Add sid to next_link for email validation

changelog.d/6097.bugfix

@@ -0,0 +1 @@
+Add sid to next_link for email validation.

synapse/handlers/identity.py

@@ -18,6 +18,7 @@
 """Utilities for interacting with Identity Servers"""
 
 import logging
+import urllib
 
 from canonicaljson import json
 
@@ -328,6 +329,15 @@ def send_threepid_validation(
             # Generate a session id
             session_id = random_string(16)
 
+        if next_link:
+            # Manipulate the next_link to add the sid, because the caller won't get
+            # it until we send a response, by which time we've sent the mail.
+            if "?" in next_link:
+                next_link += "&"
+            else:
+                next_link += "?"
+            next_link += "sid=" + urllib.parse.quote(session_id)
+
         # Generate a new validation token
         token = random_string(32)