feat: added dynamic bucket region session option

[pregnor]

Scope

Minor (Feature)

What

1. Added an AWS session initialization option for dynamically retrieving the region of the S3 bucket which is used in the helm command.
2. Updated the calls to the session constructor to use this option.
3. Added unit test cases covering all paths of this option.

Why

Without the correct region being set currently the plugin throws an error:

fetch from s3 uri=s3://pregnor-helm-s3-repo-test/stable/cadence/cadence-0.18.0.tgz: fetch object from s3: BucketRegionError: incorrect region, the bucket is not in 'us-west-2' region at endpoint ''
        status code: 301, request id: , host id: 
Error: plugin "bin/helms3" exited with error

We wanted to allow the plugin to work without manual region setting and also support multiple repositories residing in different regions without updating the regions in-between fetching charts from those repositories. This is especially relevant when the plugin is used in a Terraform environment.

How

The HEAD {{bucket}} request allows querying the bucket's actual region without prior knowledge with the proper configuration.

Unfortunately this behavior is not explicitly documented - or at least I couldn't find it - but the AWS SDK Go team confirmed in an issue comment this is an intended and supported behavior of the endpoint.

If the option runs into any issue, it has no effect on the session/configuration as a fallback mechanism. In such a case the plugin's behavior remains the same as before, namely falling back to the HELM_S3_REGION environment variable's value or to the corresponding AWS environment variable's value (AWS_REGION or AWS_DEFAULT_REGION).

Notes

The buckets used in the unit tests were found randomly, they are not mine and I do not have access to them, but they fit the test cases perfectly. They can be replaced with different bucket names if preferred.

I ran into issues when I tried to run the end to end tests, the local variant didn't work with my macOS Big Sur machine and the non-local variant didn't like the AWS users I tried to use it with so I couldn't run those tests myself.
The added unit tests should prove the solution works as expected.
Also the option can be trivially extracted to a self-contained test to run outside the plugin for more advanced testing purposes.

I also tested manually by installing the development version of the plugin and fetching a chart from an S3 repository in a different region than the set one.

I ran these tests on my buckets which had a fine-grained permission policy blocking public request to reassure the solution works for those buckets as well.

[codecov-commenter]

Codecov Report

Merging #146 (c21ba4e) into master (e208d4e) will increase coverage by 4.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
+ Coverage   40.80%   45.01%   +4.20%     
==========================================
  Files          18       18              
  Lines         272      291      +19     
==========================================
+ Hits          111      131      +20     
+ Misses        151      150       -1     
  Partials       10       10              

Impacted Files	Coverage Δ
internal/awsutil/session.go	94.59% <100.00%> (+11.26%)	⬆️

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[pregnor]

@hypnoglow please take a look at this when you can.
(I don't have access rights to set reviewers/assignees, so I'm commenting instead, don't mean to be pushy, just making sure the proper notification is done.)

Update: Couldn't wait any longer for the feature to be merged upstream, so I had to fit our fork for use, release a version there and use it instead.

[hypnoglow]

This is a great feature, thanks a lot!