test_runner: improve coverage failure diagnostics

Include the coverage filename when parsing V8 coverage data fails,
and make test-runner coverage assertions report spawned child process
output when the expected coverage report is missing.

Signed-off-by: Kamat, Trivikram <16024985+trivikr@users.noreply.github.com>
Assisted-by: openai:gpt-5.5

Author: trivikr

lib/internal/test_runner/coverage.js

@@ -33,6 +33,7 @@ const { fileURLToPath, URL } = require('internal/url');
 const { kMappings, SourceMap } = require('internal/source_map/source_map');
 const {
   codes: {
+    ERR_OPERATION_FAILED,
     ERR_SOURCE_MAP_CORRUPT,
     ERR_SOURCE_MAP_MISSING_SOURCE,
   },
@@ -402,7 +403,7 @@ class TestCoverage {
         }
 
         const coverageFile = join(this.coverageDirectory, entry.name);
-        const coverage = JSONParse(readFileSync(coverageFile, 'utf8'));
+        const coverage = readCoverageFile(coverageFile);
         this.mergeCoverage(result, this.mapCoverageWithSourceMap(coverage));
       }
 
@@ -587,6 +588,24 @@ class TestCoverage {
   }
 }
 
+function readCoverageFile(coverageFile) {
+  const rawCoverage = readFileSync(coverageFile, 'utf8');
+
+  if (rawCoverage.length === 0) {
+    throw new ERR_OPERATION_FAILED(`coverage file is empty: ${coverageFile}`);
+  }
+
+  try {
+    return JSONParse(rawCoverage);
+  } catch (err) {
+    const error = new ERR_OPERATION_FAILED(
+      `failed to parse coverage file ${coverageFile}: ${err.message}`,
+    );
+    error.cause = err;
+    throw error;
+  }
+}
+
 function toPercentage(covered, total) {
   return total === 0 ? 100 : (covered / total) * 100;
 }

test/parallel/test-runner-coverage.js

@@ -77,6 +77,22 @@ function getSpecCoverageFixtureReport() {
   return report;
 }
 
+function formatSpawnSyncResult(result) {
+  return [
+    `status: ${result.status}`,
+    `signal: ${result.signal}`,
+    `stdout:\n${result.stdout}`,
+    `stderr:\n${result.stderr}`,
+  ].join('\n');
+}
+
+function assertIncludesReport(result, report) {
+  assert(
+    result.stdout.toString().includes(report),
+    formatSpawnSyncResult(result),
+  );
+}
+
 test('test coverage report', async (t) => {
   await t.test('handles the inspector not being available', (t) => {
     if (process.features.inspector) {
@@ -111,7 +127,7 @@ test('test tap coverage reporter', skipIfNoInspector, async (t) => {
     const options = { env: { ...process.env, NODE_V8_COVERAGE: tmpdir.path } };
     const result = spawnSync(process.execPath, args, options);
     const report = getTapCoverageFixtureReport();
-    assert(result.stdout.toString().includes(report));
+    assertIncludesReport(result, report);
     assert.strictEqual(result.stderr.toString(), '');
     assert.strictEqual(result.status, 0);
     assert(findCoverageFileForPid(result.pid));
@@ -129,7 +145,7 @@ test('test tap coverage reporter', skipIfNoInspector, async (t) => {
     const result = spawnSync(process.execPath, args);
     const report = getTapCoverageFixtureReport();
 
-    assert(result.stdout.toString().includes(report));
+    assertIncludesReport(result, report);
     assert.strictEqual(result.stderr.toString(), '');
     assert.strictEqual(result.status, 0);
     assert(!findCoverageFileForPid(result.pid));
@@ -149,7 +165,7 @@ test('test spec coverage reporter', skipIfNoInspector, async (t) => {
     const result = spawnSync(process.execPath, args, options);
     const report = getSpecCoverageFixtureReport();
 
-    assert(result.stdout.toString().includes(report));
+    assertIncludesReport(result, report);
     assert.strictEqual(result.stderr.toString(), '');
     assert.strictEqual(result.status, 0);
     assert(findCoverageFileForPid(result.pid));
@@ -166,7 +182,7 @@ test('test spec coverage reporter', skipIfNoInspector, async (t) => {
     const result = spawnSync(process.execPath, args);
     const report = getSpecCoverageFixtureReport();
 
-    assert(result.stdout.toString().includes(report));
+    assertIncludesReport(result, report);
     assert.strictEqual(result.stderr.toString(), '');
     assert.strictEqual(result.status, 0);
     assert(!findCoverageFileForPid(result.pid));
@@ -187,7 +203,7 @@ test('single process coverage is the same with --test', skipIfNoInspector, () =>
   const report = getTapCoverageFixtureReport();
 
   assert.strictEqual(result.stderr.toString(), '');
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
   assert(!findCoverageFileForPid(result.pid));
 });
@@ -226,7 +242,7 @@ test('coverage is combined for multiple processes', skipIfNoInspector, () => {
   });
 
   assert.strictEqual(result.stderr.toString(), '');
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
 });
 
@@ -268,7 +284,7 @@ test.skip('coverage works with isolation=none', skipIfNoInspector, common.mustCa
   });
 
   assert.strictEqual(result.stderr.toString(), '');
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
 }, 0));
 
@@ -285,6 +301,7 @@ test('coverage reports on lines, functions, and branches', skipIfNoInspector, as
                           ]);
   assert.strictEqual(child.stderr.toString(), '');
   const stdout = child.stdout.toString();
+  assert.notStrictEqual(stdout, '', formatSpawnSyncResult(child));
   const coverage = JSON.parse(stdout);
 
   await t.test('does not include node_modules', () => {
@@ -366,7 +383,7 @@ test('coverage with ESM hook - source irrelevant', skipIfNoInspector, () => {
   const result = spawnSync(process.execPath, args, { cwd: fixture });
 
   assert.strictEqual(result.stderr.toString(), '');
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
 });
 
@@ -401,7 +418,7 @@ test('coverage with ESM hook - source transpiled', skipIfNoInspector, () => {
   const result = spawnSync(process.execPath, args, { cwd: fixture });
 
   assert.strictEqual(result.stderr.toString(), '');
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
 });
 
@@ -435,7 +452,7 @@ test('coverage with excluded files', skipIfNoInspector, () => {
     return report.replaceAll('/', '\\');
   }
 
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
   assert(!findCoverageFileForPid(result.pid));
 });
@@ -472,7 +489,7 @@ test('coverage with included files', skipIfNoInspector, () => {
     return report.replaceAll('/', '\\');
   }
 
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
   assert(!findCoverageFileForPid(result.pid));
 });
@@ -506,7 +523,7 @@ test('coverage with included and excluded files', skipIfNoInspector, () => {
     return report.replaceAll('/', '\\');
   }
 
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
   assert(!findCoverageFileForPid(result.pid));
 });
@@ -547,7 +564,7 @@ test('correctly prints the coverage report of files contained in parent director
   });
 
   assert.strictEqual(result.stderr.toString(), '');
-  assert(result.stdout.toString().includes(report));
+  assertIncludesReport(result, report);
   assert.strictEqual(result.status, 0);
 });
 
@@ -564,5 +581,5 @@ test('coverage with directory and file named "file"', skipIfNoInspector, () => {
 
   assert.strictEqual(result.stderr.toString(), '');
   assert.strictEqual(result.status, 0);
-  assert(result.stdout.toString().includes('start of coverage report'));
+  assertIncludesReport(result, 'start of coverage report');
 });