -285,6 +300,7 @@ function toTestResult(stats, tests, { path, context }) {
         title: test.title,
       };
     }),
+    v8Coverage,
   };
 }
 

yarn.lock

@@ -1031,7 +1031,7 @@ __metadata:
   languageName: node
   linkType: hard
 
-"collect-v8-coverage@npm:^1.0.0":
+"collect-v8-coverage@npm:^1.0.0, collect-v8-coverage@npm:^1.0.1":
   version: 1.0.1
   resolution: "collect-v8-coverage@npm:1.0.1"
   checksum: 4efe0a1fccd517b65478a2364b33dadd0a43fc92a56f59aaece9b6186fe5177b2de471253587de7c91516f07c7268c2f6770b6cbcffc0e0ece353b766ec87e55
@@ -1655,6 +1655,8 @@ __metadata:
   resolution: "jest-light-runner@workspace:."
   dependencies:
     "@jest/expect": ^29.0.1
+    "@jest/transform": ^29.0.1
+    collect-v8-coverage: ^1.0.1
     jest-circus: ^29.0.1
     jest-each: ^29.0.1
     jest-mock: ^29.0.1