plugin/template: support metadata (#2958)

* Enable use of metadata in templates

* Update README

* Don't stash away ctx, instead use a new func

plugin/metadata/provider.go

@@ -82,6 +82,16 @@ func Labels(ctx context.Context) []string {
 	return nil
 }
 
+// ValueFuncs returns the map[string]Func from the context, or nil if it does not exist.
+func ValueFuncs(ctx context.Context) map[string]Func {
+	if metadata := ctx.Value(key{}); metadata != nil {
+		if m, ok := metadata.(md); ok {
+			return m
+		}
+	}
+	return nil
+}
+
 // ValueFunc returns the value function of label. If none can be found nil is returned. Calling the
 // function returns the value of the label.
 func ValueFunc(ctx context.Context, label string) Func {

plugin/template/README.md

@@ -48,6 +48,8 @@ Each resource record is a full-featured [Go template](https://golang.org/pkg/tex
 * `.Group` a map of the named capture groups.
 * `.Message` the complete incoming DNS message.
 * `.Question` the matched question section.
+* `.Meta` a function that takes a metadata name and returns the value, if the
+  metadata plugin is enabled. For example, `.Meta "kubernetes/client-namespace"`
 
 The output of the template must be a [RFC 1035](https://tools.ietf.org/html/rfc1035) style resource record (commonly referred to as a "zone file").
 

plugin/template/template.go

@@ -8,6 +8,7 @@ import (
 	gotmpl "text/template"
 
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metadata"
 	"github.com/coredns/coredns/plugin/metrics"
 	"github.com/coredns/coredns/plugin/pkg/fall"
 	"github.com/coredns/coredns/plugin/pkg/upstream"
@@ -47,6 +48,19 @@ type templateData struct {
 	Type     string
 	Message  *dns.Msg
 	Question *dns.Question
+	md       map[string]metadata.Func
+}
+
+func (data *templateData) Meta(metaName string) string {
+	if data.md == nil {
+		return ""
+	}
+
+	if f, ok := data.md[metaName]; ok {
+		return f()
+	}
+
+	return ""
 }
 
 // ServeDNS implements the plugin.Handler interface.
@@ -59,7 +73,7 @@ func (h Handler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	}
 
 	for _, template := range h.Templates {
-		data, match, fthrough := template.match(state, zone)
+		data, match, fthrough := template.match(ctx, state, zone)
 		if !match {
 			if !fthrough {
 				return dns.RcodeNameError, nil
@@ -114,7 +128,7 @@ func (h Handler) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 // Name implements the plugin.Handler interface.
 func (h Handler) Name() string { return "template" }
 
-func executeRRTemplate(server, section string, template *gotmpl.Template, data templateData) (dns.RR, error) {
+func executeRRTemplate(server, section string, template *gotmpl.Template, data *templateData) (dns.RR, error) {
 	buffer := &bytes.Buffer{}
 	err := template.Execute(buffer, data)
 	if err != nil {
@@ -129,9 +143,9 @@ func executeRRTemplate(server, section string, template *gotmpl.Template, data t
 	return rr, nil
 }
 
-func (t template) match(state request.Request, zone string) (templateData, bool, bool) {
+func (t template) match(ctx context.Context, state request.Request, zone string) (*templateData, bool, bool) {
 	q := state.Req.Question[0]
-	data := templateData{}
+	data := &templateData{md: metadata.ValueFuncs(ctx)}
 
 	zone = plugin.Zones(t.zones).Matches(state.Name())
 	if zone == "" {

plugin/template/template_test.go

@@ -7,6 +7,7 @@ import (
 	"testing"
 	gotmpl "text/template"
 
+	"github.com/coredns/coredns/plugin/metadata"
 	"github.com/coredns/coredns/plugin/pkg/dnstest"
 	"github.com/coredns/coredns/plugin/pkg/fall"
 	"github.com/coredns/coredns/plugin/test"
@@ -100,6 +101,24 @@ func TestHandler(t *testing.T) {
 		fall:   fall.Root,
 		zones:  []string{"."},
 	}
+	mdTemplate := template{
+		regex:      []*regexp.Regexp{regexp.MustCompile("(^|[.])ip-10-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]example[.]$")},
+		answer:     []*gotmpl.Template{gotmpl.Must(gotmpl.New("answer").Parse(`{{ .Meta "foo" }}-{{ .Name }} 60 IN A 10.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}`))},
+		additional: []*gotmpl.Template{gotmpl.Must(gotmpl.New("additional").Parse(`{{ .Meta "bar" }}.example. IN A 203.0.113.8`))},
+		authority:  []*gotmpl.Template{gotmpl.Must(gotmpl.New("authority").Parse(`example. IN NS {{ .Meta "bar" }}.example.com.`))},
+		qclass:     dns.ClassANY,
+		qtype:      dns.TypeANY,
+		fall:       fall.Root,
+		zones:      []string{"."},
+	}
+	mdMissingTemplate := template{
+		regex:  []*regexp.Regexp{regexp.MustCompile("(^|[.])ip-10-(?P<b>[0-9]*)-(?P<c>[0-9]*)-(?P<d>[0-9]*)[.]example[.]$")},
+		answer: []*gotmpl.Template{gotmpl.Must(gotmpl.New("answer").Parse(`{{ .Meta "foofoo" }}{{ .Name }} 60 IN A 10.{{ .Group.b }}.{{ .Group.c }}.{{ .Group.d }}`))},
+		qclass: dns.ClassANY,
+		qtype:  dns.TypeANY,
+		fall:   fall.Root,
+		zones:  []string{"."},
+	}
 
 	tests := []struct {
 		tmpl           template
@@ -110,6 +129,7 @@ func TestHandler(t *testing.T) {
 		expectedCode   int
 		expectedErr    string
 		verifyResponse func(*dns.Msg) error
+		md             map[string]string
 	}{
 		{
 			name:         "RcodeServFail",
@@ -276,6 +296,78 @@ func TestHandler(t *testing.T) {
 				return nil
 			},
 		},
+		{
+			name:   "mdMatch",
+			tmpl:   mdTemplate,
+			qclass: dns.ClassINET,
+			qtype:  dns.TypeA,
+			qname:  "ip-10-95-12-8.example.",
+			verifyResponse: func(r *dns.Msg) error {
+				if len(r.Answer) != 1 {
+					return fmt.Errorf("expected 1 answer, got %v", len(r.Answer))
+				}
+				if r.Answer[0].Header().Rrtype != dns.TypeA {
+					return fmt.Errorf("expected an A record answer, got %v", dns.TypeToString[r.Answer[0].Header().Rrtype])
+				}
+				name := "myfoo-ip-10-95-12-8.example."
+				if r.Answer[0].Header().Name != name {
+					return fmt.Errorf("expected answer name %q, got %q", name, r.Answer[0].Header().Name)
+				}
+				if len(r.Extra) != 1 {
+					return fmt.Errorf("expected 1 extra record, got %v", len(r.Extra))
+				}
+				if r.Extra[0].Header().Rrtype != dns.TypeA {
+					return fmt.Errorf("expected an additional A record, got %v", dns.TypeToString[r.Extra[0].Header().Rrtype])
+				}
+				name = "mybar.example."
+				if r.Extra[0].Header().Name != name {
+					return fmt.Errorf("expected additional name %q, got %q", name, r.Extra[0].Header().Name)
+				}
+				if len(r.Ns) != 1 {
+					return fmt.Errorf("expected 1 authoritative record, got %v", len(r.Extra))
+				}
+				if r.Ns[0].Header().Rrtype != dns.TypeNS {
+					return fmt.Errorf("expected an authoritative NS record, got %v", dns.TypeToString[r.Extra[0].Header().Rrtype])
+				}
+				ns, ok := r.Ns[0].(*dns.NS)
+				if !ok {
+					return fmt.Errorf("expected NS record to be type NS, got %v", r.Ns[0])
+				}
+				rdata := "mybar.example.com."
+				if ns.Ns != rdata {
+					return fmt.Errorf("expected ns rdata %q, got %q", rdata, ns.Ns)
+				}
+				return nil
+			},
+			md: map[string]string{
+				"foo":    "myfoo",
+				"bar":    "mybar",
+				"foobar": "myfoobar",
+			},
+		},
+		{
+			name:   "mdMissing",
+			tmpl:   mdMissingTemplate,
+			qclass: dns.ClassINET,
+			qtype:  dns.TypeA,
+			qname:  "ip-10-95-12-8.example.",
+			verifyResponse: func(r *dns.Msg) error {
+				if len(r.Answer) != 1 {
+					return fmt.Errorf("expected 1 answer, got %v", len(r.Answer))
+				}
+				if r.Answer[0].Header().Rrtype != dns.TypeA {
+					return fmt.Errorf("expected an A record answer, got %v", dns.TypeToString[r.Answer[0].Header().Rrtype])
+				}
+				name := "ip-10-95-12-8.example."
+				if r.Answer[0].Header().Name != name {
+					return fmt.Errorf("expected answer name %q, got %q", name, r.Answer[0].Header().Name)
+				}
+				return nil
+			},
+			md: map[string]string{
+				"foo": "myfoo",
+			},
+		},
 	}
 
 	ctx := context.TODO()
@@ -294,6 +386,19 @@ func TestHandler(t *testing.T) {
 			}},
 		}
 		rec := dnstest.NewRecorder(&test.ResponseWriter{})
+		if tr.md != nil {
+			ctx = metadata.ContextWithMetadata(context.Background())
+
+			for k, v := range tr.md {
+				// Go requires copying to a local variable for the closure to work
+				kk := k
+				vv := v
+				metadata.SetValueFunc(ctx, kk, func() string {
+					return vv
+				})
+			}
+		}
+
 		code, err := handler.ServeDNS(ctx, rec, req)
 		if err == nil && tr.expectedErr != "" {
 			t.Errorf("Test %v expected error: %v, got nothing", tr.name, tr.expectedErr)