From 06aeeed6702f7d320baf0d80e0adc7f826dd5aaa Mon Sep 17 00:00:00 2001
From: Patrick Cloke <patrickc@matrix.org>
Date: Thu, 28 Jan 2021 12:33:36 -0500
Subject: [PATCH] Do not assume a member is still in the room when generating
 email notifications.

---
 changelog.d/9257.bugfix |  1 +
 synapse/push/mailer.py  | 13 +++++++++----
 2 files changed, 10 insertions(+), 4 deletions(-)
 create mode 100644 changelog.d/9257.bugfix

diff --git a/changelog.d/9257.bugfix b/changelog.d/9257.bugfix
new file mode 100644
index 000000000000..8e61d0b442d3
--- /dev/null
+++ b/changelog.d/9257.bugfix
@@ -0,0 +1 @@
+Fix long-standing KeyError in synapse.push.mailer in get_message_vars.
diff --git a/synapse/push/mailer.py b/synapse/push/mailer.py
index 745b1dde9459..78d5a24f6707 100644
--- a/synapse/push/mailer.py
+++ b/synapse/push/mailer.py
@@ -431,10 +431,15 @@ async def get_message_vars(
         if event.type != EventTypes.Message and event.type != EventTypes.Encrypted:
             return None
 
-        sender_state_event_id = room_state_ids[("m.room.member", event.sender)]
-        sender_state_event = await self.store.get_event(sender_state_event_id)
-        sender_name = name_from_member_event(sender_state_event)
-        sender_avatar_url = sender_state_event.content.get("avatar_url")
+        sender_state_event_id = room_state_ids.get(("m.room.member", event.sender))
+        if sender_state_event_id:
+            sender_state_event = await self.store.get_event(sender_state_event_id)
+            sender_name = name_from_member_event(sender_state_event)
+            sender_avatar_url = sender_state_event.content.get("avatar_url")
+        else:
+            # The sender is no longer in the room for some reason.
+            sender_name = event.sender
+            sender_avatar_url = None
 
         # 'hash' for deterministically picking default images: use
         # sender_hash % the number of default images to choose from