Add location header to /acme/new-reg if key is already in use

wfe/web-front-end.go

@@ -338,8 +338,9 @@ func (wfe *WebFrontEndImpl) NewRegistration(response http.ResponseWriter, reques
 		return
 	}
 
-	if _, err = wfe.SA.GetRegistrationByKey(*key); err == nil {
+	if existingReg, err := wfe.SA.GetRegistrationByKey(*key); err == nil {
 		logEvent.Error = "Registration key is already in use"
+		response.Header().Set("Location", fmt.Sprintf("%s%d", wfe.RegBase, existingReg.ID))
 		wfe.sendError(response, logEvent.Error, nil, http.StatusConflict)
 		return
 	}

wfe/web-front-end_test.go

@@ -703,6 +703,9 @@ func TestNewRegistration(t *testing.T) {
 	test.AssertEquals(t,
 		responseWriter.Body.String(),
 		"{\"type\":\"urn:acme:error:malformed\",\"detail\":\"Registration key is already in use\"}")
+	test.AssertEquals(
+		t, responseWriter.Header().Get("Location"),
+		"/acme/reg/1")
 }
 
 // Valid revocation request for existing, non-revoked cert