FA-75228 / CRDT convergence / Member archive

Replicated tree move: the cycle check tests the wrong ancestry direction · case 03

Legitimate moves under a higher ancestor are refused while cycle-forming moves are accepted.

Member previewVariant 3 · 3 implementations · 17 checks per implementation

Case contract

Moves [[counter, replica], node, parent] arrive in any order. The replica keeps a log sorted by timestamp. On arrival it undoes every logged move with a larger timestamp (restoring the old parent, or detaching a node that had none), applies the new move, then redoes the undone moves in timestamp order. A move is skipped (logged, tree unchanged) when the parent is neither "root" nor attached, or when the node is the parent or one of its ancestors. Return sorted [node, parent] pairs.

Why this case matters

Tree CRDTs must reject moves that create cycles, and must reach the same tree regardless of delivery order.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
concurrent moves that would form a cycle	`[["p", "q"], ["q", "p"]]`	`[["p", "q"], ["q", "root"]]`	Failed

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