plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/view/setup.go

@@ -0,0 +1,65 @@
+package view
+
+import (
+	"context"
+	"strings"
+
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/expression"
+
+	"github.com/antonmedv/expr"
+)
+
+func init() { plugin.Register("view", setup) }
+
+func setup(c *caddy.Controller) error {
+	cond, err := parse(c)
+	if err != nil {
+		return plugin.Error("view", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		cond.Next = next
+		return cond
+	})
+
+	return nil
+}
+
+func parse(c *caddy.Controller) (*View, error) {
+	v := new(View)
+
+	i := 0
+	for c.Next() {
+		i++
+		if i > 1 {
+			return nil, plugin.ErrOnce
+		}
+		args := c.RemainingArgs()
+		if len(args) != 1 {
+			return nil, c.ArgErr()
+		}
+		v.viewName = args[0]
+
+		for c.NextBlock() {
+			switch c.Val() {
+			case "expr":
+				args := c.RemainingArgs()
+				prog, err := expr.Compile(strings.Join(args, " "), expr.Env(expression.DefaultEnv(context.Background(), nil)))
+				if err != nil {
+					return v, err
+				}
+				v.progs = append(v.progs, prog)
+				if err != nil {
+					return nil, err
+				}
+				continue
+			default:
+				return nil, c.Errf("unknown property '%s'", c.Val())
+			}
+		}
+	}
+	return v, nil
+}