Bitcoin Core Fuzz Coverage Report

// Copyright (c) The Bitcoin Core developers

// Distributed under the MIT software license, see the accompanying

// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <compare>

#include <cstdint>

#include <functional>

#include <memory>

#include <optional>

#include <utility>

#include <vector>

#include <util/feefrac.h>

#ifndef BITCOIN_TXGRAPH_H

#define BITCOIN_TXGRAPH_H

static constexpr unsigned MAX_CLUSTER_COUNT_LIMIT{64};

/** Data structure to encapsulate fees, sizes, and dependencies for a set of transactions.

 *

 * Each TxGraph represents one or two such graphs ("main", and optionally "staging"), to allow for

 * working with batches of changes that may still be discarded.

 *

 * The connected components within each transaction graph are called clusters: whenever one

 * transaction is reachable from another, through any sequence of is-parent-of or is-child-of

 * relations, they belong to the same cluster (so clusters include parents, children, but also

 * grandparents, siblings, cousins twice removed, ...).

 *

 * For each graph, TxGraph implicitly defines an associated total ordering on its transactions

 * (its linearization) that respects topology (parents go before their children), aiming for it to

 * be close to the optimal order those transactions should be mined in if the goal is fee

 * maximization, though this is a best effort only, not a strong guarantee.

 *

 * For more explanation, see https://delvingbitcoin.org/t/introduction-to-cluster-linearization/1032

 *

 * This linearization is partitioned into chunks: groups of transactions that according to this

 * order would be mined together. Each chunk consists of the highest-feerate prefix of what remains

 * of the linearization after removing previous chunks. TxGraph guarantees that the maintained

 * linearization always results in chunks consisting of transactions that are connected. A chunk's

 * transactions always belong to the same cluster.

 *

 * The interface is designed to accommodate an implementation that only stores the transitive

 * closure of dependencies, so if B spends C, it does not distinguish between "A spending B" and

 * "A spending both B and C".

 */

class TxGraph

{

public:

    /** Internal identifier for a transaction within a TxGraph. */

    using GraphIndex = uint32_t;

    /** Data type used to reference transactions within a TxGraph.

     *

     * Every transaction within a TxGraph has exactly one corresponding TxGraph::Ref, held by users

     * of the class. Destroying the TxGraph::Ref removes the corresponding transaction (in both the

     * main and staging graphs).

     *

     * Users of the class can inherit from TxGraph::Ref. If all Refs are inherited this way, the

     * Ref* pointers returned by TxGraph functions can be cast to, and used as, this inherited type.

     */

    class Ref;

    enum class Level {

        TOP, //!< Refers to staging if it exists, main otherwise.

        MAIN //!< Always refers to the main graph, whether staging is present or not.

    };

    /** Virtual destructor, so inheriting is safe. */

    virtual ~TxGraph() = default;

    /** Initialize arg (which must be an empty Ref) to refer to a new transaction in this graph

     *  with the specified feerate.

     *

     *  If a staging graph exists, the new transaction is only created there. feerate.size must be

     *  strictly positive. In all further calls, only Refs passed to AddTransaction() are allowed

     *  to be passed to this TxGraph object (or empty Ref objects). Ref objects may outlive the

     *  TxGraph they were added to. */

    virtual void AddTransaction(Ref& arg, const FeePerWeight& feerate) noexcept = 0;

    /** Remove the specified transaction. If a staging graph exists, the removal only happens

     *  there. This is a no-op if the transaction was already removed.

     *

     * TxGraph may internally reorder transaction removals with dependency additions for

     * performance reasons. If together with any transaction removal all its descendants, or all

     * its ancestors, are removed as well (which is what always happens in realistic scenarios),

     * this reordering will not affect the behavior of TxGraph.

     *

     * As an example, imagine 3 transactions A,B,C where B depends on A. If a dependency of C on B

     * is added, and then B is deleted, C will still depend on A. If the deletion of B is reordered

     * before the C->B dependency is added, the dependency adding has no effect. If, together with

     * the deletion of B also either A or C is deleted, there is no distinction between the

     * original order case and the reordered case.

     */

    virtual void RemoveTransaction(const Ref& arg) noexcept = 0;

    /** Add a dependency between two specified transactions. If a staging graph exists, the

     *  dependency is only added there. Parent may not be a descendant of child already (but may

     *  be an ancestor of it already, in which case this is a no-op). If either transaction is

     *  already removed, this is a no-op. */

    virtual void AddDependency(const Ref& parent, const Ref& child) noexcept = 0;

    /** Modify the fee of the specified transaction, in both the main graph and the staging

     *  graph if it exists. Wherever the transaction does not exist (or was removed), this has no

     *  effect. */

    virtual void SetTransactionFee(const Ref& arg, int64_t fee) noexcept = 0;

    /** TxGraph is internally lazy, and will not compute many things until they are needed.

     *  Calling DoWork will perform some work now (controlled by max_cost) so that future operations

     *  are fast, if there is any. Returns whether all currently-available work is done. This can

     *  be invoked while oversized, but oversized graphs will be skipped by this call. */

    virtual bool DoWork(uint64_t max_cost) noexcept = 0;

    /** Create a staging graph (which cannot exist already). This acts as if a full copy of

     *  the transaction graph is made, upon which further modifications are made. This copy can

     *  be inspected, and then either discarded, or the main graph can be replaced by it by

     *  committing it. */

    virtual void StartStaging() noexcept = 0;

    /** Discard the existing active staging graph (which must exist). */

    virtual void AbortStaging() noexcept = 0;

    /** Replace the main graph with the staging graph (which must exist). */

    virtual void CommitStaging() noexcept = 0;

    /** Check whether a staging graph exists. */

    virtual bool HaveStaging() const noexcept = 0;

    /** Determine whether the graph is oversized (contains a connected component of more than the

     *  configured maximum cluster count). Some of the functions below are not available

     *  for oversized graphs. The mutators above are always available. Removing a transaction by

     *  destroying its Ref while staging exists will not clear main's oversizedness until staging

     *  is aborted or committed. */

    virtual bool IsOversized(Level level) noexcept = 0;

    /** Determine whether arg exists in the graph (i.e., was not removed). This is

     *  available even for oversized graphs. */

    virtual bool Exists(const Ref& arg, Level level) noexcept = 0;

    /** Get the individual transaction feerate of transaction arg. Returns the empty FeePerWeight

     *  if arg does not exist in either main or staging. This is available even for oversized

     *  graphs. */

    virtual FeePerWeight GetIndividualFeerate(const Ref& arg) noexcept = 0;

    /** Get the feerate of the chunk which transaction arg is in, in the main graph. Returns the

     *  empty FeePerWeight if arg does not exist in the main graph. The main graph must not be

     *  oversized. */

    virtual FeePerWeight GetMainChunkFeerate(const Ref& arg) noexcept = 0;

    /** Get pointers to all transactions in the cluster which arg is in. The transactions are

     *  returned in graph order. The queried graph must not be oversized. Returns {} if

     *  arg does not exist in the queried graph. */

    virtual std::vector<Ref*> GetCluster(const Ref& arg, Level level) noexcept = 0;

    /** Get pointers to all ancestors of the specified transaction (including the transaction

     *  itself), in unspecified order. The queried graph must not be oversized.

     *  Returns {} if arg does not exist in the graph. */

    virtual std::vector<Ref*> GetAncestors(const Ref& arg, Level level) noexcept = 0;

    /** Get pointers to all descendants of the specified transaction (including the transaction

     *  itself), in unspecified order. The queried graph must not be oversized.

     *  Returns {} if arg does not exist in the graph. */

    virtual std::vector<Ref*> GetDescendants(const Ref& arg, Level level) noexcept = 0;

    /** Like GetAncestors, but return the Refs for all transactions in the union of the provided

     *  arguments' ancestors (each transaction is only reported once). Refs that do not exist in

     *  the queried graph are ignored. Null refs are not allowed. */

    virtual std::vector<Ref*> GetAncestorsUnion(std::span<const Ref* const> args, Level level) noexcept = 0;

    /** Like GetDescendants, but return the Refs for all transactions in the union of the provided

     *  arguments' descendants (each transaction is only reported once). Refs that do not exist in

     *  the queried graph are ignored. Null refs are not allowed. */

    virtual std::vector<Ref*> GetDescendantsUnion(std::span<const Ref* const> args, Level level) noexcept = 0;

    /** Get the total number of transactions in the graph. This is available even

     *  for oversized graphs. */

    virtual GraphIndex GetTransactionCount(Level level) noexcept = 0;

    /** Compare two transactions according to their order in the main graph. Both transactions must

     *  be in the main graph. The main graph must not be oversized. */

    virtual std::strong_ordering CompareMainOrder(const Ref& a, const Ref& b) noexcept = 0;

    /** Count the number of distinct clusters that the specified transactions belong to. Refs that

     *  do not exist in the queried graph are ignored. Refs can not be null. The queried graph must

     *  not be oversized. */

    virtual GraphIndex CountDistinctClusters(std::span<const Ref* const>, Level level) noexcept = 0;

    /** For both main and staging (which must both exist and not be oversized), return the combined

     *  respective feerate diagrams, including chunks from all clusters, but excluding clusters

     *  that appear identically in both. Use FeeFrac rather than FeePerWeight so CompareChunks is

     *  usable without type-conversion. */

    virtual std::pair<std::vector<FeeFrac>, std::vector<FeeFrac>> GetMainStagingDiagrams() noexcept = 0;

    /** Remove transactions (including their own descendants) according to a fast but best-effort

     *  strategy such that the TxGraph's cluster and size limits are respected. Applies to staging

     *  if it exists, and to main otherwise. Returns the list of all removed transactions in

     *  unspecified order. This has no effect unless the relevant graph is oversized. */

    virtual std::vector<Ref*> Trim() noexcept = 0;

    /** Interface returned by GetBlockBuilder. */

    class BlockBuilder

    {

    protected:

        /** Make constructor non-public (use TxGraph::GetBlockBuilder()). */

        BlockBuilder() noexcept = default;

    public:

        /** Support safe inheritance. */

        virtual ~BlockBuilder() = default;

        /** Get the chunk that is currently suggested to be included, plus its feerate, if any. */

        virtual std::optional<std::pair<std::vector<Ref*>, FeePerWeight>> GetCurrentChunk() noexcept = 0;

        /** Mark the current chunk as included, and progress to the next one. */

        virtual void Include() noexcept = 0;

        /** Mark the current chunk as skipped, and progress to the next one. Further chunks from

         *  the same cluster as the current one will not be reported anymore. */

        virtual void Skip() noexcept = 0;

    };

    /** Construct a block builder, drawing chunks in order, from the main graph, which cannot be

     *  oversized. While the returned object exists, no mutators on the main graph are allowed.

     *  The BlockBuilder object must not outlive the TxGraph it was created with. */

    virtual std::unique_ptr<BlockBuilder> GetBlockBuilder() noexcept = 0;

    /** Get the last chunk in the main graph, i.e., the last chunk that would be returned by a

     *  BlockBuilder created now, together with its feerate. The chunk is returned in

     *  reverse-topological order, so every element is preceded by all its descendants. The main

     *  graph must not be oversized. If the graph is empty, {{}, FeePerWeight{}} is returned. */

    virtual std::pair<std::vector<Ref*>, FeePerWeight> GetWorstMainChunk() noexcept = 0;

    /** Get the approximate memory usage for this object, just counting the main graph. If a

     *  staging graph is present, return a number corresponding to memory usage after

     *  AbortStaging() would be called. BlockBuilders' memory usage, memory usage of internally

     *  queued operations, and memory due to temporary caches, is not included here. Can always be

     *  called. */

    virtual size_t GetMainMemoryUsage() noexcept = 0;

    /** Perform an internal consistency check on this object. */

    virtual void SanityCheck() const = 0;

protected:

    // Allow TxGraph::Ref to call UpdateRef and UnlinkRef.

    friend class TxGraph::Ref;

    /** Inform the TxGraph implementation that a TxGraph::Ref has moved. */

    virtual void UpdateRef(GraphIndex index, Ref& new_location) noexcept = 0;

    /** Inform the TxGraph implementation that a TxGraph::Ref was destroyed. */

    virtual void UnlinkRef(GraphIndex index) noexcept = 0;

    // Allow TxGraph implementations (inheriting from it) to access Ref internals.

    static TxGraph*& GetRefGraph(Ref& arg) noexcept { return arg.m_graph; }

    static TxGraph* GetRefGraph(const Ref& arg) noexcept { return arg.m_graph; }

    static GraphIndex& GetRefIndex(Ref& arg) noexcept { return arg.m_index; }

    static GraphIndex GetRefIndex(const Ref& arg) noexcept { return arg.m_index; }

public:

    class Ref

    {

        // Allow TxGraph's GetRefGraph and GetRefIndex to access internals.

        friend class TxGraph;

        /** Which Graph the Entry lives in. nullptr if this Ref is empty. */

        TxGraph* m_graph = nullptr;

        /** Index into the Graph's m_entries. Only used if m_graph != nullptr. */

        GraphIndex m_index = GraphIndex(-1);

    public:

        /** Construct an empty Ref. It can be initialized through TxGraph::AddTransaction. */

        Ref() noexcept = default;

        /** Destroy this Ref. If it is not empty, the corresponding transaction is removed (in both

         *  main and staging, if it exists). */

        virtual ~Ref();

        // Support move-constructing a Ref.

        Ref& operator=(Ref&& other) noexcept = delete;

        Ref(Ref&& other) noexcept;

        // Do not permit copy constructing or copy assignment. A TxGraph entry can have at most one

        // Ref pointing to it.

        Ref& operator=(const Ref&) = delete;

        Ref(const Ref&) = delete;

    };

};

/** Construct a new TxGraph with the specified limit on the number of transactions within a cluster,

 *  and on the sum of transaction sizes within a cluster.

 *

 * - max_cluster_count cannot exceed MAX_CLUSTER_COUNT_LIMIT.

 * - acceptable_cost controls how much linearization optimization work will be performed per

 *   cluster before they are considered to be of acceptable quality.

 * - fallback_order determines how to break tie-breaks between transactions:

 *   fallback_order(a, b) < 0 means a is "better" than b, and will (in case of ties) be placed

 *   first. This ordering must be stable over the transactions' lifetimes.

 */

std::unique_ptr<TxGraph> MakeTxGraph(

    unsigned max_cluster_count,

    uint64_t max_cluster_size,

    uint64_t acceptable_cost,

    const std::function<std::strong_ordering(const TxGraph::Ref&, const TxGraph::Ref&)>& fallback_order

) noexcept;

#endif // BITCOIN_TXGRAPH_H