Add a check for duplicate IdP ids

changelog.d/9184.misc

@@ -0,0 +1 @@
+Emit an error at startup if different Identity Providers are configured with the same `idp_id`.

synapse/config/oidc_config.py

@@ -15,6 +15,7 @@
 # limitations under the License.
 
 import string
+from collections import Counter
 from typing import Iterable, Optional, Tuple, Type
 
 import attr
@@ -43,6 +44,17 @@ def read_config(self, config, **kwargs):
         except DependencyException as e:
             raise ConfigError(e.message) from e
 
+        # check we don't have any duplicate idp_ids
+        # XXX: this won't detect clashes with other IdP providers using other SSO
+        # mechanisms (such as SAML or CAS); that will be detected when we set up the
+        # listeners but by then synapse will have forked, so it's not ideal.
+        c = Counter([i.idp_id for i in self.oidc_providers])
+        for idp_id, count in c.items():
+            if count > 1:
+                raise ConfigError(
+                    "Multiple OIDC providers have the idp_id %r." % idp_id
+                )
+
         public_baseurl = self.public_baseurl
         self.oidc_callback_url = public_baseurl + "_synapse/oidc/callback"