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

Code before:

    // This chain should look valid, and we should have met the proof-of-work
    // requirement.
    BOOST_CHECK(result.success);
    BOOST_CHECK(result.request_more);
    BOOST_CHECK(hss->GetState() == HeadersSyncState::State::REDOWNLOAD);

    // Try to sneakily feed back the second chain.
    result = hss->ProcessNextHeaders(second_chain, true);
    BOOST_CHECK(!result.success); // foiled!
    BOOST_CHECK(hss->GetState() == HeadersSyncState::State::FINAL);

    // Now try again, this time feeding the first chain twice.
    hss.reset(new HeadersSyncState(0, Params().GetConsensus(), chain_start, chain_work));
    (void)hss->ProcessNextHeaders(first_chain, true);
    BOOST_CHECK(hss->GetState() == HeadersSyncState::State::REDOWNLOAD);

    result = hss->ProcessNextHeaders(first_chain, true);
    BOOST_CHECK(result.success);
    BOOST_CHECK(!result.request_more);
    // All headers should be ready for acceptance:
    BOOST_CHECK(result.pow_validated_headers.size() == first_chain.size());
    // Nothing left for the sync logic to do:
    BOOST_CHECK(hss->GetState() == HeadersSyncState::State::FINAL);

    // Finally, verify that just trying to process the second chain would not
    // succeed (too little work)
    hss.reset(new HeadersSyncState(0, Params().GetConsensus(), chain_start, chain_work));
    BOOST_CHECK(hss->GetState() == HeadersSyncState::State::PRESYNC);
     // Pretend just the first message is "full", so we don't abort.
    (void)hss->ProcessNextHeaders({second_chain.front()}, true);
    BOOST_CHECK(hss->GetState() == HeadersSyncState::State::PRESYNC);

    headers_batch.clear();
    headers_batch.insert(headers_batch.end(), std::next(second_chain.begin(), 1), second_chain.end());
    // Tell the sync logic that the headers message was not full, implying no
    // more headers can be requested. For a low-work-chain, this should causes
    // the sync to end with no headers for acceptance.
    result = hss->ProcessNextHeaders(headers_batch, false);
    BOOST_CHECK(hss->GetState() == HeadersSyncState::State::FINAL);
    BOOST_CHECK(result.pow_validated_headers.empty());
    BOOST_CHECK(!result.request_more);
    // Nevertheless, no validation errors should have been detected with the
    // chain:
    BOOST_CHECK(result.success);
}

BOOST_AUTO_TEST_SUITE_END()


Review comment (suggestion):
we're reusing the previous `result` (which was brace initialized), what if we used braces to delimit the two independent code parts (+ a few other nits you might want to consider):
```suggestion
// Verify that just trying to process the second chain would not succeed (too little work).
HeadersSyncState hss{0, Params().GetConsensus(), chain_start, CHAIN_WORK};
BOOST_REQUIRE_EQUAL(hss.GetState(), HeadersSyncState::State::PRESYNC);
// Pretend just the first message is "full", so we don't abort.
{
    const auto [pow_validated_headers, success, request_more]{hss.ProcessNextHeaders({second_chain.begin(), 1}, true)};
    BOOST_REQUIRE_EQUAL(hss.GetState(), HeadersSyncState::State::PRESYNC);
    BOOST_CHECK(success);
    BOOST_CHECK(request_more);
    BOOST_CHECK_EQUAL(pow_validated_headers.size(), 0);
}

// Tell the sync logic that the headers message was not full, implying no
// more headers can be requested. For a low-work-chain, this should cause
// the sync to end with no headers for acceptance.
{
    const auto [pow_validated_headers, success, request_more]{hss.ProcessNextHeaders({second_chain.begin() + 1, second_chain.end()}, false)};
    BOOST_REQUIRE_EQUAL(hss.GetState(), HeadersSyncState::State::FINAL);
    BOOST_CHECK(!request_more);
    BOOST_CHECK_EQUAL(pow_validated_headers.size(), 0);
    // Nevertheless, no validation errors should have been detected with the chain:
    BOOST_CHECK(success);
}
```

Code after:
     * prev_time, and with a fixed merkle root hash.
     */
    std::vector<CBlockHeader> GenerateHeaders(size_t count,
            uint256 prev_hash, int32_t nVersion, uint32_t prev_time,
            const uint256& merkle_root, uint32_t nBits);
};

void HeadersGeneratorSetup::FindProofOfWork(CBlockHeader& starting_header)
{
    while (!CheckProofOfWork(starting_header.GetHash(), starting_header.nBits, Params().GetConsensus())) {
        ++starting_header.nNonce;
    }
}

std::vector<CBlockHeader> HeadersGeneratorSetup::GenerateHeaders(
        const size_t count, uint256 prev_hash, const int32_t nVersion,
        uint32_t prev_time, const uint256& merkle_root, const uint32_t nBits)
{
    std::vector<CBlockHeader> headers(count);
    for (auto& next_header : headers) {
        next_header.nVersion = nVersion;
        next_header.hashPrevBlock = prev_hash;
        next_header.hashMerkleRoot = merkle_root;
        next_header.nTime = ++prev_time;
        next_header.nBits = nBits;

        FindProofOfWork(next_header);
        prev_hash = next_header.GetHash();
    }
    return headers;
}

// 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.
//    Then we deliver the second set of headers and verify that they fail
//    processing (presumably due to commitments not matching).
// 2. Verify that repeating with the first set of headers in both phases is
//    successful.
// 3. Repeat the second set of headers in both phases to demonstrate behavior
//    when the chain a peer provides has too little work.
BOOST_FIXTURE_TEST_SUITE(headers_sync_chainwork_tests, HeadersGeneratorSetup)

BOOST_AUTO_TEST_CASE(sneaky_redownload)
{
    const auto& first_chain{FirstChain()};
    const auto& second_chain{SecondChain()};

    // Feed the first chain to HeadersSyncState, by delivering 1 header
    // initially and then the rest.