Check error before wrapping

[codesome]

This is a broken down piece of #627

During compaction, Postings() and Series() functions are called a lot of times. Checking error before wrapping saves a lot of allocs apparently.

• Add this in https://github.com/prometheus/prometheus/blob/master/CONTRIBUTING.md

[codesome]

I would not like any PRs linked with #627 to go in the next release (want them all to go in together in a release, just a personal preferance). So don't merge this yet though it's trivial!

[bwplotka]

I would remove those comments. Non nil err wrapping is a good practice overall and I would rather mention it in README in some small "Style guide" section or even on Prometheus side. We should do that everywhere - never wrap a nil error.

Can we add this to README then as a start?

"Wrap errors only on error as they increase cost some performance...."

[bwplotka]

Otherwise we will need to add this comment on every error TBH for consistency (:

[bwplotka]

Thanks @codesome 👍

[codesome]

I have removed the comment. And I think Style Guide would go in prometheus/prometheus, specifically this file https://github.com/prometheus/prometheus/blob/master/CONTRIBUTING.md. Some sections needs to start there to add specific stuff, I will make a point to add it there.

[codesome]

Oops #644 (comment) , but I guess it's fine 😅

[bwplotka]

🤦‍♂️

This one is trival though. Let's wait with others. ;p