Enable replacing use of os.Getenv and os.LookupEnv with a custom function

[gwvandesteeg]

Preflight Checklist

• I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

When trying to do configuration tests with viper and reading from the environment the tests cannot be run in parallel and any environment variables need to be set with t.Setenv, by making the function configurable we can provide many more options on how to do quick testing and parsing of configurations.

Proposed Solution

By adding the ability to change the os.LookupEnv function using an Option to the viper instance (defaulting to os.Lookupenv) we can ensure these tests can be run in parallel and can have their lookups mocked very simply.

Add a new attribute to the Viper struct

envLookupFunc func(key string) (string, bool)

set the default value in New to v.envLookupFunc = os.LookupEnv
add a new option

func EnvLookupFunction(fn func(string) (string, bool)) Option {
  return optionFunc(func(v *Viper) { v.envLookupFunc = fn })
}

Change the usage of os.Getenv to use os.LookupEnv instead, or apply the same technique as above.

Alternatives Considered

No response

Additional Information

This feature is based upon the testing system described in the below article where tests pass a custom os.Getenv function to the call to the run code to simulate different configurations.

https://grafana.com/blog/2024/02/09/how-i-write-http-services-in-go-after-13-years/

[github-actions]

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