fix(fleet-data): cycle-safe toposort::waves#162
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fixes review item from PR feedback: a cyclic depends_on graph (hand-edited plan.json, partial write, future-schema bug) used to overflow the stack in the recursive resolver and crash fleet-plan-tree / fleet-waves. Track an in-flight visiting set; on re-entry treat the back-edge as level 0 instead of recursing. New tests cover a 2-cycle and a self-cycle.
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Summary
fleet-data::toposort::wavesresolves each subtask's wave level via a recursiveresolve()helper. The module doc previously warned that cycles independs_onwould overflow the stack and trusted callers to guarantee acyclic input. That was fragile for a dashboard that parsesplan.jsonstraight off disk — a hand-edited file, a partial write, or a future schema bug could crashfleet-plan-treeandfleet-wavesoutright.This patch adds an in-flight
visiting: &mut HashSet<u32>set threaded through the recursion. On entry, ifvisiting.insert(idx)returnsfalsewe're already resolving that node further up the stack, so we return0and break the back-edge. On exit (memoised or not) wevisiting.remove(&idx)so siblings are evaluated independently.Acyclic behaviour is preserved verbatim (all six original tests still pass).
Changes
resolve()signature gainsvisiting: &mut HashSet<u32>.waves()allocates a freshHashSetper outer-loop iteration and threads it in.Test plan
cargo test -p fleet-data— 68 passed, 0 failed (60 prior + 2 new + the existing toposort suite).cargo check -p fleet-waves -p fleet-plan-tree— both compile against the new signature (call-site is internal, unchanged).cycle_broken_to_level_zerotest exercises a 2-cycle (A→B→A) and asserts no panic, both nodes emitted, wave count bounded by node count.self_cycle_brokentest exercises A→A and asserts node lands in wave 1 exactly once with no overflow.