Add Google Cloud DNS plugin (#3011)

Signed-off-by: Palash Nigam <npalash25@gmail.com>

Closes: #2822

plugin/clouddns/setup.go

@@ -0,0 +1,110 @@
+package clouddns
+
+import (
+	"context"
+	"strings"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/fall"
+	clog "github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/coredns/coredns/plugin/pkg/upstream"
+
+	"github.com/caddyserver/caddy"
+	gcp "google.golang.org/api/dns/v1"
+	"google.golang.org/api/option"
+)
+
+var log = clog.NewWithPlugin("clouddns")
+
+func init() {
+	caddy.RegisterPlugin("clouddns", caddy.Plugin{
+		ServerType: "dns",
+		Action: func(c *caddy.Controller) error {
+			f := func(ctx context.Context, opt option.ClientOption) (gcpDNS, error) {
+				var err error
+				var client *gcp.Service
+				if opt != nil {
+					client, err = gcp.NewService(ctx, opt)
+				} else {
+					// if credentials file is not provided in the Corefile
+					// authenticate the client using env variables
+					client, err = gcp.NewService(ctx)
+				}
+				return gcpClient{client}, err
+			}
+			return setup(c, f)
+		},
+	})
+}
+
+func setup(c *caddy.Controller, f func(ctx context.Context, opt option.ClientOption) (gcpDNS, error)) error {
+	for c.Next() {
+		keyPairs := map[string]struct{}{}
+		keys := map[string][]string{}
+
+		var fall fall.F
+		up := upstream.New()
+
+		args := c.RemainingArgs()
+
+		for i := 0; i < len(args); i++ {
+			parts := strings.SplitN(args[i], ":", 3)
+			if len(parts) != 3 {
+				return c.Errf("invalid zone '%s'", args[i])
+			}
+			dnsName, projectName, hostedZone := parts[0], parts[1], parts[2]
+			if dnsName == "" || projectName == "" || hostedZone == "" {
+				return c.Errf("invalid zone '%s'", args[i])
+			}
+			if _, ok := keyPairs[args[i]]; ok {
+				return c.Errf("conflict zone '%s'", args[i])
+			}
+
+			keyPairs[args[i]] = struct{}{}
+			keys[dnsName] = append(keys[dnsName], projectName+":"+hostedZone)
+		}
+
+		var opt option.ClientOption
+		for c.NextBlock() {
+			switch c.Val() {
+			case "upstream":
+				c.RemainingArgs() // eats args
+			// if filepath is provided in the Corefile use it to authenticate the dns client
+			case "credentials":
+				if c.NextArg() {
+					opt = option.WithCredentialsFile(c.Val())
+				} else {
+					return c.ArgErr()
+				}
+			case "fallthrough":
+				fall.SetZonesFromArgs(c.RemainingArgs())
+			default:
+				return c.Errf("unknown property '%s'", c.Val())
+			}
+		}
+
+		ctx := context.Background()
+		client, err := f(ctx, opt)
+		if err != nil {
+			return err
+		}
+
+		h, err := New(ctx, client, keys, up)
+		if err != nil {
+			return c.Errf("failed to create Cloud DNS plugin: %v", err)
+		}
+		h.Fall = fall
+
+		if err := h.Run(ctx); err != nil {
+			return c.Errf("failed to initialize Cloud DNS plugin: %v", err)
+		}
+
+		dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+			h.Next = next
+			return h
+		})
+	}
+
+	return nil
+}