Tarfetch: make it a button

[TimothyLoyer]

• Revise tarfetch to work as a resource button rather than as a separate UI resource
• Added tests
• Updated readme

[noelleleigh]

Can you include a section here explaining why someone might want to use this tar-based method of getting files out of a container instead of using kubectl cp?

[TimothyLoyer]

I'll explain it to you, and then try to condense an explanation for the readme.

Here is what kubectl cp --help says:

Examples:
  # !!!Important Note!!!
  # Requires that the 'tar' binary is present in your container
  # image.  If 'tar' is not present, 'kubectl cp' will fail.
  #
  # For advanced use cases, such as symlinks, wildcard expansion or
  # file mode preservation, consider using 'kubectl exec'.
  
  # Copy /tmp/foo local file to /tmp/bar in a remote pod in namespace <some-namespace>
  tar cf - /tmp/foo | kubectl exec -i -n <some-namespace> <some-pod> -- tar xf - -C /tmp/bar
  
  # Copy /tmp/foo from a remote pod to /tmp/bar locally
  kubectl exec -n <some-namespace> <some-pod> -- tar cf - /tmp/foo | tar xf - -C /tmp/bar

These more advanced examples are what tarfetch is doing - allowing a bit more flexibility than kubectl cp and puts it in a nice UI button.

I mentioned flexibility... in the less advanced examples for kubectl cp, it can only work when you know the exact name of the pod (odd since you can use a pattern like deploy/<deployment name> when using the kubectl exec examples). This means we can create a sync button which points at a deployment as a sync target.

  # Copy /tmp/foo_dir local directory to /tmp/bar_dir in a remote pod in the default namespace
  kubectl cp /tmp/foo_dir <some-pod>:/tmp/bar_dir
  
  # Copy /tmp/foo local file to /tmp/bar in a remote pod in a specific container
  kubectl cp /tmp/foo <some-pod>:/tmp/bar -c <specific-container>
  
  # Copy /tmp/foo local file to /tmp/bar in a remote pod in namespace <some-namespace>
  kubectl cp /tmp/foo <some-namespace>/<some-pod>:/tmp/bar
  
  # Copy /tmp/foo from a remote pod to /tmp/bar locally
  kubectl cp <some-namespace>/<some-pod>:/tmp/foo /tmp/bar

[TimothyLoyer]

Maybe something like...

## Alternatives

### Syncback
[Syncback](https://github.com/tilt-dev/tilt-extensions/tree/master/syncback) was the first Tilt extension to enable synchronization of files out of a Kubernetes container. Unfortunately it relies on rsync being installed on the host and container systems; a requirement which is rarely fulfilled by default (contrast this with tar, which is pre-installed on most Unix-y systems and can be made available through Windows Subsystem for Linux).

### kubectl cp
Kubectp has a [copy command](https://kubernetes.io/docs/reference/generated/kubectl/kubectl-commands#cp) which relies on tar as well (tarfetch is designed from the "advanced use cases" mentioned in `kubectl cp --help`). The biggest difference between using tarfetch and using `kubectl cp` is the constraints of the latter. Where `kubectl cp` requires an explicit pod name, tarfetch (leveraging `kubectl exec`) can derive the correct pod from a `<type>/<name>` declaration (i.e. `deployment/my-app`).

[noelleleigh]

That looks good to me (minus the "Kubectp" typo).