Fix cross-block false double-sign detection in sequential consensus

The _proposalsByView dictionary was keyed by (view, proposer) without
block number. After a view change bumps view to V for block N-1, and
then StartRound(N) sets view = N where N == V, the old entry from
block N-1 collides with the new proposal for block N. This causes false
double-sign detection because the hashes differ (different blocks).

Fix: include block number in the key → (view, block, proposer). Proposals
for different blocks at the same view no longer collide. Genuine double-
signs (same view + same block + same proposer + different hashes) are
still detected correctly.

Author: 0xZunia

src/node/Basalt.Node/NodeCoordinator.cs

@@ -98,9 +98,12 @@ public sealed class NodeCoordinator : IAsyncDisposable
     // N-14: Cap block request count to prevent resource exhaustion
     private const int MaxBlockRequestCount = 100;
 
-    // N-17: Thread-safe double-sign detection: keyed by (view, proposer) to avoid false positives
-    // when different proposers propose for the same view after a view change.
-    private readonly ConcurrentDictionary<(ulong View, PeerId Proposer), Hash256> _proposalsByView = new();
+    // N-17: Thread-safe double-sign detection: keyed by (view, block, proposer).
+    // Block number is included because view numbers can collide across blocks:
+    // after a view change bumps view to V, and then StartRound(V) reuses the same
+    // view number for the next block, old entries from the previous block would
+    // cause false double-sign detection without the block dimension.
+    private readonly ConcurrentDictionary<(ulong View, ulong Block, PeerId Proposer), Hash256> _proposalsByView = new();
 
     // Identity
     private byte[] _privateKey = [];
@@ -1146,7 +1149,7 @@ private void HandleConsensusProposal(PeerId sender, ConsensusProposalMessage pro
 
         if (proposal.BlockNumber == currentBlock)
         {
-            var proposalKey = (proposal.ViewNumber, proposal.SenderId);
+            var proposalKey = (proposal.ViewNumber, proposal.BlockNumber, proposal.SenderId);
             if (_slashingEngine != null && _proposalsByView.TryGetValue(proposalKey, out var existingHash))
             {
                 if (existingHash != proposal.BlockHash)
@@ -1155,8 +1158,8 @@ private void HandleConsensusProposal(PeerId sender, ConsensusProposalMessage pro
                     if (proposerInfo != null)
                     {
                         _slashingEngine.SlashDoubleSign(proposerInfo.Address, proposal.BlockNumber, existingHash, proposal.BlockHash);
-                        _logger.LogWarning("Double-sign detected from validator {Address} at view {View}",
-                            proposerInfo.Address, proposal.ViewNumber);
+                        _logger.LogWarning("Double-sign detected from validator {Address} at view {View} block {Block}",
+                            proposerInfo.Address, proposal.ViewNumber, proposal.BlockNumber);
                     }
                 }
             }