Provide a clear warning on startup for missing public_baseurl

changelog.d/6379.misc

@@ -0,0 +1 @@
+Complain on startup instead of 500'ing during runtime when `public_baseurl` isn't set when necessary.

synapse/config/emailconfig.py

@@ -146,6 +146,8 @@ def read_config(self, config, **kwargs):
                 if k not in email_config:
                     missing.append("email." + k)
 
+            # public_baseurl is required to build password reset and validation links that
+            # will be emailed to users
             if config.get("public_baseurl") is None:
                 missing.append("public_baseurl")
 

synapse/config/registration.py

@@ -106,6 +106,13 @@ def read_config(self, config, **kwargs):
         account_threepid_delegates = config.get("account_threepid_delegates") or {}
         self.account_threepid_delegate_email = account_threepid_delegates.get("email")
         self.account_threepid_delegate_msisdn = account_threepid_delegates.get("msisdn")
+        if self.account_threepid_delegate_msisdn and not self.public_baseurl:
+            raise ConfigError(
+                "The configuration option `public_baseurl` is required if "
+                "`account_threepid_delegate.msisdn` is set, such that "
+                "clients know where to submit validation tokens to. Please "
+                "configure `public_baseurl`."
+            )
 
         self.default_identity_server = config.get("default_identity_server")
         self.allow_guest_access = config.get("allow_guest_access", False)

tests/rest/client/v2_alpha/test_register.py

@@ -203,6 +203,7 @@ def test_advertised_flows(self):
 
     @unittest.override_config(
         {
+            "public_baseurl": "https://test_server",
             "enable_registration_captcha": True,
             "user_consent": {
                 "version": "1",