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Fix /sync missing membership in state_after #19463

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anoadragon453 merged 3 commits into from
Feb 17, 2026
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Fix /sync missing membership in state_after #19463

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7adca15
Fix missing membership in `state_after`
MadLittleMods Feb 13, 2026
01ef120
Add changelog
MadLittleMods Feb 13, 2026
ac58a18
Better changelog
MadLittleMods Feb 13, 2026
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1 change: 1 addition & 0 deletions changelog.d/19463.bugfix
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix `/sync` missing membership event in `state_after` (experimental [MSC4222](https://github.com/matrix-org/matrix-spec-proposals/pull/4222) implementation) in some scenarios.
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@MadLittleMods MadLittleMods Feb 16, 2026 •
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What are the odds of this making the next Synapse release on the 24th?

-- @dbkr, #19463 (comment)

Low based on current review rate. Would need review today in order to make the RC tomorrow (204-02-17) and then the full release on 2026-02-24.

I also don't want to rush anyone as although I think this is a sane change, could introduce a new problem that the maintainer has to then spend even more effort to put out a new RC about. Feel free to make your case in the backend team lobby room and someone may be sympathetic enough to review.

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@MadLittleMods MadLittleMods Feb 17, 2026

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Turned into some trouble: #19474 (comment)

(only because of CI though)

15 changes: 12 additions & 3 deletions synapse/handlers/sync.py
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Expand Up @@ -1041,9 +1041,18 @@ async def compute_state_delta(
if event.sender not in first_event_by_sender_map:
first_event_by_sender_map[event.sender] = event

# We need the event's sender, unless their membership was in a
# previous timeline event.
if (EventTypes.Member, event.sender) not in timeline_state:
# When using `state_after`, there is no special treatment with
# regards to state also being in the `timeline`. Always fetch
# relevant membership regardless of whether the state event is in
# the `timeline`.
if sync_config.use_state_after:
members_to_fetch.add(event.sender)
# For `state`, the client is supposed to do a flawed re-construction
# of state over time by starting with the given `state` and layering
# on state from the `timeline` as you go (flawed because state
# resolution). In this case, we only need their membership in
# `state` when their membership isn't already in the `timeline`.
elif (EventTypes.Member, event.sender) not in timeline_state:
members_to_fetch.add(event.sender)
# FIXME: we also care about invite targets etc.

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