Add client package

Signed-off-by: Dr. Stefan Schimanski <stefan.schimanski@gmail.com>

Author: sttts

client/client.go

@@ -0,0 +1,67 @@
+package client
+
+import (
+	"sync"
+
+	"github.com/hashicorp/golang-lru/v2"
+
+	"k8s.io/client-go/rest"
+	"sigs.k8s.io/controller-runtime/pkg/client"
+
+	"github.com/kcp-dev/logicalcluster/v3"
+)
+
+// ClusterClient is a cluster-aware client.
+type ClusterClient interface {
+	// Cluster returns the client for the given cluster.
+	Cluster(cluster logicalcluster.Path) (client.Client, error)
+}
+
+// clusterClient is a multi-cluster-aware client.
+type clusterClient struct {
+	baseConfig *rest.Config
+	opts       client.Options
+
+	lock  sync.RWMutex
+	cache *lru.Cache[logicalcluster.Path, client.Client]
+}
+
+// New creates a new cluster-aware client.
+func New(cfg *rest.Config, options client.Options) (ClusterClient, error) {
+	ca, err := lru.New[logicalcluster.Path, client.Client](100)
+	if err != nil {
+		return nil, err
+	}
+	return &clusterClient{
+		opts:       options,
+		baseConfig: cfg,
+		cache:      ca,
+	}, nil
+}
+
+func (c *clusterClient) Cluster(cluster logicalcluster.Path) (client.Client, error) {
+	// quick path
+	c.lock.RLock()
+	cli, ok := c.cache.Get(cluster)
+	c.lock.RUnlock()
+	if ok {
+		return cli, nil
+	}
+
+	// slow path
+	c.lock.Lock()
+	defer c.lock.Unlock()
+	if cli, ok := c.cache.Get(cluster); ok {
+		return cli, nil
+	}
+
+	// cache miss
+	cfg := rest.CopyConfig(c.baseConfig)
+	cfg.Host = cfg.Host + cluster.RequestPath()
+	cli, err := client.New(cfg, c.opts)
+	if err != nil {
+		return nil, err
+	}
+	c.cache.Add(cluster, cli)
+	return cli, nil
+}