From affbb58b8bc06b630f2cbc34b722bb586869c4b1 Mon Sep 17 00:00:00 2001
From: Patrick Cloke <patrickc@matrix.org>
Date: Thu, 19 Mar 2020 12:35:30 -0400
Subject: [PATCH] Treat an unknown alias as a BAD_ALIAS for the canonical alias
 event.

---
 changelog.d/7109.bugfix     |  1 +
 synapse/handlers/message.py | 26 ++++++++++++++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 changelog.d/7109.bugfix

diff --git a/changelog.d/7109.bugfix b/changelog.d/7109.bugfix
new file mode 100644
index 000000000000..268de9978eed
--- /dev/null
+++ b/changelog.d/7109.bugfix
@@ -0,0 +1 @@
+Return the proper error (M_BAD_ALIAS) when a non-existant canonical alias is provided.
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index b743fc2dcc16..3fb4055635b3 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -906,7 +906,18 @@ def persist_and_notify_client_event(
             directory_handler = self.hs.get_handlers().directory_handler
             if room_alias_str and room_alias_str != original_alias:
                 room_alias = RoomAlias.from_string(room_alias_str)
-                mapping = yield directory_handler.get_association(room_alias)
+                try:
+                    mapping = yield directory_handler.get_association(room_alias)
+                except SynapseError as e:
+                    # Turn M_NOT_FOUND errors into M_BAD_ALIAS errors.
+                    if e.errcode == Codes.NOT_FOUND:
+                        raise SynapseError(
+                            400,
+                            "Room alias %s does not point to the room"
+                            % (room_alias_str,),
+                            Codes.BAD_ALIAS,
+                        )
+                    raise
 
                 if mapping["room_id"] != event.room_id:
                     raise SynapseError(
@@ -932,7 +943,18 @@ def persist_and_notify_client_event(
             if new_alt_aliases:
                 for alias_str in new_alt_aliases:
                     room_alias = RoomAlias.from_string(alias_str)
-                    mapping = yield directory_handler.get_association(room_alias)
+                    try:
+                        mapping = yield directory_handler.get_association(room_alias)
+                    except SynapseError as e:
+                        # Turn M_NOT_FOUND errors into M_BAD_ALIAS errors.
+                        if e.errcode == Codes.NOT_FOUND:
+                            raise SynapseError(
+                                400,
+                                "Room alias %s does not point to the room"
+                                % (room_alias_str,),
+                                Codes.BAD_ALIAS,
+                            )
+                        raise
 
                     if mapping["room_id"] != event.room_id:
                         raise SynapseError(