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Give the correct next event when the message timestamps are the same - MSC3030 #13658

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Aug 30, 2022
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Give the correct next event when the message timestamps are the same - MSC3030 #13658

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Give the correct next event when the message timestamps are the same
MadLittleMods Aug 29, 2022
47e2a40
Add changelog
MadLittleMods Aug 29, 2022
eadfde0
Better wording in changelog
MadLittleMods Aug 30, 2022
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1 change: 1 addition & 0 deletions changelog.d/13658.bugfix
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix MSC3030 `/timestamp_to_event` endpoint to return the correct next event when the events have the same timestamp.
12 changes: 10 additions & 2 deletions synapse/storage/databases/main/events_worker.py
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Expand Up @@ -2111,7 +2111,14 @@ async def get_event_id_for_timestamp(
AND room_id = ?
/* Make sure event is not rejected */
AND rejections.event_id IS NULL
ORDER BY origin_server_ts %s
/**
* First sort by the message timestamp. If the message timestamps are the
* same, we want the message that logically comes "next" (before/after
* the given timestamp) based on the DAG and its topological order (`depth`).
* Finally, we can tie-break based on when it was received on the server
* (`stream_ordering`).
*/
ORDER BY origin_server_ts %s, depth %s, stream_ordering %s
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when it was received on the server (stream_ordering).

There is a received_ts column for this (though there may very well be tie-breaks just like the origin timestamp).

origin_server_ts and depth are untrusted fields controlled by the sender's HS. I'm not sure if that is a concern here though.

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@squahtx squahtx Aug 30, 2022 •
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I can't think of a better way to do this without ordering by depth. There is a topological_ordering column, but apparently that's just identical to depth.

We're already trusting origin_server_ts to be reasonable for this feature, so I suppose it's not much of a stretch to trust depth as well.

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@MadLittleMods MadLittleMods Aug 30, 2022 •
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origin_server_ts and depth are untrusted fields controlled by the sender's HS. I'm not sure if that is a concern here though.

We're already trusting origin_server_ts to be reasonable for this feature, so I suppose it's not much of a stretch to trust depth as well.

👍 exactly both of these

LIMIT 1;
"""

Expand All @@ -2130,7 +2137,8 @@ def get_event_id_for_timestamp_txn(txn: LoggingTransaction) -> Optional[str]:
order = "ASC"

txn.execute(
sql_template % (comparison_operator, order), (timestamp, room_id)
sql_template % (comparison_operator, order, order, order),
(timestamp, room_id),
)
row = txn.fetchone()
if row:
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