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")
 
@@ -238,6 +240,14 @@ def read_config(self, config, **kwargs):
             self.email_add_threepid_template_success_html_content = self.read_file(
                 filepath, "email.add_threepid_template_success_html"
             )
+        elif self.account_threepid_delegate_msisdn:
+            if 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`."
+                )
 
         if self.email_enable_notifs:
             required = [

synapse/handlers/identity.py

@@ -475,8 +475,6 @@ def requestMsisdnToken(
         except TimeoutError:
             raise SynapseError(500, "Timed out contacting identity server")
 
-        assert self.hs.config.public_baseurl
-
         # we need to tell the client to send the token back to us, since it doesn't
         # otherwise know where to send it, so add submit_url response parameter
         # (see also MSC2078)

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",