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/view.go

@@ -0,0 +1,48 @@
+package view
+
+import (
+	"context"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/expression"
+	"github.com/coredns/coredns/request"
+
+	"github.com/antonmedv/expr"
+	"github.com/antonmedv/expr/vm"
+	"github.com/miekg/dns"
+)
+
+// View is a plugin that enables configuring expression based advanced routing
+type View struct {
+	progs    []*vm.Program
+	viewName string
+	Next     plugin.Handler
+}
+
+// Filter implements dnsserver.Viewer.  It returns true if all View rules evaluate to true for the given state.
+func (v *View) Filter(ctx context.Context, state *request.Request) bool {
+	env := expression.DefaultEnv(ctx, state)
+	for _, prog := range v.progs {
+		result, err := expr.Run(prog, env)
+		if err != nil {
+			return false
+		}
+		if b, ok := result.(bool); ok && b {
+			continue
+		}
+		// anything other than a boolean true result is considered false
+		return false
+	}
+	return true
+}
+
+// ViewName implements dnsserver.Viewer. It returns the view name
+func (v *View) ViewName() string { return v.viewName }
+
+// Name implements the Handler interface
+func (*View) Name() string { return "view" }
+
+// ServeDNS implements the Handler interface.
+func (v *View) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	return plugin.NextOrFailure(v.Name(), v.Next, ctx, w, r)
+}