worker,coverage: support V8 coverage generation

lib/internal/process/coverage.js

@@ -1,9 +1,7 @@
 'use strict';
 const path = require('path');
 const { mkdirSync, writeFileSync } = require('fs');
-// TODO(addaleax): add support for coverage to worker threads.
-const hasInspector = process.config.variables.v8_enable_inspector === 1 &&
-  require('internal/worker').isMainThread;
+const hasInspector = process.config.variables.v8_enable_inspector === 1;
 let inspector = null;
 if (hasInspector) inspector = require('inspector');
 
@@ -14,7 +12,9 @@ function writeCoverage() {
     return;
   }
 
-  const filename = `coverage-${process.pid}-${Date.now()}.json`;
+  const { threadId } = require('internal/worker');
+
+  const filename = `coverage-${process.pid}-${Date.now()}-${threadId}.json`;
   try {
     // TODO(bcoe): switch to mkdirp once #22302 is addressed.
     mkdirSync(process.env.NODE_V8_COVERAGE);

test/fixtures/v8-coverage/worker.js

@@ -0,0 +1,5 @@
+'use strict';
+const { Worker } = require('worker_threads');
+const path = require('path');
+
+new Worker(path.resolve(__dirname, 'subprocess.js'));

test/parallel/test-v8-coverage.js

@@ -79,6 +79,23 @@ function nextdir() {
   assert.strictEqual(fixtureCoverage.functions[2].ranges[1].count, 0);
 }
 
+// outputs coverage from worker.
+{
+  const coverageDirectory = path.join(tmpdir.path, nextdir());
+  const output = spawnSync(process.execPath, [
+    '--experimental-worker',
+    require.resolve('../fixtures/v8-coverage/worker')
+  ], { env: { ...process.env, NODE_V8_COVERAGE: coverageDirectory } });
+  assert.strictEqual(output.status, 0);
+  const fixtureCoverage = getFixtureCoverage('subprocess.js',
+                                             coverageDirectory);
+  assert.ok(fixtureCoverage);
+  // first branch executed.
+  assert.strictEqual(fixtureCoverage.functions[2].ranges[0].count, 1);
+  // second branch did not execute.
+  assert.strictEqual(fixtureCoverage.functions[2].ranges[1].count, 0);
+}
+
 // does not output coverage if NODE_V8_COVERAGE is empty.
 {
   const coverageDirectory = path.join(tmpdir.path, nextdir());