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/pkg/expression/expression_test.go

@@ -0,0 +1,73 @@
+package expression
+
+import (
+	"context"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/metadata"
+	"github.com/coredns/coredns/request"
+)
+
+func TestInCidr(t *testing.T) {
+	incidr := DefaultEnv(context.Background(), &request.Request{})["incidr"]
+
+	cases := []struct {
+		ip        string
+		cidr      string
+		expected  bool
+		shouldErr bool
+	}{
+		// positive
+		{ip: "1.2.3.4", cidr: "1.2.0.0/16", expected: true, shouldErr: false},
+		{ip: "10.2.3.4", cidr: "1.2.0.0/16", expected: false, shouldErr: false},
+		{ip: "1:2::3:4", cidr: "1:2::/64", expected: true, shouldErr: false},
+		{ip: "A:2::3:4", cidr: "1:2::/64", expected: false, shouldErr: false},
+		// negative
+		{ip: "1.2.3.4", cidr: "invalid", shouldErr: true},
+		{ip: "invalid", cidr: "1.2.0.0/16", shouldErr: true},
+	}
+
+	for i, c := range cases {
+		r, err := incidr.(func(string, string) (bool, error))(c.ip, c.cidr)
+		if err != nil && !c.shouldErr {
+			t.Errorf("Test %d: unexpected error %v", i, err)
+			continue
+		}
+		if err == nil && c.shouldErr {
+			t.Errorf("Test %d: expected error", i)
+			continue
+		}
+		if c.shouldErr {
+			continue
+		}
+		if r != c.expected {
+			t.Errorf("Test %d: expected %v", i, c.expected)
+			continue
+		}
+	}
+}
+
+func TestMetadata(t *testing.T) {
+	ctx := metadata.ContextWithMetadata(context.Background())
+	metadata.SetValueFunc(ctx, "test/metadata", func() string {
+		return "success"
+	})
+	f := DefaultEnv(ctx, &request.Request{})["metadata"]
+
+	cases := []struct {
+		label     string
+		expected  string
+		shouldErr bool
+	}{
+		{label: "test/metadata", expected: "success"},
+		{label: "test/nonexistent", expected: ""},
+	}
+
+	for i, c := range cases {
+		r := f.(func(string) string)(c.label)
+		if r != c.expected {
+			t.Errorf("Test %d: expected %v", i, c.expected)
+			continue
+		}
+	}
+}