Repository: bitcoin/bitcoin
File: src/kernel/disconnected_transactions.h (C/C++)

Code before:
static const unsigned int MAX_DISCONNECTED_TX_POOL_SIZE = 20'000;
/**
 * DisconnectedBlockTransactions

 * During the reorg, it's desirable to re-add previously confirmed transactions
 * to the mempool, so that anything not re-confirmed in the new chain is
 * available to be mined. However, it's more efficient to wait until the reorg
 * is complete and process all still-unconfirmed transactions at that time,
 * since we expect most confirmed transactions to (typically) still be
 * confirmed in the new chain, and re-accepting to the memory pool is expensive
 * (and therefore better to not do in the middle of reorg-processing).
 * Instead, store the disconnected transactions (in order!) as we go, remove any
 * that are included in blocks in the new chain, and then process the remaining
 * still-unconfirmed transactions at the end.
 *
 * Order of queuedTx:
 * The front of the list should be the most recently-confirmed transactions (transactions at the
 * end of vtx of blocks closer to the tip). If memory usage grows too large, we trim from the front
 * of the list. After trimming, transactions can be re-added to the mempool from the back of the
 * list to the front without running into missing inputs.
 */
class DisconnectedBlockTransactions {
private:
    /** Cached dynamic memory usage for the CTransactions (memory for the shared pointers is
     * included in the container calculations). */
    uint64_t cachedInnerUsage = 0;
    const size_t m_max_mem_usage;
    std::list<CTransactionRef> queuedTx;
    using TxList = decltype(queuedTx);
    std::unordered_map<uint256, TxList::iterator, SaltedTxidHasher> iters_by_txid;

    /** Trim the earliest-added entries until we are within memory bounds. */
    std::vector<CTransactionRef> LimitMemoryUsage()
    {
        std::vector<CTransactionRef> evicted;

        while (!queuedTx.empty() && DynamicMemoryUsage() > m_max_mem_usage) {
            evicted.emplace_back(queuedTx.front());
            cachedInnerUsage -= RecursiveDynamicUsage(*queuedTx.front());
            iters_by_txid.erase(queuedTx.front()->GetHash());
            queuedTx.pop_front();
        }
        return evicted;
    }

public:
    DisconnectedBlockTransactions(size_t max_mem_usage) : m_max_mem_usage{max_mem_usage} {}

    // It's almost certainly a logic bug if we don't clear out queuedTx before
    // destruction, as we add to it while disconnecting blocks, and then we
    // need to re-process remaining transactions to ensure mempool consistency.

Review comment (suggestion):
A bit more accurate:
```suggestion
    /* Maximum allowed total DynamicMemoryUsage, in bytes. To be enforced in AddTransactionsFromBlock.  */
```

Code after:
 * DisconnectedBlockTransactions

 * During the reorg, it's desirable to re-add previously confirmed transactions
 * to the mempool, so that anything not re-confirmed in the new chain is
 * available to be mined. However, it's more efficient to wait until the reorg
 * is complete and process all still-unconfirmed transactions at that time,
 * since we expect most confirmed transactions to (typically) still be
 * confirmed in the new chain, and re-accepting to the memory pool is expensive
 * (and therefore better to not do in the middle of reorg-processing).
 * Instead, store the disconnected transactions (in order!) as we go, remove any
 * that are included in blocks in the new chain, and then process the remaining
 * still-unconfirmed transactions at the end.
 *
 * Order of queuedTx:
 * The front of the list should be the most recently-confirmed transactions (transactions at the
 * end of vtx of blocks closer to the tip). If memory usage grows too large, we trim from the front
 * of the list. After trimming, transactions can be re-added to the mempool from the back of the
 * list to the front without running into missing inputs.
 */
class DisconnectedBlockTransactions {
private:
    /** Cached dynamic memory usage for the `CTransactionRef`s */
    uint64_t cachedInnerUsage = 0;
    const size_t m_max_mem_usage;
    std::list<CTransactionRef> queuedTx;
    using TxList = decltype(queuedTx);
    std::unordered_map<uint256, TxList::iterator, SaltedTxidHasher> iters_by_txid;

    /** Trim the earliest-added entries until we are within memory bounds. */
    std::vector<CTransactionRef> LimitMemoryUsage();

public:
    DisconnectedBlockTransactions(size_t max_mem_usage)
        : m_max_mem_usage{max_mem_usage} {}

    ~DisconnectedBlockTransactions();

    size_t DynamicMemoryUsage() const;

    /** Add transactions from the block, iterating through vtx in reverse order. Callers should call
     * this function for blocks in descending order by block height.
     * We assume that callers never pass multiple transactions with the same txid, otherwise things
     * can go very wrong in removeForBlock due to queuedTx containing an item without a
     * corresponding entry in iters_by_txid.
     * @returns vector of transactions that were evicted for size-limiting.
     */
    [[nodiscard]] std::vector<CTransactionRef> AddTransactionsFromBlock(const std::vector<CTransactionRef>& vtx);

    /** Remove any entries that are in this block. */
    void removeForBlock(const std::vector<CTransactionRef>& vtx);