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clarify how to deterministically cut cycles

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Matthew Hodgson 2021-04-29 20:45:28 +01:00
parent 12d08ca83d
commit 00912f9c1e
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proposals/1772-groups-as-rooms.md
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@ -179,10 +179,18 @@ include:
Cycles in the parent->child and child->parent relationships are *not*
permitted, but clients (and servers) should be aware that they may be
encountered, and ignore the relationship rather than recursing infinitely.
encountered, and MUST spot and break cycles rather than infinitely looping.
XXX: we need to deterministically specify where the cycles get cut.
I think kegan found a solution for this when implementing MSC2946 in Dendrite.
In order for UI to be consistent across different clients, any cycles must be
cut deterministically. For instance, if space A points to space B as a child
but space B also points to space A as a child, then clients must be consistent
on whether A is shown in the UI as the parent of B (or vice versa). Therefore
when a client spots a loop in parent->child or child->parent relationships, it
MUST ignore the cycle by cutting it such that the oldest space-room is the
root (i.e. oldest parent). 'Oldest' means the room whose `m.room.create`
event has the numerically smallest `origin_server_ts`. If the oldest rooms
are precisely the same age, we select the root as the room with the
lexicographically smallest `room_id`.
### Suggested children