[node operator] Add GCP client

Signed-off-by: Malte Poll <mp@edgeless.systems>

operators/constellation-node-operator/internal/gcp/client/client.go

@@ -0,0 +1,83 @@
+package client
+
+import (
+	"context"
+	"math/rand"
+	"time"
+
+	compute "cloud.google.com/go/compute/apiv1"
+	"go.uber.org/multierr"
+)
+
+// Client is a client for the Google Compute Engine.
+type Client struct {
+	instanceAPI
+	instanceTemplateAPI
+	instanceGroupManagersAPI
+	diskAPI
+	// prng is a pseudo-random number generator seeded with time. Not used for security.
+	prng
+}
+
+// New creates a new client for the Google Compute Engine.
+func New(ctx context.Context) (*Client, error) {
+	var closers []closer
+	insAPI, err := compute.NewInstancesRESTClient(ctx)
+	if err != nil {
+		return nil, err
+	}
+	closers = append(closers, insAPI)
+	templAPI, err := compute.NewInstanceTemplatesRESTClient(ctx)
+	if err != nil {
+		_ = closeAll(closers)
+		return nil, err
+	}
+	closers = append(closers, templAPI)
+	groupAPI, err := compute.NewInstanceGroupManagersRESTClient(ctx)
+	if err != nil {
+		_ = closeAll(closers)
+		return nil, err
+	}
+	closers = append(closers, groupAPI)
+	diskAPI, err := compute.NewDisksRESTClient(ctx)
+	if err != nil {
+		_ = closeAll(closers)
+		return nil, err
+	}
+
+	return &Client{
+		instanceAPI:              insAPI,
+		instanceTemplateAPI:      &instanceTemplateClient{templAPI},
+		instanceGroupManagersAPI: &instanceGroupManagersClient{groupAPI},
+		diskAPI:                  diskAPI,
+		prng:                     rand.New(rand.NewSource(int64(time.Now().Nanosecond()))),
+	}, nil
+}
+
+// Close closes the client's connection.
+func (c *Client) Close() error {
+	closers := []closer{
+		c.instanceAPI,
+		c.instanceTemplateAPI,
+		c.instanceGroupManagersAPI,
+		c.diskAPI,
+	}
+	return closeAll(closers)
+}
+
+type closer interface {
+	Close() error
+}
+
+// closeAll closes all closers, even if an error occurs.
+//
+// Errors are collected and a composed error is returned.
+func closeAll(closers []closer) error {
+	// Since this function is intended to be deferred, it will always call all
+	// close operations, even if a previous operation failed.
+	var errs error
+	for _, closer := range closers {
+		errs = multierr.Append(errs, closer.Close())
+	}
+	return errs
+}