Support XDG_CACHE_HOME environment variable

[rafi]

By using the XDG_CACHE_HOME environment variable, people who set kubectl's config file elsewhere (using the KUBECONFIG environment variable) could now bypass the error that ~/.kube directory doesn't exist, and also support the XDG standard :)

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[rafi]

I signed it!

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[ahmetb]

@rafi thanks for your patch. Does kubectl actually respect XDG_CACHE_HOME today to locate your .kube/config file? If so, we can follow suit.

We should use exact same path determination logic to locate .kube/ directory. If kubectl uses another XDG_* env var, we should use that too. As far as I know, kubectl doesn't support XDG_* today.

[rafi]

@ahmetb No, it doesn't. But it supports the KUBECONFIG set a single or multiple configuration files, e.g. export KUBECONFIG="$XDG_CONFIG_HOME/kube/config.yml"

[ahmetb]

I see we're using ~/.kube for caching stuff, so this will work. But you will still need to set KUBECONFIG to actually make things work.

I'll think a bit to see if this matches XDG_CACHE_HOME, XDG_CONFIG_HOME or XDG_DATA_HOME. But I think your patch is fine, I may merge it soon.

[ahmetb]

Thanks a lot for your PR.