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feat(battle-node): bound InboundTracker with watermark-guarded sliding window

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Audit Md11 (part 1 of 2). Replace unbounded HashSet<long> _seen with a
WindowSize=256 ring (HashSet + Queue, LRU eviction). The stale-below-window
guard (pubSeq <= HighWaterMark - WindowSize) prevents window eviction from
re-admitting old seqs as novel — the load-bearing invariant.

pubSeq is client-monotonic and SIO retransmit horizons are seconds-scale, so
256 covers realistic retries by a wide margin. HighWaterMark semantics
preserved (Gungnir still reports it).

Tests: 5 new InboundTrackerTests covering below-window guard, evicted-seq
rejection, within-window dedup after eviction, memory bound, and watermark
monotonicity.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
gamer147
2026-06-02 13:06:08 -04:00
parent 898b872edd
commit 3991bcc653
2 changed files with 116 additions and 4 deletions
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45
SVSim.BattleNode/Reliability/InboundTracker.cs
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@@ -4,17 +4,54 @@ namespace SVSim.BattleNode.Reliability;
/// Per-session inbound-emit ledger. Dedupes the client's pubSeq so we never dispatch
/// a retransmitted emit twice; ack-echo (via SIO callback) is the caller's job.
/// </summary>
/// <remarks>
/// State is bounded: the ledger keeps the most recent <see cref="WindowSize"/>
/// pubSeqs in an LRU ring. Seqs below <c>HighWaterMark - WindowSize</c> are
/// treated as stale-below-window and rejected without recording — this is what
/// prevents window eviction from re-admitting an old seq as novel. The pubSeq is
/// client-assigned monotonically; the bound is sized well above the realistic
/// Socket.IO retransmit horizon, so legitimate retransmits always fall inside.
/// </remarks>
public sealed class InboundTracker
{
private readonly HashSet<long> _seen = new();
/// <summary>Sliding-window size. Anything below <c>HighWaterMark - WindowSize</c> is dropped.</summary>
public const int WindowSize = 256;
private readonly HashSet<long> _seen = new(WindowSize);
private readonly Queue<long> _order = new(WindowSize);
/// <summary>Highest pubSeq observed so far. Reported via Gungnir for diagnostics.</summary>
public long HighWaterMark { get; private set; }
/// <summary>Record an incoming pubSeq. Returns true if the caller should dispatch the envelope, false on duplicate.</summary>
/// <summary>Record an incoming pubSeq. Returns true if the caller should dispatch the envelope, false on duplicate or stale-below-window.</summary>
public bool Observe(long pubSeq)
{
if (pubSeq > HighWaterMark) HighWaterMark = pubSeq;
return _seen.Add(pubSeq);
// Stale-below-window guard. Required AFTER HighWaterMark is past the window,
// otherwise an evicted ring entry would re-admit as novel.
if (HighWaterMark > 0 && pubSeq <= HighWaterMark - WindowSize)
return false;
if (pubSeq > HighWaterMark)
{
HighWaterMark = pubSeq;
Record(pubSeq);
return true;
}
if (_seen.Contains(pubSeq))
return false;
Record(pubSeq);
return true;
}
private void Record(long pubSeq)
{
if (_order.Count >= WindowSize)
{
var evicted = _order.Dequeue();
_seen.Remove(evicted);
}
_order.Enqueue(pubSeq);
_seen.Add(pubSeq);
}
}