support kubeconfigs with more than one document

[clux]

While we now support stacked kubeconfigs (see #132), and have all the merging logic for it, we crash on multi-document kubeconfigs.

Create two k3d clusters, and save their outputs in a single file:

k3d cluster start promstack
k3d cluster start kube
k3d kubeconfig get --all > ~/.kube/k3d
export KUBECONFIG="$HOME/.kube/k3d"

You'll have a file with two ---\n delimited yaml objects, which could be merged.
kubectl handles it, kube crashes:

Error: Error loading kubeconfig: Failed to infer config.. cluster env: (Error loading kubeconfig: Unable to load in cluster config, KUBERNETES_SERVICE_HOST and KUBERNETES_SERVICE_PORT must be defined), kubeconfig: (Error loading kubeconfig: Failed to parse Kubeconfig YAML: deserializing from YAML containing more than one document is not supported)

Caused by:
    0: Failed to infer config.. cluster env: (Error loading kubeconfig: Unable to load in cluster config, KUBERNETES_SERVICE_HOST and KUBERNETES_SERVICE_PORT must be defined), kubeconfig: (Error loading kubeconfig: Failed to parse Kubeconfig YAML: deserializing from YAML containing more than one document is not supported)
    1: Error loading kubeconfig: Failed to parse Kubeconfig YAML: deserializing from YAML containing more than one document is not supported
    2: Failed to parse Kubeconfig YAML: deserializing from YAML containing more than one document is not supported
    3: deserializing from YAML containing more than one document is not supported

We could probably the yaml objects together and flatten the reads to get the same result in file_config.rs.

AFAIKT: It might be enough to change: Kubeconfig::read_from to:

• have serde_yaml::from_reader call return Vec<Kubeconfig>
• lift remapping path inside a loop over configs
• fold the kubeconfigs together using Kubeconfig::merge
• return an already merged Kubeconfig

Fold might be awkward, since you need to fold over an existing Kubeconfig, but fold_first is almost stable!

Last point; this isn't a bad lack. You can tell k3d to give you delimited path output with multiple config files (k3d config merge - just don't use it with --output) and fallback to stacked kubeconfigs. It would just be nice to not crash on what kubectl handles.

[clux]

Merging looks ok with a manual fold_first that has simplifies error handling. In the new lifted loop:

        let configs: Vec<Kubeconfig> = serde_yaml::from_reader(f).map_err(ConfigError::ParseYaml)?;

        // Remap all files we read to absolute paths.
        let mut cfg = None;
        for mut config in configs {
            // OLD REMAP LOGIC HERE INDENTED
            if let Some(c) = cfg {
                cfg = Some(Kubeconfig::merge(c, config)?);
            } else {
                cfg = Some(config);
            }
        }
        Ok(cfg.expect("Need at least one yaml document in KUBECONFIG file"))

But looks slightly more complicated due to https://www.reddit.com/r/rust/comments/ccpeh4/is_there_a_way_to_deserialize_a_multiple/

might need to split on --- ...

[clux]

Opportune pr: dtolnay/serde-yaml#189

[clux]

Released in 0.51.0