Fix context passing (#2681)

plugin/dnstap/context_test.go

@@ -6,7 +6,7 @@ import (
 )
 
 func TestDnstapContext(t *testing.T) {
-	ctx := tapContext{context.TODO(), Dnstap{}}
+	ctx := ContextWithTapper(context.TODO(), Dnstap{})
 	tapper := TapperFromContext(ctx)
 
 	if tapper == nil {

plugin/dnstap/gocontext.go

@@ -0,0 +1,23 @@
+package dnstap
+
+import "context"
+
+type contextKey struct{}
+
+var dnstapKey = contextKey{}
+
+// ContextWithTapper returns a new `context.Context` that holds a reference to
+// `t`'s Tapper.
+func ContextWithTapper(ctx context.Context, t Tapper) context.Context {
+	return context.WithValue(ctx, dnstapKey, t)
+}
+
+// TapperFromContext returns the `Tapper` previously associated with `ctx`, or
+// `nil` if no such `Tapper` could be found.
+func TapperFromContext(ctx context.Context) Tapper {
+	val := ctx.Value(dnstapKey)
+	if sp, ok := val.(Tapper); ok {
+		return sp
+	}
+	return nil
+}

plugin/dnstap/handler.go

@@ -44,12 +44,6 @@ const (
 	DnstapSendOption ContextKey = "dnstap-send-option"
 )
 
-// TapperFromContext will return a Tapper if the dnstap plugin is enabled.
-func TapperFromContext(ctx context.Context) (t Tapper) {
-	t, _ = ctx.(Tapper)
-	return
-}
-
 // TapMessage implements Tapper.
 func (h Dnstap) TapMessage(m *tap.Message) {
 	t := tap.Dnstap_MESSAGE
@@ -71,6 +65,7 @@ func (h Dnstap) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	// message to be sent out
 	sendOption := taprw.SendOption{Cq: true, Cr: true}
 	newCtx := context.WithValue(ctx, DnstapSendOption, &sendOption)
+	newCtx = ContextWithTapper(newCtx, h)
 
 	rw := &taprw.ResponseWriter{
 		ResponseWriter: w,
@@ -80,7 +75,7 @@ func (h Dnstap) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 		QueryEpoch:     time.Now(),
 	}
 
-	code, err := plugin.NextOrFailure(h.Name(), h.Next, tapContext{newCtx, h}, rw, r)
+	code, err := plugin.NextOrFailure(h.Name(), h.Next, newCtx, rw, r)
 	if err != nil {
 		// ignore dnstap errors
 		return code, err