feat: Wait for msgs to be processed before emitting stopped

[barwin]

Resolves #410

Description:

Optionally delay the emit('stopped') event until in-flight messages
from the most recent poll have been processed.

Thank you for creating and maintaining a great module. sqs-consumer has been
pleasant tool to work with.

I'm entirely open to other suggestions of how to address the problem. Maybe I'm missing something that already exists to accomplish this. Thanks in advance.

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 change)

Why is this change required?:

We are implementing a graceful shutdown of queue consumers just before issuing a process.exit() in our app

I'd like to support waiting for in-flight messages to be finalized before exiting, but it seems
the current implementation emits stopped without that guarantee.

Code changes:

• Add optional pollingCompleteWaitTimeMs option for Consumer.create() to enable the waiting behavior on shutdown.
• Add method Consumer.waitForPollingToComplete() that loops with 1s delay until all messages are processed, when feature is enabled. The current Consumer.stop() method would use this to facilitate the waiting prior to emitting stopped

---

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[barwin]

With fresh eyes, this implementation has some flaws I want to work out before review. I also now see there is a draft PR with a concept of polling status, which would be useful for this feature ..

updates pushed

[nicholasgriffintn]

Notes:

• I wonder what people's thoughts might be on if abort should also wait for polling to complete? Side thought mainly, possibly one for a discussion, just a note :).

• We may also need to move this line: https://github.com/bbc/sqs-consumer/blob/main/src/consumer.ts#L132 to when it has actually stopped, and maybe add a this.stopping state in instead.

With a similar check for stopping to this: https://github.com/bbc/sqs-consumer/blob/main/src/consumer.ts#L126

Then we might want to adjust this function to return all three states:

https://github.com/bbc/sqs-consumer/blob/main/src/consumer.ts#L151

(This comment would make it breaking though, so maybe for the future...)

[nicholasgriffintn]

Thanks for this PR btw! Looking good!

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Code Climate has analyzed commit bbfb1cc and detected 0 issues on this pull request.

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[barwin]

I have read the CLA Document and I hereby sign the CLA

[barwin]

Thanks for the feedback, @nicholasgriffintn - I've pushed changes that I think address all the requests, but left one clarification question about the date stuff.

I like your suggestion in #454 (comment) but I agree it may be nice as a follow up for later since it would be a breaking change

[nicholasgriffintn]

This is looking good btw, sorry for the delay, it's on my list :).

[barwin]

This is looking good btw, sorry for the delay, it's on my list :).

All good, thanks!