Add rules--presently disabled--for using GetMedianTimePast as endpoint for lock-time calculations

The lock-time code currently uses CBlock::nTime as the cutoff point for time based locked transactions. This has the unfortunate outcome of creating a perverse incentive for miners to lie about the time of a block in order to collect more fees by including transactions that by wall clock determination have not yet matured. By using CBlockIndex::GetMedianTimePast from the prior block instead, the self-interested miner no longer gains from generating blocks with fraudulent timestamps. Users can compensate for this change by simply adding an hour (3600 seconds) to their time-based lock times.

If enforced, this would be a soft-fork change. This commit only adds the functionality on an unexecuted code path, without changing the behaviour of Bitcoin Core.

src/consensus/consensus.h

@@ -13,4 +13,10 @@ static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
 /** Coinbase transaction outputs can only be spent after this number of new blocks (network rule) */
 static const int COINBASE_MATURITY = 100;
 
+/** Flags for LockTime() */
+enum {
+    /* Use GetMedianTimePast() instead of nTime for end point timestamp. */
+    LOCKTIME_MEDIAN_TIME_PAST = (1 << 1),
+};
+
 #endif // BITCOIN_CONSENSUS_CONSENSUS_H

src/main.cpp

@@ -650,10 +650,35 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime)
     return true;
 }
 
-bool CheckFinalTx(const CTransaction &tx)
+bool CheckFinalTx(const CTransaction &tx, int flags)
 {
     AssertLockHeld(cs_main);
-    return IsFinalTx(tx, chainActive.Height() + 1, GetAdjustedTime());
+
+    // By convention a negative value for flags indicates that the
+    // current network-enforced consensus rules should be used. In
+    // a future soft-fork scenario that would mean checking which
+    // rules would be enforced for the next block and setting the
+    // appropriate flags. At the present time no soft-forks are
+    // scheduled, so no flags are set.
+    flags = std::max(flags, 0);
+
+    // CheckFinalTx() uses chainActive.Height()+1 to evaluate
+    // nLockTime because when IsFinalTx() is called within
+    // CBlock::AcceptBlock(), the height of the block *being*
+    // evaluated is what is used. Thus if we want to know if a
+    // transaction can be part of the *next* block, we need to call
+    // IsFinalTx() with one more than chainActive.Height().
+    const int nBlockHeight = chainActive.Height() + 1;
+
+    // Timestamps on the other hand don't get any special treatment,
+    // because we can't know what timestamp the next block will have,
+    // and there aren't timestamp applications where it matters.
+    // However this changes once median past time-locks are enforced:
+    const int64_t nBlockTime = (flags & LOCKTIME_MEDIAN_TIME_PAST)
+                             ? chainActive.Tip()->GetMedianTimePast()
+                             : GetAdjustedTime();
+
+    return IsFinalTx(tx, nBlockHeight, nBlockTime);
 }
 
 unsigned int GetLegacySigOpCount(const CTransaction& tx)
@@ -797,7 +822,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     // 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.
-    if (!CheckFinalTx(tx))
+    if (!CheckFinalTx(tx, STANDARD_LOCKTIME_VERIFY_FLAGS))
         return state.DoS(0, false, REJECT_NONSTANDARD, "non-final");
 
     // is it already in the memory pool?
@@ -2723,10 +2748,15 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, CBlockIn
     const Consensus::Params& consensusParams = Params().GetConsensus();
 
     // Check that all transactions are finalized
-    BOOST_FOREACH(const CTransaction& tx, block.vtx)
+    BOOST_FOREACH(const CTransaction& tx, block.vtx) {
-        if (!IsFinalTx(tx, nHeight, block.GetBlockTime())) {
+        int nLockTimeFlags = 0;
+        int64_t nLockTimeCutoff = (nLockTimeFlags & LOCKTIME_MEDIAN_TIME_PAST)
+                                ? pindexPrev->GetMedianTimePast()
+                                : block.GetBlockTime();
+        if (!IsFinalTx(tx, nHeight, nLockTimeCutoff)) {
             return state.DoS(10, error("%s: contains a non-final transaction", __func__), REJECT_INVALID, "bad-txns-nonfinal");
         }
+    }
 
     // Enforce block.nVersion=2 rule that the coinbase starts with serialized block height
     // if 750 of the last 1,000 blocks are version 2 or greater (51/100 if testnet):

src/main.h

@@ -308,8 +308,10 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime);
  * Check if transaction will be final in the next block to be created.
  *
  * Calls IsFinalTx() with current block height and appropriate block time.
+ *
+ * See consensus/consensus.h for flag definitions.
  */
-bool CheckFinalTx(const CTransaction &tx);
+bool CheckFinalTx(const CTransaction &tx, int flags = -1);
 
 /** 
  * Closure representing one script verification

src/miner.cpp

@@ -148,6 +148,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         CBlockIndex* pindexPrev = chainActive.Tip();
         const int nHeight = pindexPrev->nHeight + 1;
         pblock->nTime = GetAdjustedTime();
+        const int64_t nMedianTimePast = pindexPrev->GetMedianTimePast();
         CCoinsViewCache view(pcoinsTip);
 
         // Priority order to process transactions
@@ -162,7 +163,12 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
              mi != mempool.mapTx.end(); ++mi)
         {
             const CTransaction& tx = mi->GetTx();
-            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, pblock->nTime))
+
+            int64_t nLockTimeCutoff = (STANDARD_LOCKTIME_VERIFY_FLAGS & LOCKTIME_MEDIAN_TIME_PAST)
+                                    ? nMedianTimePast
+                                    : pblock->GetBlockTime();
+
+            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, nLockTimeCutoff))
                 continue;
 
             COrphan* porphan = NULL;

src/policy/policy.h

@@ -43,6 +43,9 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
 /** For convenience, standard but not mandatory verify flags. */
 static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;
 
+/** Used as the flags parameter to CheckFinalTx() in non-consensus code */
+static const unsigned int STANDARD_LOCKTIME_VERIFY_FLAGS = 0;
+
 bool IsStandard(const CScript& scriptPubKey, txnouttype& whichType);
     /**
      * Check for standard transaction types