Prevent sending response headers if already sent in default error han…

[suhailgupta03]

Summary

This pull request introduces a safeguard in the defaultProcessEventErrorHandler to prevent attempts to send HTTP response headers after they have already been transmitted. This situation can occur when ack() is invoked before an error is thrown during event processing, leading to potential conflicts in the response handling.

The goal of this PR is to enhance the robustness of the error handling mechanism by ensuring that the server does not encounter ERR_HTTP_HEADERS_SENT errors, which can disrupt the normal flow of Slack event processing. By checking the response.headersSent flag before setting headers or ending the response, we maintain the integrity of the HTTP transaction and provide a more reliable experience for developers using Bolt.js.

Example Scenario

Consider a custom event handler that acknowledges an event and then performs an asynchronous operation which fails:

app.event('custom_event', async ({ ack, say }) => {
  try {
    await ack();
    // An asynchronous operation that may fail
    await someAsyncOperation();
    await say('Event processed successfully!');
  } catch (error) {
    // Error handling logic that might inadvertently try to re-ack or send a response
    console.error('An error occurred:', error);
    // If the following statement crashes, it will cause a server crash
    // because the headers have already been sent when ack() was called.
    await performCleaning();
  }
});

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @suhailgupta03 to sign the Salesforce Inc. Contributor License Agreement.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (3142f9a) 82.21% compared to head (9a10a61) 82.00%.

Files	Patch %	Lines
src/receivers/HTTPModuleFunctions.ts	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2006      +/-   ##
==========================================
- Coverage   82.21%   82.00%   -0.22%     
==========================================
  Files          18       18              
  Lines        1524     1528       +4     
  Branches      438      439       +1     
==========================================
  Hits         1253     1253              
- Misses        175      178       +3     
- Partials       96       97       +1     

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[seratch]

This is a good catch! Thanks for sending this PR. Could you amend the logging part a little?

[seratch]

Can you adjust the logging for consistency with the following logic?

Suggested change

	logger.error('Headers already sent, cannot send another response');
	logger.error('An unhandled error occurred after ack() called in a listener');
	logger.debug(`Error details: ${error}, storedResponse: ${storedResponse}`);

[suhailgupta03]

@seratch Updated.

[seratch]

Also, perhap adding unit tests for this pattern may not be an easy task. It's okay to skip the codecov error this time

[seratch]

Thank you! Looks good to me 👍