fix(instrumentation-router): prevent MaxListenersExceededWarning on response stream by bhuvan-somisetty · Pull Request #3637 · open-telemetry/opentelemetry-js-contrib

bhuvan-somisetty · 2026-07-23T15:29:51Z

Which problem is this PR solving?

When using streams or async handlers with @opentelemetry/instrumentation-router and @opentelemetry/instrumentation-express, each pending layer registers a close listener on the response object via ensureFallbackListener(). When multiple async handlers or stream events stack up, Node emits a MaxListenersExceededWarning: Possible EventEmitter memory leak detected because res.getMaxListeners() is exceeded.

Short description of the changes

Increment res.getMaxListeners() by 1 whenever attaching a fallback close listener in both @opentelemetry/instrumentation-router and @opentelemetry/instrumentation-express.
Decrement res.getMaxListeners() back down when the close event fires or when removeListener('close', ...) is called via wrappedNext / callback completion.
Added unit tests in instrumentation-router verifying that async middleware with streaming responses and pre-populated listeners do not trigger MaxListenersExceededWarning.

Type of change

Bug fix (non-breaking change which fixes an issue)
New feature (non-breaking change which adds functionality)
Breaking change (fix or feature that would cause existing functionality to not work as expected)
This change requires a documentation update

How Has This Been Tested?

Added unit tests in packages/instrumentation-router/test/index.test.ts covering async middleware and response streaming scenarios to assert no MaxListenersExceededWarning is emitted.

Checklist

Followed the style guidelines of this project
Unit tests added/updated for the change
No new warnings introduced

opentelemetry-pr-dashboard · 2026-07-23T15:30:40Z

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github-actions · 2026-07-24T07:20:43Z

This package does not have an assigned component owner and is considered unmaintained. As such this package is in feature-freeze and this PR will be closed with 14 days unless a new owner or a sponsor (a member of @open-telemetry/javascript-approvers) for the feature is found. It is the responsibility of the author to find a sponsor for this feature.
Are you familiar with this package? Consider becoming a component owner.

pichlermarc · 2026-07-31T14:58:02Z

+// `glob` always wants forward slashes in its patterns, even on Windows,
+// where `path.join` produces backslashes.
+function toGlobPattern(...parts) {
+  return path
+    .join(...parts)
+    .split(path.sep)
+    .join('/');
+}
+
 let numProbs = 0;
 function problem(...args) {
  numProbs += 1;
  if (USE_COLOR) {
    process.stdout.write('\x1b[31m');
  }
  args.unshift('bitrot error:');
  console.log(...args);
  if (USE_COLOR) {
    process.stdout.write('\x1b[39m');
  }
 }

 function warn(...args) {
  if (USE_COLOR) {
    process.stdout.write('\x1b[33m');
  }
  args.unshift('bitrot warn:');
  console.warn(...args);
  if (USE_COLOR) {
    process.stdout.write('\x1b[39m');
  }
 }

 function gitCloneSync(repo, dir) {
  execSync(`git clone ${repo} "${dir}"`);
 }
 function gitPullSync(cwd) {
  execSync('git pull', { cwd });
 }

 function isPublicPackage(pj) {
  if (pj.private === true) {
    return false;
  } else if (pj.publishConfig?.access) {
    return pj.publishConfig.access === 'public';
  } else {
    // Default is *false* for scoped packages.
    return !pj.name.startsWith('@');
  }
 }

 /**
 * BITROT: Check that the `"groupName": "OTel Core experimental",`
 * group in renovate.json matches the actual current set of experimental
 * packages from the core repo.
 */
 function bitrotRenovateCoreExperimental() {
  const renovateJson = path.join(TOP, 'renovate.json');
  const renovate = JSON.parse(fs.readFileSync(renovateJson));
  const group = renovate.packageRules.filter(
    r => r.groupName === 'OTel Core experimental'
  )[0];
  assert.ok(group, `found "OTel Core experimental" group in ${renovateJson}`);

  const ojDir = path.join(BUILD_DIR, 'opentelemetry-js');
  if (fs.existsSync(ojDir)) {
    gitPullSync(ojDir);
  } else {
    gitCloneSync(
      'https://github.com/open-telemetry/opentelemetry-js.git',
      ojDir
    );
  }

  const pkgNames = globSync(
-    path.join(ojDir, 'experimental/packages/*/package.json')
+    toGlobPattern(ojDir, 'experimental/packages/*/package.json')
  )
    .map(packageJson => JSON.parse(fs.readFileSync(packageJson)))
    .filter(pj => isPublicPackage(pj))
    .map(pj => pj.name);

  const pkgNamesSet = new Set(pkgNames);
  const renovateSet = new Set(group.matchPackageNames);
  const missing = pkgNamesSet.difference(renovateSet); // requires Node.js >=22
  const extraneous = renovateSet.difference(pkgNamesSet);

  const issues = [];
  if (missing.size) {
    issues.push(`missing entries: ${JSON.stringify(Array.from(missing))}`);
  }
  if (extraneous.size) {
    issues.push(
      `extraneous entries: ${JSON.stringify(Array.from(extraneous))}`
    );
  }
  if (issues.length) {
    problem(
      `${renovateJson}: "matchPackageNames" in the "OTel Core experimental" group does not match the current set experimental packages from the opentelemetry-js.git repo:\n  - ${issues.join(
        '\n  - '
      )}\nThe "matchPackageNames" should be:\n${JSON.stringify(
        pkgNames.sort(),
        null,
        2
      )}`
    );
  }
 }

 function getNpmInfo(name) {
  const CACHE_TIMEOUT_MS = 30 * 60 * 1000; // 30 minutes
  const cache = ensureCacheLoaded('npmInfo');
  const cacheEntry = cache[name];
  if (cacheEntry) {
    if (cacheEntry.timestamp + CACHE_TIMEOUT_MS > Date.now()) {
      return cacheEntry.value;
    } else {
      delete cache[name];
    }
  }

  // Limited security guard on exec'ing given `name`.
  const PKG_NAME_RE = /^(@[\w_.-]+\/)?([\w_.-]+)$/;
  if (!PKG_NAME_RE.test(name)) {
    throw new Error(
      `${JSON.stringify(name)} does not look like a valid npm package name`
    );
  }

  const stdout = execSync(`npm info --json "${name}"`);
  const npmInfo = JSON.parse(stdout);

  cache[name] = {
    timestamp: Date.now(),
    value: npmInfo,
  };
  saveCache();
  return npmInfo;
 }

 /**
 * BITROT: Check if instrumentations are missing support for major releases of
 * the target package.
 *
 * Limitations:
 * - This doesn't currently handle both `pg` and `pg-pool` from instr-pg.
 * - This doesn't currently support instr-aws-sdk, because of the wildcard in
 *   the packages supported: `@aws-sdk/client-*`.
 */
 function bitrotInstrumentations() {
  const instrReadmes = globSync(
-    path.join(TOP, 'packages/instrumentation-*/README.md')
+    toGlobPattern(TOP, 'packages/instrumentation-*/README.md')
  );


please remove these unrelated changes.

Hi @pichlermarc, thanks for catching that! I've rebased the branch on latest main and removed the unrelated changes.

…esponse stream

bhuvan-somisetty requested a review from a team as a code owner July 23, 2026 15:29

github-actions Bot assigned JamieDanielson, pkanal and raphael-theriault-swi Jul 23, 2026

github-actions Bot requested review from JamieDanielson, pkanal and raphael-theriault-swi July 23, 2026 15:30

github-actions Bot added pkg:instrumentation-express pkg:instrumentation-router pkg-status:unmaintained This package is unmaintained. Only bugfixes may be acceped until a new owner has been found. labels Jul 23, 2026

opentelemetry-pr-dashboard Bot mentioned this pull request Jul 23, 2026

Pull Request Dashboard #3579

Open

github-actions Bot added the pkg-status:unmaintained:autoclose-scheduled label Jul 24, 2026

pichlermarc requested changes Jul 31, 2026

View reviewed changes

fix(instrumentation-router): prevent MaxListenersExceededWarning on r…

5a57b29

…esponse stream

bhuvan-somisetty force-pushed the fix/router-stream-max-listeners-3547 branch from 011decc to 5a57b29 Compare July 31, 2026 16:10

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fix(instrumentation-router): prevent MaxListenersExceededWarning on response stream - #3637

fix(instrumentation-router): prevent MaxListenersExceededWarning on response stream#3637
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bhuvan-somisetty:fix/router-stream-max-listeners-3547

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bhuvan-somisetty commented Jul 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Which problem is this PR solving?

Short description of the changes

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Checklist

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bhuvan-somisetty commented Jul 23, 2026 •

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