[validation] make CheckSequenceLocks context-free

Allow CheckSequenceLocks to use heights and coins from any CoinsView and
CBlockIndex provided. This means that CheckSequenceLocks() doesn't need
to hold the mempool lock or cs_main. The caller is responsible for
ensuring the CoinsView and CBlockIndex are consistent before passing
them in. The typical usage is still to create a CCoinsViewMemPool from
the mempool and grab the CBlockIndex from the chainstate tip.

src/test/miner_tests.cpp

@@ -28,7 +28,8 @@ struct MinerTestingSetup : public TestingSetup {
     void TestPackageSelection(const CChainParams& chainparams, const CScript& scriptPubKey, const std::vector<CTransactionRef>& txFirst) EXCLUSIVE_LOCKS_REQUIRED(::cs_main, m_node.mempool->cs);
     bool TestSequenceLocks(const CTransaction& tx, int flags) EXCLUSIVE_LOCKS_REQUIRED(::cs_main, m_node.mempool->cs)
     {
-        return CheckSequenceLocks(::ChainstateActive(), *m_node.mempool, tx, flags);
+        CCoinsViewMemPool viewMempool(&m_node.chainman->ActiveChainstate().CoinsTip(), *m_node.mempool);
+        return CheckSequenceLocks(m_node.chainman->ActiveChain().Tip(), viewMempool, tx, flags);
     }
     BlockAssembler AssemblerForTest(const CChainParams& params);
 };

src/txmempool.cpp

@@ -515,7 +515,9 @@ void CTxMemPool::removeForReorg(CChainState& active_chainstate, int flags)
         LockPoints lp = it->GetLockPoints();
         assert(std::addressof(::ChainstateActive()) == std::addressof(active_chainstate));
         bool validLP =  TestLockPointValidity(active_chainstate.m_chain, &lp);
-        if (!CheckFinalTx(active_chainstate.m_chain.Tip(), tx, flags) || !CheckSequenceLocks(active_chainstate, *this, tx, flags, &lp, validLP)) {
+        CCoinsViewMemPool viewMempool(&active_chainstate.CoinsTip(), *this);
+        if (!CheckFinalTx(active_chainstate.m_chain.Tip(), tx, flags)
+            || !CheckSequenceLocks(active_chainstate.m_chain.Tip(), viewMempool, tx, flags, &lp, validLP)) {
             // Note if CheckSequenceLocks fails the LockPoints may still be invalid
             // So it's critical that we remove the tx and not depend on the LockPoints.
             txToRemove.insert(it);

src/validation.cpp

@@ -245,18 +245,13 @@ bool TestLockPointValidity(CChain& active_chain, const LockPoints* lp)
     return true;
 }
 
-bool CheckSequenceLocks(CChainState& active_chainstate,
-                        const CTxMemPool& pool,
+bool CheckSequenceLocks(CBlockIndex* tip,
+                        const CCoinsView& coins_view,
                         const CTransaction& tx,
                         int flags,
                         LockPoints* lp,
                         bool useExistingLockPoints)
 {
-    AssertLockHeld(cs_main);
-    AssertLockHeld(pool.cs);
-    assert(std::addressof(::ChainstateActive()) == std::addressof(active_chainstate));
-
-    CBlockIndex* tip = active_chainstate.m_chain.Tip();
     assert(tip != nullptr);
 
     CBlockIndex index;
@@ -276,14 +271,12 @@ bool CheckSequenceLocks(CChainState& active_chainstate,
         lockPair.second = lp->time;
     }
     else {
-        // CoinsTip() contains the UTXO set for active_chainstate.m_chain.Tip()
-        CCoinsViewMemPool viewMemPool(&active_chainstate.CoinsTip(), pool);
         std::vector<int> prevheights;
         prevheights.resize(tx.vin.size());
         for (size_t txinIndex = 0; txinIndex < tx.vin.size(); txinIndex++) {
             const CTxIn& txin = tx.vin[txinIndex];
             Coin coin;
-            if (!viewMemPool.GetCoin(txin.prevout, coin)) {
+            if (!coins_view.GetCoin(txin.prevout, coin)) {
                 return error("%s: Missing input", __func__);
             }
             if (coin.nHeight == MEMPOOL_HEIGHT) {
@@ -686,10 +679,10 @@ bool MemPoolAccept::PreChecks(ATMPArgs& args, Workspace& ws)
     // Only accept BIP68 sequence locked transactions that can be mined in the next
     // block; we don't want our mempool filled up with transactions that can't
     // be mined yet.
-    // Must keep pool.cs for this unless we change CheckSequenceLocks to take a
-    // CoinsViewCache instead of create its own
+    // Pass in m_view which has all of the relevant inputs cached. Note that, since m_view's
+    // backend was removed, it no longer pulls coins from the mempool.
     assert(std::addressof(::ChainstateActive()) == std::addressof(m_active_chainstate));
-    if (!CheckSequenceLocks(m_active_chainstate, m_pool, tx, STANDARD_LOCKTIME_VERIFY_FLAGS, &lp))
+    if (!CheckSequenceLocks(m_active_chainstate.m_chain.Tip(), m_view, tx, STANDARD_LOCKTIME_VERIFY_FLAGS, &lp))
         return state.Invalid(TxValidationResult::TX_PREMATURE_SPEND, "non-BIP68-final");
 
     assert(std::addressof(g_chainman.m_blockman) == std::addressof(m_active_chainstate.m_blockman));

src/validation.h

@@ -226,9 +226,13 @@ bool CheckFinalTx(const CBlockIndex* active_chain_tip, const CTransaction &tx, i
 bool TestLockPointValidity(CChain& active_chain, const LockPoints* lp) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
 /**
- * Check if transaction will be BIP 68 final in the next block to be created.
- *
- * Simulates calling SequenceLocks() with data from the tip of the current active chain.
+ * Check if transaction will be BIP68 final in the next block to be created on top of tip.
+ * @param[in]   tip             Chain tip to check tx sequence locks against. For example,
+ *                              the tip of the current active chain.
+ * @param[in]   coins_view      Any CCoinsView that provides access to the relevant coins
+ *                              for checking sequence locks. Any CCoinsView can be passed in;
+ *                              it is assumed to be consistent with the tip.
+ * Simulates calling SequenceLocks() with data from the tip passed in.
  * Optionally stores in LockPoints the resulting height and time calculated and the hash
  * of the block needed for calculation or skips the calculation and uses the LockPoints
  * passed in for evaluation.
@@ -236,12 +240,12 @@ bool TestLockPointValidity(CChain& active_chain, const LockPoints* lp) EXCLUSIVE
  *
  * See consensus/consensus.h for flag definitions.
  */
-bool CheckSequenceLocks(CChainState& active_chainstate,
-                        const CTxMemPool& pool,
+bool CheckSequenceLocks(CBlockIndex* tip,
+                        const CCoinsView& coins_view,
                         const CTransaction& tx,
                         int flags,
                         LockPoints* lp = nullptr,
-                        bool useExistingLockPoints = false) EXCLUSIVE_LOCKS_REQUIRED(::cs_main, pool.cs);
+                        bool useExistingLockPoints = false);
 
 /**
  * Closure representing one script verification