package kubesource

import (
	"fmt"
	"os"

	"k8s.io/client-go/dynamic"
	"k8s.io/client-go/rest"
	"k8s.io/client-go/tools/clientcmd"
)

// ClientOptions say where the cluster is.
type ClientOptions struct {
	// Kubeconfig is a kubeconfig path. Empty means: the in-cluster service
	// account if there is one, else the standard loading rules ($KUBECONFIG,
	// then ~/.kube/config). The same binary therefore runs in a pod and on a
	// workstation with no flag at all.
	Kubeconfig string
	// Context selects a kubeconfig context; empty is the file's current one.
	Context string
}

// Namespace returns the namespace to watch when none was configured: the
// pod's own when running in-cluster, else "" (every namespace the credentials
// can list).
func Namespace() string {
	b, err := os.ReadFile("/var/run/secrets/kubernetes.io/serviceaccount/namespace")
	if err != nil {
		return ""
	}
	return string(b)
}

// NewClient builds a dynamic client and reports which source it used.
func NewClient(o ClientOptions) (dynamic.Interface, string, error) {
	cfg, from, err := restConfig(o)
	if err != nil {
		return nil, "", err
	}
	// The runtime lists and watches a handful of resources; the defaults
	// (5 QPS) are fine for that, but a hung apiserver must not hang us.
	cfg.UserAgent = "fancy-dns"
	c, err := dynamic.NewForConfig(cfg)
	if err != nil {
		return nil, "", fmt.Errorf("kubernetes client: %w", err)
	}
	return c, from, nil
}

func restConfig(o ClientOptions) (*rest.Config, string, error) {
	if o.Kubeconfig == "" && o.Context == "" {
		if cfg, err := rest.InClusterConfig(); err == nil {
			return cfg, "in-cluster service account", nil
		}
	}
	rules := clientcmd.NewDefaultClientConfigLoadingRules()
	if o.Kubeconfig != "" {
		rules.ExplicitPath = o.Kubeconfig
	}
	cfg, err := clientcmd.NewNonInteractiveDeferredLoadingClientConfig(
		rules, &clientcmd.ConfigOverrides{CurrentContext: o.Context}).ClientConfig()
	if err != nil {
		return nil, "", fmt.Errorf("no usable Kubernetes credentials (in-cluster or kubeconfig): %w", err)
	}
	return cfg, "kubeconfig", nil
}