Repository: bitcoin/bitcoin
File: src/test/headers_sync_chainwork_tests.cpp (C++)

Code before:
        next_header.nTime = prev_time+1;
        next_header.nBits = nBits;

        FindProofOfWork(next_header);
        prev_hash = next_header.GetHash();
        prev_time = next_header.nTime;
    }
    return;
}

BOOST_FIXTURE_TEST_SUITE(headers_sync_chainwork_tests, HeadersGeneratorSetup)

// In this test, we construct two sets of headers from genesis, one with
// sufficient proof of work and one without.
// 1. We deliver the first set of headers and verify that the headers sync state
//    updates to the REDOWNLOAD phase successfully.
// 2. Then we deliver the second set of headers and verify that they fail
//    processing (presumably due to commitments not matching).
// 3. Finally, we verify that repeating with the first set of headers in both
//    phases is successful.
BOOST_AUTO_TEST_CASE(headers_sync_state)
{
    std::vector<CBlockHeader> first_chain;
    std::vector<CBlockHeader> second_chain;

    std::unique_ptr<HeadersSyncState> hss;

    const int target_blocks = 15000;
    arith_uint256 chain_work = target_blocks*2;

    // Generate headers for two different chains (using differing merkle roots
    // to ensure the headers are different).
    GenerateHeaders(first_chain, target_blocks-1, Params().GenesisBlock().GetHash(),
            Params().GenesisBlock().nVersion, Params().GenesisBlock().nTime,
            ArithToUint256(0), Params().GenesisBlock().nBits);

    GenerateHeaders(second_chain, target_blocks-2, Params().GenesisBlock().GetHash(),
            Params().GenesisBlock().nVersion, Params().GenesisBlock().nTime,
            ArithToUint256(1), Params().GenesisBlock().nBits);

    const CBlockIndex* chain_start = WITH_LOCK(::cs_main, return m_node.chainman->m_blockman.LookupBlockIndex(Params().GenesisBlock().GetHash()));
    std::vector<CBlockHeader> headers_batch;

    // Feed the first chain to HeadersSyncState, by delivering 1 header
    // initially and then the rest.
    headers_batch.insert(headers_batch.end(), std::next(first_chain.begin()), first_chain.end());

    hss.reset(new HeadersSyncState(0, Params().GetConsensus(), chain_start, chain_work));
    (void)hss->ProcessNextHeaders({first_chain.front()}, true);
    // Pretend the first header is still "full", so we don't abort.
    auto result = hss->ProcessNextHeaders(headers_batch, true);

Review comment (refactor):
I think we could do these in the previous refactor commit instead - to separate low risk changes from ones where we have to may more attention, while keeping the first one about move-only changes + minor ones. Or do the first commit as strictly move-only and add a second one with these tiny refactors (I'd like the latter more).

Code after:
// value (123) so our redownload buffer is well below the number of blocks
// required to reach the CHAIN_WORK threshold, to behave similarly to mainnet.
constexpr size_t REDOWNLOAD_BUFFER_SIZE{TARGET_BLOCKS - (MAX_HEADERS_RESULTS + 123)};
constexpr size_t COMMITMENT_PERIOD{600}; // Somewhat close to mainnet.

struct HeadersGeneratorSetup : public RegTestingSetup {
    const CBlock& genesis{Params().GenesisBlock()};
    const CBlockIndex* chain_start{WITH_LOCK(::cs_main, return m_node.chainman->m_blockman.LookupBlockIndex(genesis.GetHash()))};

    // Generate headers for two different chains (using differing merkle roots
    // to ensure the headers are different).
    const std::vector<CBlockHeader>& FirstChain()
    {
        // Block header hash target is half of max uint256 (2**256 / 2), expressible
        // roughly as the coefficient 0x7fffff with the exponent 0x20 (32 bytes).
        // This implies around every 2nd hash attempt should succeed, which
        // is why CHAIN_WORK == TARGET_BLOCKS * 2.
        assert(genesis.nBits == 0x207fffff);

        // Subtract 1 since the genesis block also contributes work so we reach
        // the CHAIN_WORK target.
        static const auto first_chain{GenerateHeaders(/*count=*/TARGET_BLOCKS - 1, genesis.GetHash(),
                genesis.nVersion, genesis.nTime, /*merkle_root=*/uint256::ZERO, genesis.nBits)};
        return first_chain;
    }
    const std::vector<CBlockHeader>& SecondChain()
    {
        // Subtract 2 to keep total work below the target.
        static const auto second_chain{GenerateHeaders(/*count=*/TARGET_BLOCKS - 2, genesis.GetHash(),
                genesis.nVersion, genesis.nTime, /*merkle_root=*/uint256::ONE, genesis.nBits)};
        return second_chain;
    }

    HeadersSyncState CreateState()
    {
        return {/*id=*/0,
                Params().GetConsensus(),
                HeadersSyncParams{
                    .commitment_period = COMMITMENT_PERIOD,
                    .redownload_buffer_size = REDOWNLOAD_BUFFER_SIZE,
                },
                chain_start,
                /*minimum_required_work=*/CHAIN_WORK};
    }

private:
    /** Search for a nonce to meet (regtest) proof of work */
    void FindProofOfWork(CBlockHeader& starting_header);
    /**
     * Generate headers in a chain that build off a given starting hash, using
     * the given nVersion, advancing time by 1 second from the starting