Inside Bitcoin

1// Copyright (c) The Bitcoin Core developers
2// Distributed under the MIT software license, see the accompanying
3// file COPYING or http://www.opensource.org/licenses/mit-license.php.
4
5#include <compare>
6#include <cstdint>
7#include <functional>
8#include <memory>
9#include <optional>
10#include <utility>
11#include <vector>
12
13#include <util/feefrac.h>
14
15#ifndef BITCOIN_TXGRAPH_H
16#define BITCOIN_TXGRAPH_H
17
18inline constexpr unsigned MAX_CLUSTER_COUNT_LIMIT{64};
19
20/** Data structure to encapsulate fees, sizes, and dependencies for a set of transactions.
21 *
22 * Each TxGraph represents one or two such graphs ("main", and optionally "staging"), to allow for
23 * working with batches of changes that may still be discarded.
24 *
25 * The connected components within each transaction graph are called clusters: whenever one
26 * transaction is reachable from another, through any sequence of is-parent-of or is-child-of
27 * relations, they belong to the same cluster (so clusters include parents, children, but also
28 * grandparents, siblings, cousins twice removed, ...).
29 *
30 * For each graph, TxGraph implicitly defines an associated total ordering on its transactions
31 * (its linearization) that respects topology (parents go before their children), aiming for it to
32 * be close to the optimal order those transactions should be mined in if the goal is fee
33 * maximization, though this is a best effort only, not a strong guarantee.
34 *
35 * For more explanation, see https://delvingbitcoin.org/t/introduction-to-cluster-linearization/1032
36 *
37 * This linearization is partitioned into chunks: groups of transactions that according to this
38 * order would be mined together. Each chunk consists of the highest-feerate prefix of what remains
39 * of the linearization after removing previous chunks. TxGraph guarantees that the maintained
40 * linearization always results in chunks consisting of transactions that are connected. A chunk's
41 * transactions always belong to the same cluster.
42 *
43 * The interface is designed to accommodate an implementation that only stores the transitive
44 * closure of dependencies, so if B spends C, it does not distinguish between "A spending B" and
45 * "A spending both B and C".
46 */
47class TxGraph
48{
49public:
50    /** Internal identifier for a transaction within a TxGraph. */
51    using GraphIndex = uint32_t;
52
53    /** Data type used to reference transactions within a TxGraph.
54     *
55     * Every transaction within a TxGraph has exactly one corresponding TxGraph::Ref, held by users
56     * of the class. Destroying the TxGraph::Ref removes the corresponding transaction (in both the
57     * main and staging graphs).
58     *
59     * Users of the class can inherit from TxGraph::Ref. If all Refs are inherited this way, the
60     * Ref* pointers returned by TxGraph functions can be cast to, and used as, this inherited type.
61     */
62    class Ref;
63
64    enum class Level {
65        TOP, //!< Refers to staging if it exists, main otherwise.
66        MAIN //!< Always refers to the main graph, whether staging is present or not.
67    };
68
69    /** Virtual destructor, so inheriting is safe. */
70    virtual ~TxGraph() = default;
71    /** Initialize arg (which must be an empty Ref) to refer to a new transaction in this graph
72     *  with the specified feerate.
73     *
74     *  If a staging graph exists, the new transaction is only created there. feerate.size must be
75     *  strictly positive. In all further calls, only Refs passed to AddTransaction() are allowed
76     *  to be passed to this TxGraph object (or empty Ref objects). Ref objects may outlive the
77     *  TxGraph they were added to. */
78    virtual void AddTransaction(Ref& arg, const FeePerWeight& feerate) noexcept = 0;
79    /** Remove the specified transaction. If a staging graph exists, the removal only happens
80     *  there. This is a no-op if the transaction was already removed.
81     *
82     * TxGraph may internally reorder transaction removals with dependency additions for
83     * performance reasons. If together with any transaction removal all its descendants, or all
84     * its ancestors, are removed as well (which is what always happens in realistic scenarios),
85     * this reordering will not affect the behavior of TxGraph.
86     *
87     * As an example, imagine 3 transactions A,B,C where B depends on A. If a dependency of C on B
88     * is added, and then B is deleted, C will still depend on A. If the deletion of B is reordered
89     * before the C->B dependency is added, the dependency adding has no effect. If, together with
90     * the deletion of B also either A or C is deleted, there is no distinction between the
91     * original order case and the reordered case.
92     */
93    virtual void RemoveTransaction(const Ref& arg) noexcept = 0;
94    /** Add a dependency between two specified transactions. If a staging graph exists, the
95     *  dependency is only added there. Parent may not be a descendant of child already (but may
96     *  be an ancestor of it already, in which case this is a no-op). If either transaction is
97     *  already removed, this is a no-op. */
98    virtual void AddDependency(const Ref& parent, const Ref& child) noexcept = 0;
99    /** Modify the fee of the specified transaction, in both the main graph and the staging
100     *  graph if it exists. Wherever the transaction does not exist (or was removed), this has no
101     *  effect. */
102    virtual void SetTransactionFee(const Ref& arg, int64_t fee) noexcept = 0;
103
104    /** TxGraph is internally lazy, and will not compute many things until they are needed.
105     *  Calling DoWork will perform some work now (controlled by max_cost) so that future operations
106     *  are fast, if there is any. Returns whether all currently-available work is done. This can
107     *  be invoked while oversized, but oversized graphs will be skipped by this call. */
108    virtual bool DoWork(uint64_t max_cost) noexcept = 0;
109
110    /** Create a staging graph (which cannot exist already). This acts as if a full copy of
111     *  the transaction graph is made, upon which further modifications are made. This copy can
112     *  be inspected, and then either discarded, or the main graph can be replaced by it by
113     *  committing it. */
114    virtual void StartStaging() noexcept = 0;
115    /** Discard the existing active staging graph (which must exist). */
116    virtual void AbortStaging() noexcept = 0;
117    /** Replace the main graph with the staging graph (which must exist). */
118    virtual void CommitStaging() noexcept = 0;
119    /** Check whether a staging graph exists. */
120    virtual bool HaveStaging() const noexcept = 0;
121
122    /** Determine whether the graph is oversized (contains a connected component of more than the
123     *  configured maximum cluster count). Some of the functions below are not available
124     *  for oversized graphs. The mutators above are always available. Removing a transaction by
125     *  destroying its Ref while staging exists will not clear main's oversizedness until staging
126     *  is aborted or committed. */
127    virtual bool IsOversized(Level level) noexcept = 0;
128    /** Determine whether arg exists in the graph (i.e., was not removed). This is
129     *  available even for oversized graphs. */
130    virtual bool Exists(const Ref& arg, Level level) noexcept = 0;
131    /** Get the individual transaction feerate of transaction arg. Returns the empty FeePerWeight
132     *  if arg does not exist in either main or staging. This is available even for oversized
133     *  graphs. */
134    virtual FeePerWeight GetIndividualFeerate(const Ref& arg) noexcept = 0;
135    /** Get the feerate of the chunk which transaction arg is in, in the main graph. Returns the
136     *  empty FeePerWeight if arg does not exist in the main graph. The main graph must not be
137     *  oversized. */
138    virtual FeePerWeight GetMainChunkFeerate(const Ref& arg) noexcept = 0;
139    /** Get pointers to all transactions in the cluster which arg is in. The transactions are
140     *  returned in graph order. The queried graph must not be oversized. Returns {} if
141     *  arg does not exist in the queried graph. */
142    virtual std::vector<Ref*> GetCluster(const Ref& arg, Level level) noexcept = 0;
143    /** Get pointers to all ancestors of the specified transaction (including the transaction
144     *  itself), in unspecified order. The queried graph must not be oversized.
145     *  Returns {} if arg does not exist in the graph. */
146    virtual std::vector<Ref*> GetAncestors(const Ref& arg, Level level) noexcept = 0;
147    /** Get pointers to all descendants of the specified transaction (including the transaction
148     *  itself), in unspecified order. The queried graph must not be oversized.
149     *  Returns {} if arg does not exist in the graph. */
150    virtual std::vector<Ref*> GetDescendants(const Ref& arg, Level level) noexcept = 0;
151    /** Like GetAncestors, but return the Refs for all transactions in the union of the provided
152     *  arguments' ancestors (each transaction is only reported once). Refs that do not exist in
153     *  the queried graph are ignored. Null refs are not allowed. */
154    virtual std::vector<Ref*> GetAncestorsUnion(std::span<const Ref* const> args, Level level) noexcept = 0;
155    /** Like GetDescendants, but return the Refs for all transactions in the union of the provided
156     *  arguments' descendants (each transaction is only reported once). Refs that do not exist in
157     *  the queried graph are ignored. Null refs are not allowed. */
158    virtual std::vector<Ref*> GetDescendantsUnion(std::span<const Ref* const> args, Level level) noexcept = 0;
159    /** Get the total number of transactions in the graph. This is available even
160     *  for oversized graphs. */
161    virtual GraphIndex GetTransactionCount(Level level) noexcept = 0;
162    /** Compare two transactions according to their order in the main graph. Both transactions must
163     *  be in the main graph. The main graph must not be oversized. */
164    virtual std::strong_ordering CompareMainOrder(const Ref& a, const Ref& b) noexcept = 0;
165    /** Count the number of distinct clusters that the specified transactions belong to. Refs that
166     *  do not exist in the queried graph are ignored. Refs can not be null. The queried graph must
167     *  not be oversized. */
168    virtual GraphIndex CountDistinctClusters(std::span<const Ref* const>, Level level) noexcept = 0;
169    /** For both main and staging (which must both exist and not be oversized), return the combined
170     *  respective feerate diagrams, including chunks from all clusters, but excluding clusters
171     *  that appear identically in both. Use FeeFrac rather than FeePerWeight so CompareChunks is
172     *  usable without type-conversion. */
173    virtual std::pair<std::vector<FeeFrac>, std::vector<FeeFrac>> GetMainStagingDiagrams() noexcept = 0;
174    /** Remove transactions (including their own descendants) according to a fast but best-effort
175     *  strategy such that the TxGraph's cluster and size limits are respected. Applies to staging
176     *  if it exists, and to main otherwise. Returns the list of all removed transactions in
177     *  unspecified order. This has no effect unless the relevant graph is oversized. */
178    virtual std::vector<Ref*> Trim() noexcept = 0;
179
180    /** Interface returned by GetBlockBuilder. */
181    class BlockBuilder
182    {
183    protected:
184        /** Make constructor non-public (use TxGraph::GetBlockBuilder()). */
185        BlockBuilder() noexcept = default;
186    public:
187        /** Support safe inheritance. */
188        virtual ~BlockBuilder() = default;
189        /** Get the chunk that is currently suggested to be included, plus its feerate, if any. */
190        virtual std::optional<std::pair<std::vector<Ref*>, FeePerWeight>> GetCurrentChunk() noexcept = 0;
191        /** Mark the current chunk as included, and progress to the next one. */
192        virtual void Include() noexcept = 0;
193        /** Mark the current chunk as skipped, and progress to the next one. Further chunks from
194         *  the same cluster as the current one will not be reported anymore. */
195        virtual void Skip() noexcept = 0;
196    };
197
198    /** Construct a block builder, drawing chunks in order, from the main graph, which cannot be
199     *  oversized. While the returned object exists, no mutators on the main graph are allowed.
200     *  The BlockBuilder object must not outlive the TxGraph it was created with. */
201    virtual std::unique_ptr<BlockBuilder> GetBlockBuilder() noexcept = 0;
202    /** Get the last chunk in the main graph, i.e., the last chunk that would be returned by a
203     *  BlockBuilder created now, together with its feerate. The chunk is returned in
204     *  reverse-topological order, so every element is preceded by all its descendants. The main
205     *  graph must not be oversized. If the graph is empty, {{}, FeePerWeight{}} is returned. */
206    virtual std::pair<std::vector<Ref*>, FeePerWeight> GetWorstMainChunk() noexcept = 0;
207
208    /** Get the approximate memory usage for this object, just counting the main graph. If a
209     *  staging graph is present, return a number corresponding to memory usage after
210     *  AbortStaging() would be called. BlockBuilders' memory usage, memory usage of internally
211     *  queued operations, and memory due to temporary caches, is not included here. Can always be
212     *  called. */
213    virtual size_t GetMainMemoryUsage() noexcept = 0;
214
215    /** Perform an internal consistency check on this object. */
216    virtual void SanityCheck() const = 0;
217
218protected:
219    // Allow TxGraph::Ref to call UpdateRef and UnlinkRef.
220    friend class TxGraph::Ref;
221    /** Inform the TxGraph implementation that a TxGraph::Ref has moved. */
222    virtual void UpdateRef(GraphIndex index, Ref& new_location) noexcept = 0;
223    /** Inform the TxGraph implementation that a TxGraph::Ref was destroyed. */
224    virtual void UnlinkRef(GraphIndex index) noexcept = 0;
225    // Allow TxGraph implementations (inheriting from it) to access Ref internals.
226    static TxGraph*& GetRefGraph(Ref& arg) noexcept { return arg.m_graph; }
227    static TxGraph* GetRefGraph(const Ref& arg) noexcept { return arg.m_graph; }
228    static GraphIndex& GetRefIndex(Ref& arg) noexcept { return arg.m_index; }
229    static GraphIndex GetRefIndex(const Ref& arg) noexcept { return arg.m_index; }
230
231public:
232    class Ref
233    {
234        // Allow TxGraph's GetRefGraph and GetRefIndex to access internals.
235        friend class TxGraph;
236        /** Which Graph the Entry lives in. nullptr if this Ref is empty. */
237        TxGraph* m_graph = nullptr;
238        /** Index into the Graph's m_entries. Only used if m_graph != nullptr. */
239        GraphIndex m_index = GraphIndex(-1);
240    public:
241        /** Construct an empty Ref. It can be initialized through TxGraph::AddTransaction. */
242        Ref() noexcept = default;
243        /** Destroy this Ref. If it is not empty, the corresponding transaction is removed (in both
244         *  main and staging, if it exists). */
245        virtual ~Ref();
246        // Support move-constructing a Ref.
247        Ref& operator=(Ref&& other) noexcept = delete;
248        Ref(Ref&& other) noexcept;
249        // Do not permit copy constructing or copy assignment. A TxGraph entry can have at most one
250        // Ref pointing to it.