Show a confirmation page during user password reset (#8004)

This PR adds a confirmation step to resetting your user password between clicking the link in your email and your password actually being reset.

This is to better align our password reset flow with the industry standard of requiring a confirmation from the user after email validation.

tests/rest/client/v2_alpha/test_account.py

@@ -19,6 +19,7 @@ import os
 import re
 from email.parser import Parser
 from typing import Optional
+from urllib.parse import urlencode
 
 import pkg_resources
 
@@ -27,6 +28,7 @@ from synapse.api.constants import LoginType, Membership
 from synapse.api.errors import Codes
 from synapse.rest.client.v1 import login, room
 from synapse.rest.client.v2_alpha import account, register
+from synapse.rest.synapse.client.password_reset import PasswordResetSubmitTokenResource
 
 from tests import unittest
 from tests.unittest import override_config
@@ -70,6 +72,7 @@ class PasswordResetTestCase(unittest.HomeserverTestCase):
 
     def prepare(self, reactor, clock, hs):
         self.store = hs.get_datastore()
+        self.submit_token_resource = PasswordResetSubmitTokenResource(hs)
 
     def test_basic_password_reset(self):
         """Test basic password reset flow
@@ -251,8 +254,32 @@ class PasswordResetTestCase(unittest.HomeserverTestCase):
         # Remove the host
         path = link.replace("https://example.com", "")
 
+        # Load the password reset confirmation page
         request, channel = self.make_request("GET", path, shorthand=False)
-        self.render(request)
+        request.render(self.submit_token_resource)
+        self.pump()
+        self.assertEquals(200, channel.code, channel.result)
+
+        # Now POST to the same endpoint, mimicking the same behaviour as clicking the
+        # password reset confirm button
+
+        # Send arguments as url-encoded form data, matching the template's behaviour
+        form_args = []
+        for key, value_list in request.args.items():
+            for value in value_list:
+                arg = (key, value)
+                form_args.append(arg)
+
+        # Confirm the password reset
+        request, channel = self.make_request(
+            "POST",
+            path,
+            content=urlencode(form_args).encode("utf8"),
+            shorthand=False,
+            content_is_form=True,
+        )
+        request.render(self.submit_token_resource)
+        self.pump()
         self.assertEquals(200, channel.code, channel.result)
 
     def _get_link_from_email(self):