Fix context passing (#2681)

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
+}