feat: use longest common prefix for determining tx replay set

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to the versioning scheme outlined in the [README.md](README.md).
 
+## Unreleased
+
+### Added
+
+- When determining a global transaction replay set, the state evaluator now uses a longest-common-prefix algorithm to find a replay set in the case where a single replay set has less than 70% of signer weight.
+
 ## [3.2.0.0.1]
 ### Added
 

libsigner/src/tests/signer_state.rs

@@ -29,6 +29,117 @@ use crate::v0::messages::{
 };
 use crate::v0::signer_state::{GlobalStateEvaluator, ReplayTransactionSet, SignerStateMachine};
 
+/// Test setup helper struct containing common test data
+struct SignerStateTest {
+    global_eval: GlobalStateEvaluator,
+    addresses: Vec<StacksAddress>,
+    burn_block: ConsensusHash,
+    burn_block_height: u64,
+    current_miner: StateMachineUpdateMinerState,
+    local_supported_signer_protocol_version: u64,
+    active_signer_protocol_version: u64,
+    tx_a: StacksTransaction,
+    tx_b: StacksTransaction,
+    tx_c: StacksTransaction,
+    tx_d: StacksTransaction,
+}
+
+impl SignerStateTest {
+    fn new(num_signers: u32) -> Self {
+        let global_eval = generate_global_state_evaluator(num_signers);
+        let addresses: Vec<_> = global_eval.address_weights.keys().cloned().collect();
+        let local_address = addresses[0].clone();
+
+        let burn_block = ConsensusHash([20u8; 20]);
+        let burn_block_height = 100;
+        let current_miner = StateMachineUpdateMinerState::ActiveMiner {
+            current_miner_pkh: Hash160([0xab; 20]),
+            tenure_id: ConsensusHash([0x44; 20]),
+            parent_tenure_id: ConsensusHash([0x22; 20]),
+            parent_tenure_last_block: StacksBlockId([0x33; 32]),
+            parent_tenure_last_block_height: 1,
+        };
+
+        let local_supported_signer_protocol_version = 1;
+        let active_signer_protocol_version = 1;
+
+        // Create test transactions with different memos for uniqueness
+        let pk1 = StacksPrivateKey::random();
+        let pk2 = StacksPrivateKey::random();
+        let pk3 = StacksPrivateKey::random();
+        let pk4 = StacksPrivateKey::random();
+
+        let make_tx = |pk: &StacksPrivateKey, memo: [u8; 34]| StacksTransaction {
+            version: TransactionVersion::Testnet,
+            chain_id: 0x80000000,
+            auth: TransactionAuth::from_p2pkh(pk).unwrap(),
+            anchor_mode: TransactionAnchorMode::Any,
+            post_condition_mode: TransactionPostConditionMode::Allow,
+            post_conditions: vec![],
+            payload: TransactionPayload::TokenTransfer(
+                local_address.clone().into(),
+                100,
+                TokenTransferMemo(memo),
+            ),
+        };
+
+        let tx_a = make_tx(&pk1, [1u8; 34]);
+        let tx_b = make_tx(&pk2, [2u8; 34]);
+        let tx_c = make_tx(&pk3, [3u8; 34]);
+        let tx_d = make_tx(&pk4, [4u8; 34]);
+
+        Self {
+            global_eval,
+            addresses,
+            burn_block,
+            burn_block_height,
+            current_miner,
+            local_supported_signer_protocol_version,
+            active_signer_protocol_version,
+            tx_a,
+            tx_b,
+            tx_c,
+            tx_d,
+        }
+    }
+
+    /// Create a replay transaction update message
+    fn create_replay_update(
+        &self,
+        transactions: Vec<StacksTransaction>,
+    ) -> StateMachineUpdateMessage {
+        StateMachineUpdateMessage::new(
+            self.active_signer_protocol_version,
+            self.local_supported_signer_protocol_version,
+            StateMachineUpdateContent::V1 {
+                burn_block: self.burn_block,
+                burn_block_height: self.burn_block_height,
+                current_miner: self.current_miner.clone(),
+                replay_transactions: transactions,
+            },
+        )
+        .unwrap()
+    }
+
+    /// Update multiple signers with the same replay transaction set
+    fn update_signers(&mut self, signer_indices: &[usize], transactions: Vec<StacksTransaction>) {
+        let update = self.create_replay_update(transactions);
+        for &index in signer_indices {
+            self.global_eval
+                .insert_update(self.addresses[index].clone(), update.clone());
+        }
+    }
+
+    /// Get the global state replay set
+    fn get_global_replay_set(&mut self) -> Vec<StacksTransaction> {
+        self.global_eval
+            .determine_global_state()
+            .unwrap()
+            .tx_replay_set
+            .unwrap_or_default()
+    }
+}
+
 fn generate_global_state_evaluator(num_addresses: u32) -> GlobalStateEvaluator {
     let address_weights = generate_random_address_with_equal_weights(num_addresses);
     let active_protocol_version = 0;
@@ -417,3 +528,181 @@ fn determine_global_states_with_tx_replay_set() {
         tx_replay_state_machine
     );
 }
+
+#[test]
+/// Case: One signer has [A,B,C], another has [A,B] - should find common prefix [A,B]
+fn test_replay_set_common_prefix_coalescing() {
+    let mut state_test = SignerStateTest::new(5);
+
+    // Signers 0, 1: [A,B,C] (40% weight)
+    state_test.update_signers(
+        &[0, 1],
+        vec![
+            state_test.tx_a.clone(),
+            state_test.tx_b.clone(),
+            state_test.tx_c.clone(),
+        ],
+    );
+
+    // Signers 2, 3, 4: [A,B] (60% weight - should win)
+    state_test.update_signers(
+        &[2, 3, 4],
+        vec![state_test.tx_a.clone(), state_test.tx_b.clone()],
+    );
+
+    let transactions = state_test.get_global_replay_set();
+
+    // Should find common prefix [A,B] since it's the longest prefix with majority support
+    assert_eq!(transactions.len(), 2);
+    assert_eq!(transactions[0], state_test.tx_a); // Order matters!
+    assert_eq!(transactions[1], state_test.tx_b);
+    assert!(!transactions.contains(&state_test.tx_c));
+}
+
+#[test]
+/// Case: One sequence has clear majority - should use that sequence
+fn test_replay_set_majority_prefix_selection() {
+    let mut state_test = SignerStateTest::new(5);
+
+    // Signer 0: [A] (20% weight)
+    state_test.update_signers(&[0], vec![state_test.tx_a.clone()]);
+
+    // Signers 1, 2, 3, 4: [C] (80% weight - above threshold)
+    state_test.update_signers(&[1, 2, 3, 4], vec![state_test.tx_c.clone()]);
+
+    let transactions = state_test.get_global_replay_set();
+
+    // Should use [C] since it has majority support (80% > 70%)
+    assert_eq!(transactions.len(), 1);
+    assert_eq!(transactions[0], state_test.tx_c);
+}
+
+#[test]
+/// Case: Exact agreement should be prioritized over subset coalescing
+fn test_replay_set_exact_agreement_prioritized() {
+    let mut state_test = SignerStateTest::new(5);
+
+    // 4 signers agree on [A,B] exactly (80% - above threshold)
+    state_test.update_signers(
+        &[0, 1, 2, 3],
+        vec![state_test.tx_a.clone(), state_test.tx_b.clone()],
+    );
+
+    // 1 signer has just [A] (20%)
+    state_test.update_signers(&[4], vec![state_test.tx_a.clone()]);
+
+    let transactions = state_test.get_global_replay_set();
+
+    // Should use exact agreement [A,B] rather than common prefix [A]
+    assert_eq!(transactions.len(), 2);
+    assert_eq!(transactions[0], state_test.tx_a); // Order matters!
+    assert_eq!(transactions[1], state_test.tx_b);
+}
+
+#[test]
+/// Case: Complete disagreement - no overlap and no majority
+fn test_replay_set_no_agreement_returns_empty() {
+    let mut state_test = SignerStateTest::new(5);
+
+    // Signer 0: [A] (20% weight)
+    state_test.update_signers(&[0], vec![state_test.tx_a.clone()]);
+
+    // Signer 1: [B] (20% weight)
+    state_test.update_signers(&[1], vec![state_test.tx_b.clone()]);
+
+    // Signer 2: [C] (20% weight)
+    state_test.update_signers(&[2], vec![state_test.tx_c.clone()]);
+
+    // Signers 3, 4: empty sets (40% weight)
+    state_test.update_signers(&[3, 4], vec![]);
+
+    let transactions = state_test.get_global_replay_set();
+
+    // Should return empty set to prioritize liveness when no agreement
+    assert_eq!(transactions.len(), 0);
+}
+
+#[test]
+/// Case: Same transactions in different order have no common prefix
+fn test_replay_set_order_matters_no_common_prefix() {
+    let mut state_test = SignerStateTest::new(4);
+
+    // Signers 0, 1: [A,B] (50% weight)
+    state_test.update_signers(
+        &[0, 1],
+        vec![state_test.tx_a.clone(), state_test.tx_b.clone()],
+    );
+
+    // Signers 2, 3: [B,A] (50% weight)
+    state_test.update_signers(
+        &[2, 3],
+        vec![state_test.tx_b.clone(), state_test.tx_a.clone()],
+    );
+
+    let transactions = state_test.get_global_replay_set();
+
+    // Should return empty set since [A,B] and [B,A] have no common prefix
+    // Even though both contain the same transactions, order matters for replay
+    assert_eq!(transactions.len(), 0);
+}
+
+#[test]
+/// Case: [A,B,C] vs [A,B,D] should find common prefix [A,B]
+fn test_replay_set_partial_prefix_match() {
+    let mut state_test = SignerStateTest::new(5);
+
+    // Signer 0, 1: [A,B,C] (40% weight)
+    state_test.update_signers(
+        &[0, 1],
+        vec![
+            state_test.tx_a.clone(),
+            state_test.tx_b.clone(),
+            state_test.tx_c.clone(),
+        ],
+    );
+
+    // Signers 2, 3, 4: [A,B,D] (60% weight)
+    state_test.update_signers(
+        &[2, 3, 4],
+        vec![
+            state_test.tx_a.clone(),
+            state_test.tx_b.clone(),
+            state_test.tx_d.clone(),
+        ],
+    );
+
+    let transactions = state_test.get_global_replay_set();
+
+    // Should find [A,B] as the longest common prefix with majority support
+    assert_eq!(transactions.len(), 2);
+    assert_eq!(transactions[0], state_test.tx_a);
+    assert_eq!(transactions[1], state_test.tx_b);
+}
+
+#[test]
+/// Edge case: Equal-weight competing prefixes should find common prefix
+fn test_replay_set_equal_weight_competing_prefixes() {
+    let mut state_test = SignerStateTest::new(6);
+
+    // Signers 0, 1, 2: [A,B] (50% weight - not enough alone)
+    state_test.update_signers(
+        &[0, 1, 2],
+        vec![state_test.tx_a.clone(), state_test.tx_b.clone()],
+    );
+
+    // Signers 3, 4, 5: [A,C] (50% weight - not enough alone)
+    state_test.update_signers(
+        &[3, 4, 5],
+        vec![state_test.tx_a.clone(), state_test.tx_c.clone()],
+    );
+
+    let transactions = state_test.get_global_replay_set();
+
+    // Should find common prefix [A] since both [A,B] and [A,C] start with [A]
+    // and [A] has 100% support (above the 70% threshold)
+    assert_eq!(transactions.len(), 1, "Should find common prefix [A]");
+    assert_eq!(
+        transactions[0], state_test.tx_a,
+        "Should contain transaction A"
+    );
+}