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fix: Ensure backfilled events cannot become forward extremities #2119

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nex merged 1 commit from eleboucher/continuwuity:fix/late-events-as-forward-extremities into main 2026-08-18 20:22:21 +00:00
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eleboucher commented 2026-08-08 11:57:20 +00:00
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Events arriving after a child of theirs has already been backfilled are no longer installed as forward extremities. Backfilled events now mark their prev_events as referenced, and a backfilled event that nothing references can itself act as a forward extremity, so the branch it sits on stays reachable. Also corrected an inverted debug assertion in get_missing_events that made debug builds panic when an incoming event had none of its prev_events locally.

Fixes: #2115

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<!-- In order to help reviewers know what your pull request does at a glance, you should ensure that 1. Your PR title is a short, single sentence describing what you changed 2. You have described in more detail what you have changed, why you have changed it, what the intended effect is, and why you think this will be beneficial to the project. If you have made any potentially strange/questionable design choices, but didn't feel they'd benefit from code comments, please don't mention them here - after opening your pull request, go to "files changed", and click on the "+" symbol in the line number gutter, and attach comments to the lines that you think would benefit from some clarification. --> Events arriving after a child of theirs has already been backfilled are no longer installed as forward extremities. Backfilled events now mark their prev_events as referenced, and a backfilled event that nothing references can itself act as a forward extremity, so the branch it sits on stays reachable. Also corrected an inverted debug assertion in `get_missing_events` that made debug builds panic when an incoming event had none of its prev_events locally. <!-- Example: This pull request allows us to warp through time and space ten times faster than before by double-inverting the warp drive with hyperheated jump fluid, both making the drive faster and more efficient. This resolves the common issue where we have to wait more than 10 milliseconds to engage, use, and disengage the warp drive when travelling between galaxies. --> <!-- Closes: #... --> Fixes: #2115 <!-- Uncomment the above line(s) if your pull request fixes an issue or closes another pull request by superseding it. 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fix: Do not make late-arriving events forward extremities
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eleboucher force-pushed fix/late-events-as-forward-extremities from 7e1c8e74aa
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2026-08-08 13:02:16 +00:00 Compare
nex changed title from fix: Do not make late-arriving events forward extremities to fix: Ensure backfilled events cannot become forward extremities 2026-08-11 14:04:02 +00:00
eleboucher force-pushed fix/late-events-as-forward-extremities from c1d0fbbe64
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nex requested changes 2026-08-11 14:09:34 +00:00
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src/service/rooms/timeline/append.rs Outdated
@ -198,0 +202,4 @@
.pdu_metadata
.is_event_referenced(&incoming_pdu.room_id_or_hash(), incoming_pdu.event_id())
.await;
if !has_child || forward_extremities.is_empty() {
nex commented 2026-08-11 14:06:19 +00:00
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I'm icky about this being possible in the first place, but i guess it's not a terrible idea to have a break-glass workaround

I'm icky about this being possible in the first place, but i guess it's not a terrible idea to have a break-glass workaround
nex marked this conversation as resolved
src/service/rooms/timeline/backfill.rs Outdated
@ -216,0 +220,4 @@
.mark_as_referenced(&room_id, pdu.prev_events().map(AsRef::as_ref));
// If nothing references this event, it is a leaf, so it belongs in the forward
// extremities.
nex commented 2026-08-11 14:08:45 +00:00
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There's never a situation in which a backfilled event is legally supposed to become a forward extremity as far as I'm aware - the server only starts tracking extremities after it finishes joining a room, at which point the membership event is the only extremity it cares about. Other servers should deal with unreferenced forks post factum if said forks aren't already convened by the join event itself

There's never a situation in which a backfilled event is legally supposed to become a forward extremity as far as I'm aware - the server only starts tracking extremities after it finishes joining a room, at which point the membership event is the only extremity it cares about. Other servers should deal with unreferenced forks post factum if said forks aren't already convened by the join event itself
nex commented 2026-08-11 14:09:32 +00:00
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Furthermore, we're expected to be able to serve state at for events we reference in prev events, which we cannot do for backfilled events yet (I'm working on this), so this might also potentially cause our subsequent events to be proactively dropped

Furthermore, we're expected to be able to serve state at for events we reference in prev events, which we cannot do for backfilled events yet (I'm working on this), so this might also potentially cause our subsequent events to be proactively dropped
eleboucher marked this conversation as resolved
eleboucher force-pushed fix/late-events-as-forward-extremities from f98f4ea8d7
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Aranjedeath reviewed 2026-08-15 16:23:07 +00:00
src/service/rooms/event_handler/fetch_and_handle_outliers.rs Outdated
@ -158,2 +157,3 @@
} else {
i
} else {
i.expected_add(1)
Aranjedeath commented 2026-08-15 16:23:07 +00:00
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idk anything about rust but these are usually saturating ? should this be expected ?

idk anything about rust but these are usually saturating ? should this be expected ?
nex commented 2026-08-15 16:23:48 +00:00
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An expected add is fine because the value will never overflow (u8 can store up to 255, but there can only be 20 prev events, effectively clamping the max value at 20)

An expected add is fine because the value will never overflow (u8 can store up to 255, but there can only be 20 prev events, effectively clamping the max value at 20)
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eleboucher requested review from nex 2026-08-17 06:51:20 +00:00
eleboucher force-pushed fix/late-events-as-forward-extremities from c96f22da3f
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nex approved these changes 2026-08-18 20:22:17 +00:00
nex merged commit 3efb66c76c into main 2026-08-18 20:22:21 +00:00
nex deleted branch fix/late-events-as-forward-extremities 2026-08-18 20:22:21 +00:00
nex added this to the 26.8.0 milestone 2026-08-20 01:01:07 +00:00
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