tests: functional: Don't timeout when web server restarts

Likely helps: #2271.

When web server restarts we are shown an error page. Trying to lookup and
element in this error page still raises StaleElementReferenceException. However,
if the page was reloaded with driver.visit(), then trying to lookup the old
element does not throw StaleElementReferenceException. Instead the
NoSuchElementException is thrown. For this case, ensure that we stop waiting
appropriately. This is likely to solve the large waits and timeouts when testing
dynamicdns.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/tests/functional/__init__.py

@@ -16,6 +16,7 @@ from contextlib import contextmanager
 import pytest
 import requests
 from selenium.common.exceptions import (ElementClickInterceptedException,
+                                        NoSuchElementException,
                                         StaleElementReferenceException,
                                         WebDriverException)
 from selenium.webdriver.support.ui import WebDriverWait
@@ -99,19 +100,47 @@ class _PageLoaded():
     def __init__(self, element, expected_url=None):
         self.element = element
         self.expected_url = expected_url
+        self.loaded_new_page = False
 
     def __call__(self, driver):
         is_stale = False
         try:
             self.element.has_class('whatever_class')
+        # XXX: There is still another unhandled case where the webserver
+        # restarts after submission of a form and the browser does not switch
+        # to error page. It continues to wait for a response from the server
+        # indefinitely until timeout.
+        except NoSuchElementException:
+            # This is still the old page, wait for it to change.
+            if not self.loaded_new_page:
+                return False
+
+            # This is the new page and we are not waiting for a specific URL.
+            # Stop waiting.
+            if not self.expected_url:
+                return True
+
+            # This is the new page and expected URL has not reached, wait for
+            # the page to spontaneously change.
+            if not driver.url.endswith(self.expected_url):
+                return False
+
+            # This is the new page and we have reached the expected URL. Stop
+            # waiting.
+            return True
         except (StaleElementReferenceException, TypeError):
             # After a domain name change, Let's Encrypt will restart the web
             # server and could cause a connection failure.
             if driver.find_by_id('netErrorButtonContainer'):
                 try:
                     driver.visit(driver.url)
+                    # After this for some unknown reason,
+                    # StaleElementReferenceException is no longer thrown.
+                    self.loaded_new_page = True
                 except WebDriverException:
+                    # Web server is not yet available.
                     pass
+
                 return False
 
             is_fully_loaded = driver.execute_script(