比特币区块检查

比特币采用Pow共识机制，即不断调整Nonce值，对区块头做双重SHA256哈希运算，使得结果满足给定数量前导0的哈希值的过程。其中前导0的个数，取决于挖矿难度，前导0的个数越多，挖矿难度越大。

1 块儿产生时的检查

首先，生成铸币交易，并与其它所有准备打包进区块的交易组成交易列表，生成Merkle根哈希值；

其次，将Merkle根哈希值，与区块头其它字段组成区块头，80字节长度的区块头作为Pow算法的输入；

再次，不断变更区块头的随机数Nonce，对变更后的区块头做SHA256哈希运算，与当前难度的目标值做对比，如果小于目标难度，即Pow完成；

最后，Pow完成的区块向全网广播，其它节点将验证其是否符合规则，如果验证有效，其它节点将接收此区块，并附加在已有区块链之后；

之后，将进入下一轮挖矿。

当新的区块产生或者其他节点发送过来一个新的区块时(generateBlocks/net_processing)，都会进行合法性校验。

static UniValue generateBlocks(const CTxMemPool& mempool, const CScript& coinbase_script, int nGenerate, uint64_t nMaxTries)
{
    int nHeightEnd = 0;
    int nHeight = 0;

    {   // Don't keep cs_main locked
        LOCK(cs_main);
        nHeight = ::ChainActive().Height();
        nHeightEnd = nHeight+nGenerate;
    }
    unsigned int nExtraNonce = 0;
    UniValue blockHashes(UniValue::VARR);
    while (nHeight < nHeightEnd && !ShutdownRequested())
    {
        std::unique_ptr<CBlockTemplate> pblocktemplate(BlockAssembler(mempool, Params()).CreateNewBlock(coinbase_script));
        if (!pblocktemplate.get())
            throw JSONRPCError(RPC_INTERNAL_ERROR, "Couldn't create new block");
        CBlock *pblock = &pblocktemplate->block;
        {
            LOCK(cs_main);
            IncrementExtraNonce(pblock, ::ChainActive().Tip(), nExtraNonce);
        }
        while (nMaxTries > 0 && pblock->nNonce < std::numeric_limits<uint32_t>::max() && !CheckProofOfWork(pblock->GetHash(), pblock->nBits, Params().GetConsensus()) && !ShutdownRequested()) {
            ++pblock->nNonce;
            --nMaxTries;
        }
        if (nMaxTries == 0 || ShutdownRequested()) {
            break;
        }
        if (pblock->nNonce == std::numeric_limits<uint32_t>::max()) {
            continue;
        }
        std::shared_ptr<const CBlock> shared_pblock = std::make_shared<const CBlock>(*pblock);
        if (!ProcessNewBlock(Params(), shared_pblock, true, nullptr))
            throw JSONRPCError(RPC_INTERNAL_ERROR, "ProcessNewBlock, block not accepted");
        ++nHeight;
        blockHashes.push_back(pblock->GetHash().GetHex());
    }
    return blockHashes;
}

以上源码第21行IncrementExtraNonce函数中，会通过修改铸币交易中的ExtraNonce字段来更新coinbase交易，从而更新区块MerkleRoot。

void IncrementExtraNonce(CBlock* pblock, const CBlockIndex* pindexPrev, unsigned int& nExtraNonce)
{
    // Update nExtraNonce
    static uint256 hashPrevBlock;
    if (hashPrevBlock != pblock->hashPrevBlock)
    {
        nExtraNonce = 0;
        hashPrevBlock = pblock->hashPrevBlock;
    }
    ++nExtraNonce;
    unsigned int nHeight = pindexPrev->nHeight+1; // Height first in coinbase required for block.version=2
    CMutableTransaction txCoinbase(*pblock->vtx[0]);
    txCoinbase.vin[0].scriptSig = (CScript() << nHeight << CScriptNum(nExtraNonce));
    assert(txCoinbase.vin[0].scriptSig.size() <= 100);

    pblock->vtx[0] = MakeTransactionRef(std::move(txCoinbase));
    pblock->hashMerkleRoot = BlockMerkleRoot(*pblock);
}

第23~26行通过不断更新nNonce来检查区块的double sha256哈希值是否满足难度要求。

第34行函数ProcessNewBlock中将对生成的新区块进行检查，主要功能参见CheckBlock函数。

bool CheckBlock(const CBlock& block, BlockValidationState& state, const Consensus::Params& consensusParams, bool fCheckPOW, bool fCheckMerkleRoot)
{
    // These are checks that are independent of context.

    if (block.fChecked)
        return true;

    // Check that the header is valid (particularly PoW).  This is mostly
    // redundant with the call in AcceptBlockHeader.
    if (!CheckBlockHeader(block, state, consensusParams, fCheckPOW))
        return false;

    // Check the merkle root.
    if (fCheckMerkleRoot) {
        bool mutated;
        uint256 hashMerkleRoot2 = BlockMerkleRoot(block, &mutated);
        if (block.hashMerkleRoot != hashMerkleRoot2)
            return state.Invalid(BlockValidationResult::BLOCK_MUTATED, "bad-txnmrklroot", "hashMerkleRoot mismatch");

        // Check for merkle tree malleability (CVE-2012-2459): repeating sequences
        // of transactions in a block without affecting the merkle root of a block,
        // while still invalidating it.
        if (mutated)
            return state.Invalid(BlockValidationResult::BLOCK_MUTATED, "bad-txns-duplicate", "duplicate transaction");
    }

    // All potential-corruption validation must be done before we do any
    // transaction validation, as otherwise we may mark the header as invalid
    // because we receive the wrong transactions for it.
    // Note that witness malleability is checked in ContextualCheckBlock, so no
    // checks that use witness data may be performed here.

    // Size limits
    if (block.vtx.empty() || block.vtx.size() * WITNESS_SCALE_FACTOR > MAX_BLOCK_WEIGHT || ::GetSerializeSize(block, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) * WITNESS_SCALE_FACTOR > MAX_BLOCK_WEIGHT)
        return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "bad-blk-length", "size limits failed");

    // First transaction must be coinbase, the rest must not be
    if (block.vtx.empty() || !block.vtx[0]->IsCoinBase())
        return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "bad-cb-missing", "first tx is not coinbase");
    for (unsigned int i = 1; i < block.vtx.size(); i++)
        if (block.vtx[i]->IsCoinBase())
            return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "bad-cb-multiple", "more than one coinbase");

    // Check transactions
    // Must check for duplicate inputs (see CVE-2018-17144)
    for (const auto& tx : block.vtx) {
        TxValidationState tx_state;
        if (!CheckTransaction(*tx, tx_state)) {
            // CheckBlock() does context-free validation checks. The only
            // possible failures are consensus failures.
            assert(tx_state.GetResult() == TxValidationResult::TX_CONSENSUS);
            return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, tx_state.GetRejectReason(),
                                 strprintf("Transaction check failed (tx hash %s) %s", tx->GetHash().ToString(), tx_state.GetDebugMessage()));
        }
    }
    unsigned int nSigOps = 0;
    for (const auto& tx : block.vtx)
    {
        nSigOps += GetLegacySigOpCount(*tx);
    }
    if (nSigOps * WITNESS_SCALE_FACTOR > MAX_BLOCK_SIGOPS_COST)
        return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS, "bad-blk-sigops", "out-of-bounds SigOpCount");

    if (fCheckPOW && fCheckMerkleRoot)
        block.fChecked = true;

    return true;
}

在该函数中，会对区块进行POW检查、merkleroot检查、块儿size limits检查、coinbase检查、transactions检查、签名检查等。

2 块儿接收及本地存储时的检查

主要结合AcceptBlockHeader函数进行讲解，其他细节请参考源码。

bool BlockManager::AcceptBlockHeader(const CBlockHeader& block, BlockValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex)
{
    AssertLockHeld(cs_main);
    // Check for duplicate
    uint256 hash = block.GetHash();
    BlockMap::iterator miSelf = m_block_index.find(hash);
    CBlockIndex *pindex = nullptr;
    if (hash != chainparams.GetConsensus().hashGenesisBlock) {
        if (miSelf != m_block_index.end()) {
            // Block header is already known.
            pindex = miSelf->second;
            if (ppindex)
                *ppindex = pindex;
            if (pindex->nStatus & BLOCK_FAILED_MASK) {
                LogPrintf("ERROR: %s: block %s is marked invalid\n", __func__, hash.ToString());
                return state.Invalid(BlockValidationResult::BLOCK_CACHED_INVALID, "duplicate");
            }
            return true;
        }

        if (!CheckBlockHeader(block, state, chainparams.GetConsensus()))
            return error("%s: Consensus::CheckBlockHeader: %s, %s", __func__, hash.ToString(), state.ToString());

        // Get prev block index
        CBlockIndex* pindexPrev = nullptr;
        BlockMap::iterator mi = m_block_index.find(block.hashPrevBlock);
        if (mi == m_block_index.end()) {
            LogPrintf("ERROR: %s: prev block not found\n", __func__);
            return state.Invalid(BlockValidationResult::BLOCK_MISSING_PREV, "prev-blk-not-found");
        }
        pindexPrev = (*mi).second;
        if (pindexPrev->nStatus & BLOCK_FAILED_MASK) {
            LogPrintf("ERROR: %s: prev block invalid\n", __func__);
            return state.Invalid(BlockValidationResult::BLOCK_INVALID_PREV, "bad-prevblk");
        }
        if (!ContextualCheckBlockHeader(block, state, chainparams, pindexPrev, GetAdjustedTime()))
            return error("%s: Consensus::ContextualCheckBlockHeader: %s, %s", __func__, hash.ToString(), state.ToString());

        /* Determine if this block descends from any block which has been found
         * invalid (m_failed_blocks), then mark pindexPrev and any blocks between
         * them as failed. For example:
         *
         *                D3
         *              /
         *      B2 - C2
         *    /         \
         *  A             D2 - E2 - F2
         *    \
         *      B1 - C1 - D1 - E1
         *
         * In the case that we attempted to reorg from E1 to F2, only to find
         * C2 to be invalid, we would mark D2, E2, and F2 as BLOCK_FAILED_CHILD
         * but NOT D3 (it was not in any of our candidate sets at the time).
         *
         * In any case D3 will also be marked as BLOCK_FAILED_CHILD at restart
         * in LoadBlockIndex.
         */
        if (!pindexPrev->IsValid(BLOCK_VALID_SCRIPTS)) {
            // The above does not mean "invalid": it checks if the previous block
            // hasn't been validated up to BLOCK_VALID_SCRIPTS. This is a performance
            // optimization, in the common case of adding a new block to the tip,
            // we don't need to iterate over the failed blocks list.
            for (const CBlockIndex* failedit : m_failed_blocks) {
                if (pindexPrev->GetAncestor(failedit->nHeight) == failedit) {
                    assert(failedit->nStatus & BLOCK_FAILED_VALID);
                    CBlockIndex* invalid_walk = pindexPrev;
                    while (invalid_walk != failedit) {
                        invalid_walk->nStatus |= BLOCK_FAILED_CHILD;
                        setDirtyBlockIndex.insert(invalid_walk);
                        invalid_walk = invalid_walk->pprev;
                    }
                    LogPrintf("ERROR: %s: prev block invalid\n", __func__);
                    return state.Invalid(BlockValidationResult::BLOCK_INVALID_PREV, "bad-prevblk");
                }
            }
        }
    }
    if (pindex == nullptr)
        pindex = AddToBlockIndex(block);

    if (ppindex)
        *ppindex = pindex;

    return true;
}

第21行CheckBlockHeader函数主要进行POW检查。

第36行ContextualCheckBlockHeader函数进行上下文检查。

static bool ContextualCheckBlockHeader(const CBlockHeader& block, BlockValidationState& state, const CChainParams& params, const CBlockIndex* pindexPrev, int64_t nAdjustedTime) EXCLUSIVE_LOCKS_REQUIRED(cs_main)
{
    assert(pindexPrev != nullptr);
    const int nHeight = pindexPrev->nHeight + 1;

    // Check proof of work
    const Consensus::Params& consensusParams = params.GetConsensus();
    if (block.nBits != GetNextWorkRequired(pindexPrev, &block, consensusParams))
        return state.Invalid(BlockValidationResult::BLOCK_INVALID_HEADER, "bad-diffbits", "incorrect proof of work");

    // Check against checkpoints
    if (fCheckpointsEnabled) {
        // Don't accept any forks from the main chain prior to last checkpoint.
        // GetLastCheckpoint finds the last checkpoint in MapCheckpoints that's in our
        // g_blockman.m_block_index.
        CBlockIndex* pcheckpoint = GetLastCheckpoint(params.Checkpoints());
        if (pcheckpoint && nHeight < pcheckpoint->nHeight) {
            LogPrintf("ERROR: %s: forked chain older than last checkpoint (height %d)\n", __func__, nHeight);
            return state.Invalid(BlockValidationResult::BLOCK_CHECKPOINT, "bad-fork-prior-to-checkpoint");
        }
    }

    // Check timestamp against prev
    if (block.GetBlockTime() <= pindexPrev->GetMedianTimePast())
        return state.Invalid(BlockValidationResult::BLOCK_INVALID_HEADER, "time-too-old", "block's timestamp is too early");

    // Check timestamp
    if (block.GetBlockTime() > nAdjustedTime + MAX_FUTURE_BLOCK_TIME)
        return state.Invalid(BlockValidationResult::BLOCK_TIME_FUTURE, "time-too-new", "block timestamp too far in the future");

    // Reject outdated version blocks when 95% (75% on testnet) of the network has upgraded:
    // check for version 2, 3 and 4 upgrades
    if((block.nVersion < 2 && nHeight >= consensusParams.BIP34Height) ||
       (block.nVersion < 3 && nHeight >= consensusParams.BIP66Height) ||
       (block.nVersion < 4 && nHeight >= consensusParams.BIP65Height))
            return state.Invalid(BlockValidationResult::BLOCK_INVALID_HEADER, strprintf("bad-version(0x%08x)", block.nVersion),
                                 strprintf("rejected nVersion=0x%08x block", block.nVersion));

    return true;
}

在该函数中会依次进行nBits检查、checkpoints检查、时间戳检查、nVersion检查。

第58行之后代码将会进行父块儿有效性检查。

下面详细说明时间戳检查

2.1 当前产生块儿时间戳

首先确定当前产生区块儿的时间戳nTime，它并不是直接取本地系统时间，而是当前时间+偏移量(可以理解成网络延迟)。

偏移量的计算方法：

1 取网络上其他节点的系统时间x(建立节点时发出version请求)
2 计算出x与本地系统时间的差值y
3 取n个y并排序，n的值在5-200中间
4 取上述排序的中位数即为偏移量(网络延迟)
5 如其他节点数量小于5个，则偏移量为0

该部分内容详情可查看源码timedata.cpp中AddTimeData函数实现，之后还会调用UpdateTime函数进一步更新该时间戳，取nTime和前一块儿medianTime的较大值。

medianTime时间由GetMedianTimePast函数获取，该函数从当前块儿开始，取nMedianTimeSpan(默认11)个区块时间排序的中位值。

2.2 时间戳检查

时间戳检查时，从两个方面进行检查

1 过去时间中值（MPT）规则

向前比较，时间戳必须比过去 11 个区块的中值更大，否则认为块儿太老了（错误信息：block's timestamp is too early）。

2 未来区块时间规则

向后比较，根据MAX_FUTURE_BLOCK_TIME（2 * 60 * 60）常量，相比当前节点的中值时间，块儿时间戳不能出现在未来2个小时以上。

否则认为块儿太新了（错误信息：block timestamp too far in the future）。

参考：

https://blog.bitmex.com/zh_cn-bitcoins-block-timestamp-protection-rules/