KeyStreamerAt

[thomascoquet]

The PR concerns the following changes:

• introduction of the KeyStreamerAt interface to stream blocks instead of fetching them using KeyReaderAt. This has the benefits of reducing lock contention and allocation of large byte buffers.
• refacto of existing handlers so that they implement both KeyStreamerAt and the original API KeyReaderAt using a wrapper.

[coveralls]

Pull Request Test Coverage Report for Build 1241102608

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 29 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.9%) to 88.624%

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Files with Coverage Reduction	New Missed Lines	%
s3.go	1	72.88%
gcs.go	3	65.91%
http.go	7	52.24%
adapter.go	18	95.96%

Totals
Change from base Build 1097200852:	-0.9%
Covered Lines:	631
Relevant Lines:	712

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💛 - Coveralls

[tbonfort]

Thanks for the contribution, there are a few changes I'm requesting, feel free to reach out if they aren't clear enough.

[thomascoquet]

Hey, did the requested changes!
I wanted to make existing code benefit from the streaming hence the awkwardness in the handlers, but I agree it is cleaner to let the existing NewAdapter as is and create another one for streaming.

[tbonfort]

actually, after having thought about it a bit more, I believe we should internally only rely on the streamAt interface. KeyReaderAt and RegisterAdapter should be marked as deprecated and fall back to wrapping the provided KeyReaderAt as a KeyStreamerAt. Makes sense?

[thomascoquet]

Yes, this is what I originally thought as well. But it means the original external API needs to be modified?
It makes sense to go from KeyStreamerAt -> KeyReaderAt; the opposite is less logical.

[tbonfort]

I've tried to clarify through these comments. The suggested changes do not modify the existing API, and transparently handles the case when the given reader only implements KeyReaderAt

[tbonfort]

hmm, the handler.ReadAt methods can't be removed actually to maintain KeyReaderAt interface. They can just panic as they will never be used

[thomascoquet]

Made the changes and deprecated all KeyReaderAt methods.
I tried to update the comments regarding error conventions, could use a thorough second look!
The linting is failing because of io.NopCloser on golang 1.15, do you prefer I use ioutil?