Fix `/sync` missing membership in `state_after` by MadLittleMods · Pull Request #19463 · element-hq/synapse

MadLittleMods · 2026-02-13T19:18:36Z

Fix /sync missing membership in state_after.

This applies to any scenario where the first membership has a different sender compared to the state_key and then the second membership has the same sender/state_key. Like someone inviting another person and then them joining. Or someone being kicked and then they leave.

This bug has been present since the MSC4222 implementation was introduced into the codebase (#17888).

Fix #19455
Fix https://github.com/element-hq/customer-success/issues/656

I have a feeling, this might also fix these issues (will close and see how people report back):

Fix #18182

Testing strategy

Complement tests: #19460 (matrix-org/complement#842)

We will need #19460 to merge in order to enable the Complement tests in Synapse but this PR should be merged first so they pass in the first place. I've tested locally that the Complement tests pass with this fix.

Dev notes

In the same ballpark:

Pull Request Checklist

Pull request is based on the develop branch
Pull request includes a changelog file. The entry should:
- Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
- Use markdown where necessary, mostly for code blocks.
- End with either a period (.) or an exclamation mark (!).
- Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
Code style is correct (run the linters)

Fix #19455

MadLittleMods · 2026-02-13T22:32:54Z

synapse/handlers/sync.py

+                    elif (EventTypes.Member, event.sender) not in timeline_state:
                        members_to_fetch.add(event.sender)
                    # FIXME: we also care about invite targets etc.

                    if event.is_state():
                        timeline_state[(event.type, event.state_key)] = event.event_id


Yes, there is another bug here, #19464

We should be adding to the timeline_state right away before checking (EventTypes.Member, event.sender) not in timeline_state

dbkr · 2026-02-16T13:48:28Z

What are the odds of this making the next Synapse release on the 24th?

MadLittleMods · 2026-02-16T17:10:33Z

changelog.d/19463.bugfix

@@ -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.


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.

Turned into some trouble: #19474 (comment)

(only because of CI though)

anoadragon453

Looks reasonable. Thanks for providing Complement tests, along with detailed context!

anoadragon453 · 2026-02-17T13:04:14Z

Merging so we can get tests passing on #19460.

This reverts commit b0b4203.

Reverts #19463 The complement tests haven't been reviewed and require more testing. Discussed in the internal [backend team lobby](https://matrix.to/#/!SGNQGPGUwtcPBUotTL:matrix.org/$XDARK2u2iLL5wWaxiL6tJYkLg80Sn6yWWEQib8ahl5Q?via=jki.re&via=element.io&via=matrix.org) room.

MadLittleMods · 2026-02-17T17:22:52Z

For reference, this PR was reverted, see #19474 (comment)

Fixes being re-introduced in #19460

*This PR was originally only to enable [MSC4222](matrix-org/matrix-spec-proposals#4222) Complement tests (`/sync` `state_after`) but after merging the [fix PR](#19463), we discovered that while the tests pass locally, [fail in CI](#19460 (comment)). To unblock the RC, we decided to revert the fix PR (see #19474 (comment) for more info). To better ensure tests actually pass in CI, we're re-introducing the fix here in the same PR that we enable the tests in.* --- Fix `/sync` missing membership in `state_after`. This applies to any scenario where the first membership has a different `sender` compared to the `state_key` and then the second membership has the same `sender`/`state_key`. Like someone inviting another person and then them joining. Or someone being kicked and then they leave. This bug has been present since the MSC4222 implementation was introduced into the codebase (#17888). --- Fix #19455 Fix element-hq/customer-success#656 I have a feeling, this might also fix these issues (will close and see how people report back): Fix #18182 Fix #19478 ### Testing strategy Complement tests: matrix-org/complement#842 We will need #19460 to merge in order to enable the Complement tests in Synapse but this PR should be merged first so they pass in the first place. I've tested locally that the Complement tests pass with this fix. ### Dev notes [MSC4222](matrix-org/matrix-spec-proposals#4222) has already been merged into the spec and is already part of Matrix v1.16 but we haven't [stabilized support in Synapse yet](#19414). --- In the same ballpark: - #19455 - #17050 - #17430 - #16940 - #18182 - #18793 - #19478 --- Docker builds preferring remote image over the local image we just built, #19460 (comment) `containerd` image store (storage driver, driver type) -> #19475 ### Todo - [x] Wait for #19463 to merge so the Complement tests all pass - [x] Wait for #19475 to merge ### Pull Request Checklist  * [x] Pull request is based on the develop branch * [x] Pull request includes a [changelog file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog). The entry should: - Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from `EventStore` to `EventWorkerStore`.". - Use markdown where necessary, mostly for `code blocks`. - End with either a period (.) or an exclamation mark (!). - Start with a capital letter. - Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry. * [x] [Code style](https://element-hq.github.io/synapse/latest/code_style.html) is correct (run the [linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters)) --------- Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com> Co-authored-by: Andrew Ferrazzutti <andrewf@element.io>

Fix missing membership in state_after

7adca15

Fix #19455

MadLittleMods added the A-Sync label Feb 13, 2026

MadLittleMods mentioned this pull request Feb 13, 2026

Fix /sync missing membership in state_after (re-introduce) #19460

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Add changelog

01ef120

MadLittleMods mentioned this pull request Feb 13, 2026

Fix /sync timeline state duplicated in state when lazy-loading #19464

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MadLittleMods added A-Membership A-Lazy-Loading labels Feb 13, 2026

MadLittleMods commented Feb 13, 2026

View reviewed changes

MadLittleMods changed the title ~~Fix missing membership in state_after~~ Fix missing membership in /sync state_after Feb 13, 2026

MadLittleMods changed the title ~~Fix missing membership in /sync state_after~~ Fix /sync missing membership in state_after Feb 13, 2026

Better changelog

ac58a18

MadLittleMods mentioned this pull request Feb 13, 2026

Member events for sender of events in sync timeline missing from state_after #19455

Closed

MadLittleMods marked this pull request as ready for review February 13, 2026 23:45

MadLittleMods requested a review from a team as a code owner February 13, 2026 23:46

MadLittleMods commented Feb 16, 2026

View reviewed changes

anoadragon453 approved these changes Feb 17, 2026

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anoadragon453 merged commit b0b4203 into develop Feb 17, 2026
79 of 81 checks passed

anoadragon453 deleted the madlittlemods/19455-fix-state-after-missing-membership branch February 17, 2026 13:04

MadLittleMods mentioned this pull request Feb 17, 2026

Add MSC4222 /sync state_after test for initial sync lazy-loading room members matrix-org/complement#842

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sandhose added a commit that referenced this pull request Feb 17, 2026

Revert "Fix /sync missing membership in state_after (#19463)"

8b9f290

This reverts commit b0b4203.

sandhose mentioned this pull request Feb 17, 2026

Revert "Fix /sync missing membership in state_after" #19474

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Fix `/sync` missing membership in `state_after`#19463

Fix `/sync` missing membership in `state_after`#19463
anoadragon453 merged 3 commits intodevelopfrom
madlittlemods/19455-fix-state-after-missing-membership

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		@@ -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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