From 0b56f31f8b80fb87e02f3c159e0bf3713a4be681 Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Fri, 22 May 2026 10:34:14 +0100
Subject: [PATCH] Correct misleading logging in
 `_add_messages_to_local_device_inbox_txn`

---
 synapse/storage/databases/main/deviceinbox.py | 56 +++++++++++--------
 1 file changed, 34 insertions(+), 22 deletions(-)

diff --git a/synapse/storage/databases/main/deviceinbox.py b/synapse/storage/databases/main/deviceinbox.py
index fc61f46c1c..04e3057464 100644
--- a/synapse/storage/databases/main/deviceinbox.py
+++ b/synapse/storage/databases/main/deviceinbox.py
@@ -897,7 +897,8 @@ class DeviceInboxWorkerStore(SQLBaseStore):
     ) -> None:
         assert self._can_write_to_device
 
-        local_by_user_then_device = {}
+        # A map from user id, to device id, to a pair of (serialized message, msgid).
+        local_by_user_then_device: dict[str, dict[str, tuple[str, str]]] = {}
         for user_id, messages_by_device in messages_by_user_then_device.items():
             messages_json_for_user = {}
             devices = list(messages_by_device.keys())
@@ -912,11 +913,13 @@ class DeviceInboxWorkerStore(SQLBaseStore):
                     retcol="device_id",
                 )
 
-                message_json = json_encoder.encode(messages_by_device["*"])
+                message_json, msgid = _serialize_to_device_message(
+                    user_id, "*", messages_by_device["*"]
+                )
                 for device_id in devices:
                     # Add the message for all devices for this user on this
                     # server.
-                    messages_json_for_user[device_id] = message_json
+                    messages_json_for_user[device_id] = (message_json, msgid)
             else:
                 if not devices:
                     continue
@@ -938,19 +941,11 @@ class DeviceInboxWorkerStore(SQLBaseStore):
                 for (device_id,) in rows:
                     # Only insert into the local inbox if the device exists on
                     # this server
-                    with start_active_span("serialise_to_device_message"):
-                        msg = messages_by_device[device_id]
-                        set_tag(SynapseTags.TO_DEVICE_TYPE, msg["type"])
-                        set_tag(SynapseTags.TO_DEVICE_SENDER, msg["sender"])
-                        set_tag(SynapseTags.TO_DEVICE_RECIPIENT, user_id)
-                        set_tag(SynapseTags.TO_DEVICE_RECIPIENT_DEVICE, device_id)
-                        set_tag(
-                            SynapseTags.TO_DEVICE_MSGID,
-                            msg["content"].get(EventContentFields.TO_DEVICE_MSGID),
-                        )
-                        message_json = json_encoder.encode(msg)
-
-                    messages_json_for_user[device_id] = message_json
+                    msg = messages_by_device[device_id]
+                    message_json, msgid = _serialize_to_device_message(
+                        user_id, device_id, msg
+                    )
+                    messages_json_for_user[device_id] = (message_json, msgid)
 
             if messages_json_for_user:
                 local_by_user_then_device[user_id] = messages_json_for_user
@@ -965,22 +960,21 @@ class DeviceInboxWorkerStore(SQLBaseStore):
             values=[
                 (user_id, device_id, stream_id, message_json, self._instance_name)
                 for user_id, messages_by_device in local_by_user_then_device.items()
-                for device_id, message_json in messages_by_device.items()
+                for device_id, (message_json, _msgid) in messages_by_device.items()
             ],
         )
 
         if issue9533_logger.isEnabledFor(logging.DEBUG):
             issue9533_logger.debug(
-                "Stored to-device messages with stream_id %i: %s",
+                "Storing to-device messages with stream_id %i: %s",
                 stream_id,
                 [
-                    f"{user_id}/{device_id} (msgid "
-                    f"{msg['content'].get(EventContentFields.TO_DEVICE_MSGID)})"
+                    f"{user_id}/{device_id} (msgid {msgid})"
                     for (
                         user_id,
                         messages_by_device,
-                    ) in messages_by_user_then_device.items()
-                    for (device_id, msg) in messages_by_device.items()
+                    ) in local_by_user_then_device.items()
+                    for (device_id, (_msg, msgid)) in messages_by_device.items()
                 ],
             )
 
@@ -1066,6 +1060,24 @@ class DeviceInboxWorkerStore(SQLBaseStore):
         return results
 
 
+def _serialize_to_device_message(
+    user_id: str, device_id: str, msg: JsonDict
+) -> tuple[str, str]:
+    """Serialiize a to-device message, ready to add to the device_inbox table.
+
+    Returns a tuple (message_json, msgid).
+    """
+    with start_active_span("serialise_to_device_message"):
+        msgid: str = msg["content"].get(EventContentFields.TO_DEVICE_MSGID, "")
+        set_tag(SynapseTags.TO_DEVICE_TYPE, msg["type"])
+        set_tag(SynapseTags.TO_DEVICE_SENDER, msg["sender"])
+        set_tag(SynapseTags.TO_DEVICE_RECIPIENT, user_id)
+        set_tag(SynapseTags.TO_DEVICE_RECIPIENT_DEVICE, device_id)
+        set_tag(SynapseTags.TO_DEVICE_MSGID, msgid)
+        message_json = json_encoder.encode(msg)
+    return message_json, msgid
+
+
 class DeviceInboxBackgroundUpdateStore(SQLBaseStore):
     DEVICE_INBOX_STREAM_ID = "device_inbox_stream_drop"
     REMOVE_DEAD_DEVICES_FROM_INBOX = "remove_dead_devices_from_device_inbox"