Provide access to response writer in Context

Signed-off-by: Stephen J Day <stephen.day@docker.com>

context/http.go

@@ -16,6 +16,7 @@ import (
 // Common errors used with this package.
 var (
 	ErrNoRequestContext        = errors.New("no http request in context")
+	ErrNoResponseWriterContext = errors.New("no http response in context")
 )
 
 func parseIP(ipStr string) net.IP {
@@ -110,6 +111,20 @@ func WithResponseWriter(ctx Context, w http.ResponseWriter) (Context, http.Respo
 	return irw, irw
 }
 
+// GetResponseWriter returns the http.ResponseWriter from the provided
+// context. If not present, ErrNoResponseWriterContext is returned. The
+// returned instance provides instrumentation in the context.
+func GetResponseWriter(ctx Context) (http.ResponseWriter, error) {
+	v := ctx.Value("http.response")
+
+	rw, ok := v.(http.ResponseWriter)
+	if !ok || rw == nil {
+		return nil, ErrNoResponseWriterContext
+	}
+
+	return rw, nil
+}
+
 // getVarsFromRequest let's us change request vars implementation for testing
 // and maybe future changes.
 var getVarsFromRequest = mux.Vars