Add Support for FlatcarContainerLinux

[bephinix]

Fixes #189.

[bephinix]

@kke I am currently working on providing support for FCL using #183 as a reference. Although everything is built correctly, setting os: flatcarcontainerlinux still results in unsupported os.

[kke]

@kke I am currently working on providing support for FCL using #183 as a reference. Although everything is built correctly, setting os: flatcarcontainerlinux still results in unsupported os.

You need to at least implement InstallPackage, I think that is the bare minimum for distro support.

func (l FlatcarContainerLinux) InstallPackage(h os.Host, pkg ...string) error {
	return h.Execf("SOME_COMMAND_TO_INSTALL_PACKAGES_HERE %s", strings.Join(pkg, " "), exec.Sudo(h))
}

[bephinix]

@kke Ok, OS detection works and Flatcar is now a valid OS for k0sctl but the new path is still not used although the correct method is reimplemented.

[kke]

Hmm. I'm not entirely sure why the BaseLinux type exists. This composing / type embedding stuff in Go is tricky stuff.

I think it should cause a compilation error if there are ambiguous functions 🤔 It may be required that the K0sBinaryPath() is removed from configurer/linux.go, maybe creating something like:

package linux

type writableUSR struct {}

func (w writableUSR) K0sBinaryPath() string {
  return "/usr/local/bin/k0s"
}

and then embedding that to all the other linuxes:

type Debian struct {
	linux.Ubuntu
	configurer.Linux
       writableUSR
}

[bephinix]

@kke I solved the problem by using a dedicated interface for functions returning a static string such as the K0S Binary Path. The MR now consists of three logical commits:

1. Introduction of dedicated interface to support overridding static string functions.
2. Fix for download method to create destination directory if it does not exists.
3. Add support for Flatcar Container Linux.

It works now as intended. 🎉
Pleases let me know if I am missing something. 😉

[kke]

Shaping up nicely, some wizardry going on, need to tinker it with a bit myself before yanking the lever :)

[bephinix]

I have just changed the default path for flatcar to /opt/bin/k0s as /opt/bin seems to be the default workarround for flatcar systems. Nevertheless, the path creation seems to be a good idea.

[bephinix]

@kke Did you already found time to check the changes? It would be great to use the official k0sctl binaries for my deployments.
So if you need further information or assistance, please let me know. 👍🏻

[kke]

Sorry, not yet, trying to look into it asap.

[kke]

It's a pretty clever way to override the functions 🤔

[bephinix]

@kke Okay, let me update the PR to include bespoked ideas. :)

[bephinix]

@kke Updated. 🚀