[consensus] Require large block at SegWit+BIP102HeightDelta fork point

Implements issue #29
Supercedes #35

Author: jgarzik

src/chainparams.cpp

@@ -75,6 +75,7 @@ class CMainParams : public CChainParams {
         consensus.BIP34Hash = uint256S("0x000000000000024b89b42a942fe0d9fea3bb44ab7bd1b19115dd6a759c0808b8");
         consensus.BIP65Height = 388381; // 000000000000000004c2b624ed5d7756c508d90fd0da2c7c679febfa6c4735f0
         consensus.BIP66Height = 363725; // 00000000000000000379eaa19dce8c9b722d46ae6a57c2f1a988119488b50931
+        consensus.BIP102HeightDelta = 144 * 90;
         consensus.powLimit = uint256S("00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
         consensus.nPowTargetTimespan = 14 * 24 * 60 * 60; // two weeks
         consensus.nPowTargetSpacing = 10 * 60;
@@ -186,6 +187,7 @@ class CTestNetParams : public CChainParams {
         consensus.BIP34Hash = uint256S("0x00000000fb7c0a2aeb5f1244e81921b84b7ac770004543144e10c2284f89bfd8");
         consensus.BIP65Height = 10001; // 00000000fb7c0a2aeb5f1244e81921b84b7ac770004543144e10c2284f89bfd8
         consensus.BIP66Height = 10001; // 00000000fb7c0a2aeb5f1244e81921b84b7ac770004543144e10c2284f89bfd8
+        consensus.BIP102HeightDelta = 144 * 90; // TODO: update re test plan
         consensus.powLimit = uint256S("00000000ffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
         consensus.nPowTargetTimespan = 14 * 24 * 60 * 60; // two weeks
         consensus.nPowTargetSpacing = 10 * 60;
@@ -276,6 +278,7 @@ class CRegTestParams : public CChainParams {
         consensus.BIP34Hash = uint256();
         consensus.BIP65Height = 1351; // BIP65 activated on regtest (Used in rpc activation tests)
         consensus.BIP66Height = 1251; // BIP66 activated on regtest (Used in rpc activation tests)
+        consensus.BIP102HeightDelta = 144 * 90; // TODO: update
         consensus.powLimit = uint256S("7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff");
         consensus.nPowTargetTimespan = 14 * 24 * 60 * 60; // two weeks
         consensus.nPowTargetSpacing = 10 * 60;

src/consensus/consensus.h

@@ -8,19 +8,13 @@
 
 #include <stdint.h>
 
-/** BIP102 block size increase - time until HF activated */
-static const unsigned int BIP102_FORK_BUFFER = (144 * 90);
-
 /** The maximum allowed size for a serialized block, in bytes (only for buffer size limits) */
 static const unsigned int MAX_BLOCK_SERIALIZED_SIZE = (8 * 1000 * 1000);
 
 /** The maximum allowed size for a block excluding witness data, in bytes (network rule) */
 static inline bool BIP102active(bool fSegwitSeasoned)
 {
-    if (!fSegwitSeasoned)
-        return false;
-
-    return true;
+    return fSegwitSeasoned;
 }
 
 static const unsigned int MAX_LEGACY_BLOCK_SIZE = (1 * 1000 * 1000);

src/consensus/params.h

@@ -51,6 +51,9 @@ struct Params {
     int BIP65Height;
     /** Block height at which BIP66 becomes active */
     int BIP66Height;
+    /** Block height delta at which BIP102 becomes active */
+    int BIP102HeightDelta;
+
     /**
      * Minimum blocks including miner confirmation of the total of 2016 blocks in a retargeting period,
      * (nPowTargetTimespan / nPowTargetSpacing) which is also used for BIP9 deployments.

src/validation.cpp

@@ -2938,10 +2938,7 @@ bool IsWitnessSeasoned(const CBlockIndex* pindexPrev, const Consensus::Params& p
 
     const int nHeight = pindexPrev->nHeight + 1;
 
-    if (nHeight < (int)BIP102_FORK_BUFFER)
-      return false;
-
-    const CBlockIndex* pindexForkBuffer = pindexPrev->GetAncestor(nHeight - BIP102_FORK_BUFFER);
+    const CBlockIndex* pindexForkBuffer = pindexPrev->GetAncestor(nHeight - params.BIP102HeightDelta);
 
     return (VersionBitsState(pindexForkBuffer, params, Consensus::DEPLOYMENT_SEGWIT, versionbitscache) == THRESHOLD_ACTIVE);
 }
@@ -3069,6 +3066,7 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, const Co
     //   multiple, the last one is used.
     bool fHaveWitness = false;
     bool fSegwitSeasoned = false;
+    bool fBIP102FirstBlock = false;
     bool fSegWitActive = (VersionBitsState(pindexPrev, consensusParams, Consensus::DEPLOYMENT_SEGWIT, versionbitscache) == THRESHOLD_ACTIVE);
     if (fSegWitActive) {
         int commitpos = GetWitnessCommitmentIndex(block);
@@ -3088,12 +3086,28 @@ bool ContextualCheckBlock(const CBlock& block, CValidationState& state, const Co
             fHaveWitness = true;
         }
 
-        fSegwitSeasoned = IsWitnessSeasoned(pindexPrev, consensusParams);
+        // Look back to test SegWit activation period
+        const CBlockIndex* pindexForkBuffer = pindexPrev ? pindexPrev->GetAncestor(nHeight - consensusParams.BIP102HeightDelta) : NULL;
+        fSegwitSeasoned = (VersionBitsState(pindexForkBuffer, consensusParams, Consensus::DEPLOYMENT_SEGWIT, versionbitscache) == THRESHOLD_ACTIVE);
+
+        // Look back one more block, to detect edge
+        if (fSegwitSeasoned) {
+            assert(pindexForkBuffer);
+            const CBlockIndex* pindexLastSeason = pindexForkBuffer->pprev;
+            fBIP102FirstBlock = (VersionBitsState(pindexLastSeason, consensusParams, Consensus::DEPLOYMENT_SEGWIT, versionbitscache) != THRESHOLD_ACTIVE);
+        }
     }
 
+    // Max block base size limit
     if (::GetSerializeSize(block, SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) > MaxBlockBaseSize(fSegwitSeasoned))
         return state.DoS(100, false, REJECT_INVALID, "bad-blk-length", false, "size limits failed");
 
+    // First block at fork must be large
+    if (fBIP102FirstBlock) {
+        if (::GetSerializeSize(block, SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) <= MAX_LEGACY_BLOCK_SIZE)
+            return state.DoS(100, false, REJECT_INVALID, "bad-blk-length-toosmall", false, "size limits failed");
+    }
+
     // No witness data is allowed in blocks that don't commit to witness data, as this would otherwise leave room for spam
     if (!fHaveWitness) {
         for (size_t i = 0; i < block.vtx.size(); i++) {