Version 2 transactions remain non-standard until CSV activates

Before activation, such transactions might not be mined, so don't
allow into the mempool.

src/main.cpp

@@ -1025,6 +1025,14 @@ bool AcceptToMemoryPoolWorker(CTxMemPool& pool, CValidationState& state, const C
     if (fRequireStandard && !IsStandardTx(tx, reason))
         return state.DoS(0, false, REJECT_NONSTANDARD, reason);
 
+    // Don't relay version 2 transactions until CSV is active, and we can be
+    // sure that such transactions will be mined (unless we're on
+    // -testnet/-regtest).
+    const CChainParams& chainparams = Params();
+    if (fRequireStandard && tx.nVersion >= 2 && VersionBitsTipState(chainparams.GetConsensus(), Consensus::DEPLOYMENT_CSV) != THRESHOLD_ACTIVE) {
+        return state.DoS(0, false, REJECT_NONSTANDARD, "premature-version2-tx");
+    }
+
     // Only accept nLockTime-using 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.