Bug: State leaks when throwOnEmptyMetrics throws — dimensions and metadata carry over to next publish

[svozza]

Expected Behavior

When publishStoredMetrics() throws due to throwOnEmptyMetrics, the internal state (dimensions, metadata) should still be cleaned up. A subsequent publishStoredMetrics() call should start with a clean slate.

Current Behavior

In publishStoredMetrics() at Metrics.ts:603-622:

if (!this.disabled) {
  const emfOutput = this.serializeMetrics();  // ← throws here (throwOnEmptyMetrics)
  hasMetrics && this.console.log(JSON.stringify(emfOutput));
}

this.clearMetrics();      // ← never reached
this.clearDimensions();   // ← never reached
this.clearMetadata();     // ← never reached

When serializeMetrics() throws (due to throwOnEmptyMetrics), the cleanup code on lines 618-620 never runs. Dimensions and metadata from the failed publish are retained and leak into the next publishStoredMetrics() call.

Code snippet

import { Metrics, MetricUnit } from '@aws-lambda-powertools/metrics';

const metrics = new Metrics({ namespace: 'test' });
metrics.setThrowOnEmptyMetrics(true);
metrics.addDimension('request_id', 'abc-123');
metrics.addMetadata('trace_id', 'xyz-789');

// First publish throws — no metrics added
try {
  metrics.publishStoredMetrics();
} catch {
  // expected
}

// Second publish — dimensions and metadata from the failed publish leak through
metrics.setThrowOnEmptyMetrics(false);
metrics.addMetric('test', MetricUnit.Count, 1);
metrics.publishStoredMetrics();
// ❌ The EMF output contains request_id: 'abc-123' and trace_id: 'xyz-789'
//    even though they should have been cleared

Steps to Reproduce

1. Create a Metrics instance, enable throwOnEmptyMetrics
2. Add a dimension and metadata, but no metrics
3. Call publishStoredMetrics() — it throws
4. Catch the error, disable throwOnEmptyMetrics, add a metric, publish again
5. Observe the leaked dimension and metadata in the second EMF output

Possible Solution

Wrap the serialize/emit block in try/finally to guarantee cleanup:

try {
  if (!this.disabled) {
    const emfOutput = this.serializeMetrics();
    hasMetrics && this.console.log(JSON.stringify(emfOutput));
  }
} finally {
  this.clearMetrics();
  this.clearDimensions();
  this.clearMetadata();
}

Powertools for AWS Lambda (TypeScript) version

2.33.0

AWS Lambda function runtime

22.x

Packaging format used

npm

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