Only assert valid next_link params when provided (#8417)

Broken in https://github.com/matrix-org/synapse/pull/8275 and has yet to be put in a release. Fixes https://github.com/matrix-org/synapse/issues/8418.

`next_link` is an optional parameter. However, we were checking whether the `next_link` param was valid, even if it wasn't provided. In that case, `next_link` was `None`, which would clearly not be a valid URL.

This would prevent password reset and other operations if `next_link` was not provided, and the `next_link_domain_whitelist` config option was set.

changelog.d/8417.feature

@@ -0,0 +1 @@
+Add a config option to specify a whitelist of domains that a user can be redirected to after validating their email or phone number.

synapse/rest/client/v2_alpha/account.py

@@ -103,8 +103,9 @@ class EmailPasswordRequestTokenRestServlet(RestServlet):
                 Codes.THREEPID_DENIED,
             )
 
-        # Raise if the provided next_link value isn't valid
+        if next_link:
-        assert_valid_next_link(self.hs, next_link)
+            # Raise if the provided next_link value isn't valid
+            assert_valid_next_link(self.hs, next_link)
 
         # The email will be sent to the stored address.
         # This avoids a potential account hijack by requesting a password reset to
@@ -379,8 +380,9 @@ class EmailThreepidRequestTokenRestServlet(RestServlet):
                 Codes.THREEPID_DENIED,
             )
 
-        # Raise if the provided next_link value isn't valid
+        if next_link:
-        assert_valid_next_link(self.hs, next_link)
+            # Raise if the provided next_link value isn't valid
+            assert_valid_next_link(self.hs, next_link)
 
         existing_user_id = await self.store.get_user_id_by_threepid("email", email)
 
@@ -453,8 +455,9 @@ class MsisdnThreepidRequestTokenRestServlet(RestServlet):
                 Codes.THREEPID_DENIED,
             )
 
-        # Raise if the provided next_link value isn't valid
+        if next_link:
-        assert_valid_next_link(self.hs, next_link)
+            # Raise if the provided next_link value isn't valid
+            assert_valid_next_link(self.hs, next_link)
 
         existing_user_id = await self.store.get_user_id_by_threepid("msisdn", msisdn)
 

tests/rest/client/v2_alpha/test_account.py

@@ -732,6 +732,12 @@ class ThreepidEmailRestTestCase(unittest.HomeserverTestCase):
     @override_config({"next_link_domain_whitelist": ["example.com", "example.org"]})
     def test_next_link_domain_whitelist(self):
         """Tests next_link parameters must fit the whitelist if provided"""
+
+        # Ensure not providing a next_link parameter still works
+        self._request_token(
+            "something@example.com", "some_secret", next_link=None, expect_code=200,
+        )
+
         self._request_token(
             "something@example.com",
             "some_secret",