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Add txrequest module

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This adds a new module (unused for now) which defines TxRequestTracker, a data
structure that maintains all information about transaction requests, and coordinates
requests.
This commit is contained in:
Pieter Wuille 2020-09-20 21:17:29 -07:00
parent 0b2abaa666
commit da3b8fde03
5 changed files with 807 additions and 0 deletions
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2
src/Makefile.am
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@ -215,6 +215,7 @@ BITCOIN_CORE_H = \
timedata.h \
torcontrol.h \
txdb.h \
txrequest.h \
txmempool.h \
undo.h \
util/asmap.h \
@ -327,6 +328,7 @@ libbitcoin_server_a_SOURCES = \
timedata.cpp \
torcontrol.cpp \
txdb.cpp \
txrequest.cpp \
txmempool.cpp \
validation.cpp \
validationinterface.cpp \

606
src/txrequest.cpp Normal file
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@ -0,0 +1,606 @@
// Copyright (c) 2020 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 <txrequest.h>
#include <crypto/siphash.h>
#include <net.h>
#include <primitives/transaction.h>
#include <random.h>
#include <uint256.h>
#include <util/memory.h>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <chrono>
#include <unordered_map>
#include <utility>
#include <assert.h>
namespace {
/** The various states a (txhash,peer) pair can be in.
*
* Note that CANDIDATE is split up into 3 substates (DELAYED, BEST, READY), allowing more efficient implementation.
* Also note that the sorting order of ByTxHashView relies on the specific order of values in this enum.
*
* Expected behaviour is:
* - When first announced by a peer, the state is CANDIDATE_DELAYED until reqtime is reached.
* - Announcements that have reached their reqtime but not been requested will be either CANDIDATE_READY or
* CANDIDATE_BEST. Neither of those has an expiration time; they remain in that state until they're requested or
* no longer needed. CANDIDATE_READY announcements are promoted to CANDIDATE_BEST when they're the best one left.
* - When requested, an announcement will be in state REQUESTED until expiry is reached.
* - If expiry is reached, or the peer replies to the request (either with NOTFOUND or the tx), the state becomes
* COMPLETED.
*/
enum class State : uint8_t {
/** A CANDIDATE announcement whose reqtime is in the future. */
CANDIDATE_DELAYED,
/** A CANDIDATE announcement that's not CANDIDATE_DELAYED or CANDIDATE_BEST. */
CANDIDATE_READY,
/** The best CANDIDATE for a given txhash; only if there is no REQUESTED announcement already for that txhash.
* The CANDIDATE_BEST is the highest-priority announcement among all CANDIDATE_READY (and _BEST) ones for that
* txhash. */
CANDIDATE_BEST,
/** A REQUESTED announcement. */
REQUESTED,
/** A COMPLETED announcement. */
COMPLETED,
};
//! Type alias for sequence numbers.
using SequenceNumber = uint64_t;
/** An announcement. This is the data we track for each txid or wtxid that is announced to us by each peer. */
struct Announcement {
/** Txid or wtxid that was announced. */
const uint256 m_txhash;
/** For CANDIDATE_{DELAYED,BEST,READY} the reqtime; for REQUESTED the expiry. */
std::chrono::microseconds m_time;
/** What peer the request was from. */
const NodeId m_peer;
/** What sequence number this announcement has. */
const SequenceNumber m_sequence : 59;
/** Whether the request is preferred. */
const bool m_preferred : 1;
/** Whether this is a wtxid request. */
const bool m_is_wtxid : 1;
/** What state this announcement is in. */
State m_state : 3;
/** Whether this announcement is selected. There can be at most 1 selected peer per txhash. */
bool IsSelected() const
{
return m_state == State::CANDIDATE_BEST || m_state == State::REQUESTED;
}
/** Whether this announcement is waiting for a certain time to pass. */
bool IsWaiting() const
{
return m_state == State::REQUESTED || m_state == State::CANDIDATE_DELAYED;
}
/** Whether this announcement can feasibly be selected if the current IsSelected() one disappears. */
bool IsSelectable() const
{
return m_state == State::CANDIDATE_READY || m_state == State::CANDIDATE_BEST;
}
/** Construct a new announcement from scratch, initially in CANDIDATE_DELAYED state. */
Announcement(const GenTxid& gtxid, NodeId peer, bool preferred, std::chrono::microseconds reqtime,
SequenceNumber sequence) :
m_txhash(gtxid.GetHash()), m_time(reqtime), m_peer(peer), m_sequence(sequence), m_preferred(preferred),
m_is_wtxid(gtxid.IsWtxid()), m_state(State::CANDIDATE_DELAYED) {}
};
//! Type alias for priorities.
using Priority = uint64_t;
/** A functor with embedded salt that computes priority of an announcement.
*
* Higher priorities are selected first.
*/
class PriorityComputer {
const uint64_t m_k0, m_k1;
public:
explicit PriorityComputer(bool deterministic) :
m_k0{deterministic ? 0 : GetRand(0xFFFFFFFFFFFFFFFF)},
m_k1{deterministic ? 0 : GetRand(0xFFFFFFFFFFFFFFFF)} {}
Priority operator()(const uint256& txhash, NodeId peer, bool preferred) const
{
uint64_t low_bits = CSipHasher(m_k0, m_k1).Write(txhash.begin(), txhash.size()).Write(peer).Finalize() >> 1;
return low_bits | uint64_t{preferred} << 63;
}
Priority operator()(const Announcement& ann) const
{
return operator()(ann.m_txhash, ann.m_peer, ann.m_preferred);
}
};
// Definitions for the 3 indexes used in the main data structure.
//
// Each index has a By* type to identify it, a By*View data type to represent the view of announcement it is sorted
// by, and an By*ViewExtractor type to convert an announcement into the By*View type.
// See https://www.boost.org/doc/libs/1_58_0/libs/multi_index/doc/reference/key_extraction.html#key_extractors
// for more information about the key extraction concept.
// The ByPeer index is sorted by (peer, state == CANDIDATE_BEST, txhash)
//
// Uses:
// * Looking up existing announcements by peer/txhash, by checking both (peer, false, txhash) and
// (peer, true, txhash).
// * Finding all CANDIDATE_BEST announcements for a given peer in GetRequestable.
struct ByPeer {};
using ByPeerView = std::tuple<NodeId, bool, const uint256&>;
struct ByPeerViewExtractor
{
using result_type = ByPeerView;
result_type operator()(const Announcement& ann) const
{
return ByPeerView{ann.m_peer, ann.m_state == State::CANDIDATE_BEST, ann.m_txhash};
}
};
// The ByTxHash index is sorted by (txhash, state, priority).
//
// Note: priority == 0 whenever state != CANDIDATE_READY.
//
// Uses:
// * Deleting all announcements with a given txhash in ForgetTxHash.
// * Finding the best CANDIDATE_READY to convert to CANDIDATE_BEST, when no other CANDIDATE_READY or REQUESTED
// announcement exists for that txhash.
// * Determining when no more non-COMPLETED announcements for a given txhash exist, so the COMPLETED ones can be
// deleted.
struct ByTxHash {};
using ByTxHashView = std::tuple<const uint256&, State, Priority>;
class ByTxHashViewExtractor {
const PriorityComputer& m_computer;
public:
ByTxHashViewExtractor(const PriorityComputer& computer) : m_computer(computer) {}
using result_type = ByTxHashView;
result_type operator()(const Announcement& ann) const
{
const Priority prio = (ann.m_state == State::CANDIDATE_READY) ? m_computer(ann) : 0;
return ByTxHashView{ann.m_txhash, ann.m_state, prio};
}
};
enum class WaitState {
//! Used for announcements that need efficient testing of "is their timestamp in the future?".
FUTURE_EVENT,
//! Used for announcements whose timestamp is not relevant.
NO_EVENT,
//! Used for announcements that need efficient testing of "is their timestamp in the past?".
PAST_EVENT,
};
WaitState GetWaitState(const Announcement& ann)
{
if (ann.IsWaiting()) return WaitState::FUTURE_EVENT;
if (ann.IsSelectable()) return WaitState::PAST_EVENT;
return WaitState::NO_EVENT;
}
// The ByTime index is sorted by (wait_state, time).
//
// All announcements with a timestamp in the future can be found by iterating the index forward from the beginning.
// All announcements with a timestamp in the past can be found by iterating the index backwards from the end.
//
// Uses:
// * Finding CANDIDATE_DELAYED announcements whose reqtime has passed, and REQUESTED announcements whose expiry has
// passed.
// * Finding CANDIDATE_READY/BEST announcements whose reqtime is in the future (when the clock time went backwards).
struct ByTime {};
using ByTimeView = std::pair<WaitState, std::chrono::microseconds>;
struct ByTimeViewExtractor
{
using result_type = ByTimeView;
result_type operator()(const Announcement& ann) const
{
return ByTimeView{GetWaitState(ann), ann.m_time};
}
};
/** Data type for the main data structure (Announcement objects with ByPeer/ByTxHash/ByTime indexes). */
using Index = boost::multi_index_container<
Announcement,
boost::multi_index::indexed_by<
boost::multi_index::ordered_unique<boost::multi_index::tag<ByPeer>, ByPeerViewExtractor>,
boost::multi_index::ordered_non_unique<boost::multi_index::tag<ByTxHash>, ByTxHashViewExtractor>,
boost::multi_index::ordered_non_unique<boost::multi_index::tag<ByTime>, ByTimeViewExtractor>
>
>;
/** Helper type to simplify syntax of iterator types. */
template<typename Tag>
using Iter = typename Index::index<Tag>::type::iterator;
/** Per-peer statistics object. */
struct PeerInfo {
size_t m_total = 0; //!< Total number of announcements for this peer.
size_t m_completed = 0; //!< Number of COMPLETED announcements for this peer.
size_t m_requested = 0; //!< Number of REQUESTED announcements for this peer.
};
} // namespace
/** Actual implementation for TxRequestTracker's data structure. */
class TxRequestTracker::Impl {
//! The current sequence number. Increases for every announcement. This is used to sort txhashes returned by
//! GetRequestable in announcement order.
SequenceNumber m_current_sequence{0};
//! This tracker's priority computer.
const PriorityComputer m_computer;
//! This tracker's main data structure.
Index m_index;
//! Map with this tracker's per-peer statistics.
std::unordered_map<NodeId, PeerInfo> m_peerinfo;
//! Wrapper around Index::...::erase that keeps m_peerinfo up to date.
template<typename Tag>
Iter<Tag> Erase(Iter<Tag> it)
{
auto peerit = m_peerinfo.find(it->m_peer);
peerit->second.m_completed -= it->m_state == State::COMPLETED;
peerit->second.m_requested -= it->m_state == State::REQUESTED;
if (--peerit->second.m_total == 0) m_peerinfo.erase(peerit);
return m_index.get<Tag>().erase(it);
}
//! Wrapper around Index::...::modify that keeps m_peerinfo up to date.
template<typename Tag, typename Modifier>
void Modify(Iter<Tag> it, Modifier modifier)
{
auto peerit = m_peerinfo.find(it->m_peer);
peerit->second.m_completed -= it->m_state == State::COMPLETED;
peerit->second.m_requested -= it->m_state == State::REQUESTED;
m_index.get<Tag>().modify(it, std::move(modifier));
peerit->second.m_completed += it->m_state == State::COMPLETED;
peerit->second.m_requested += it->m_state == State::REQUESTED;
}
//! Convert a CANDIDATE_DELAYED announcement into a CANDIDATE_READY. If this makes it the new best
//! CANDIDATE_READY (and no REQUESTED exists) and better than the CANDIDATE_BEST (if any), it becomes the new
//! CANDIDATE_BEST.
void PromoteCandidateReady(Iter<ByTxHash> it)
{
assert(it != m_index.get<ByTxHash>().end());
assert(it->m_state == State::CANDIDATE_DELAYED);
// Convert CANDIDATE_DELAYED to CANDIDATE_READY first.
Modify<ByTxHash>(it, [](Announcement& ann){ ann.m_state = State::CANDIDATE_READY; });
// The following code relies on the fact that the ByTxHash is sorted by txhash, and then by state (first
// _DELAYED, then _READY, then _BEST/REQUESTED). Within the _READY announcements, the best one (highest
// priority) comes last. Thus, if an existing _BEST exists for the same txhash that this announcement may
// be preferred over, it must immediately follow the newly created _READY.
auto it_next = std::next(it);
if (it_next == m_index.get<ByTxHash>().end() || it_next->m_txhash != it->m_txhash ||
it_next->m_state == State::COMPLETED) {
// This is the new best CANDIDATE_READY, and there is no IsSelected() announcement for this txhash
// already.
Modify<ByTxHash>(it, [](Announcement& ann){ ann.m_state = State::CANDIDATE_BEST; });
} else if (it_next->m_state == State::CANDIDATE_BEST) {
Priority priority_old = m_computer(*it_next);
Priority priority_new = m_computer(*it);
if (priority_new > priority_old) {
// There is a CANDIDATE_BEST announcement already, but this one is better.
Modify<ByTxHash>(it_next, [](Announcement& ann){ ann.m_state = State::CANDIDATE_READY; });
Modify<ByTxHash>(it, [](Announcement& ann){ ann.m_state = State::CANDIDATE_BEST; });
}
}
}
//! Change the state of an announcement to something non-IsSelected(). If it was IsSelected(), the next best
//! announcement will be marked CANDIDATE_BEST.
void ChangeAndReselect(Iter<ByTxHash> it, State new_state)
{
assert(new_state == State::COMPLETED || new_state == State::CANDIDATE_DELAYED);
assert(it != m_index.get<ByTxHash>().end());
if (it->IsSelected() && it != m_index.get<ByTxHash>().begin()) {
auto it_prev = std::prev(it);
// The next best CANDIDATE_READY, if any, immediately precedes the REQUESTED or CANDIDATE_BEST
// announcement in the ByTxHash index.
if (it_prev->m_txhash == it->m_txhash && it_prev->m_state == State::CANDIDATE_READY) {
// If one such CANDIDATE_READY exists (for this txhash), convert it to CANDIDATE_BEST.
Modify<ByTxHash>(it_prev, [](Announcement& ann){ ann.m_state = State::CANDIDATE_BEST; });
}
}
Modify<ByTxHash>(it, [new_state](Announcement& ann){ ann.m_state = new_state; });
}
//! Check if 'it' is the only announcement for a given txhash that isn't COMPLETED.
bool IsOnlyNonCompleted(Iter<ByTxHash> it)
{
assert(it != m_index.get<ByTxHash>().end());
assert(it->m_state != State::COMPLETED); // Not allowed to call this on COMPLETED announcements.
// This announcement has a predecessor that belongs to the same txhash. Due to ordering, and the
// fact that 'it' is not COMPLETED, its predecessor cannot be COMPLETED here.
if (it != m_index.get<ByTxHash>().begin() && std::prev(it)->m_txhash == it->m_txhash) return false;
// This announcement has a successor that belongs to the same txhash, and is not COMPLETED.
if (std::next(it) != m_index.get<ByTxHash>().end() && std::next(it)->m_txhash == it->m_txhash &&
std::next(it)->m_state != State::COMPLETED) return false;
return true;
}
/** Convert any announcement to a COMPLETED one. If there are no non-COMPLETED announcements left for this
* txhash, they are deleted. If this was a REQUESTED announcement, and there are other CANDIDATEs left, the
* best one is made CANDIDATE_BEST. Returns whether the announcement still exists. */
bool MakeCompleted(Iter<ByTxHash> it)
{
assert(it != m_index.get<ByTxHash>().end());
// Nothing to be done if it's already COMPLETED.
if (it->m_state == State::COMPLETED) return true;
if (IsOnlyNonCompleted(it)) {
// This is the last non-COMPLETED announcement for this txhash. Delete all.
uint256 txhash = it->m_txhash;
do {
it = Erase<ByTxHash>(it);
} while (it != m_index.get<ByTxHash>().end() && it->m_txhash == txhash);
return false;
}
// Mark the announcement COMPLETED, and select the next best announcement (the first CANDIDATE_READY) if
// needed.
ChangeAndReselect(it, State::COMPLETED);
return true;
}
//! Make the data structure consistent with a given point in time:
//! - REQUESTED annoucements with expiry <= now are turned into COMPLETED.
//! - CANDIDATE_DELAYED announcements with reqtime <= now are turned into CANDIDATE_{READY,BEST}.
//! - CANDIDATE_{READY,BEST} announcements with reqtime > now are turned into CANDIDATE_DELAYED.
void SetTimePoint(std::chrono::microseconds now)
{
// Iterate over all CANDIDATE_DELAYED and REQUESTED from old to new, as long as they're in the past,
// and convert them to CANDIDATE_READY and COMPLETED respectively.
while (!m_index.empty()) {
auto it = m_index.get<ByTime>().begin();
if (it->m_state == State::CANDIDATE_DELAYED && it->m_time <= now) {
PromoteCandidateReady(m_index.project<ByTxHash>(it));
} else if (it->m_state == State::REQUESTED && it->m_time <= now) {
MakeCompleted(m_index.project<ByTxHash>(it));
} else {
break;
}
}
while (!m_index.empty()) {
// If time went backwards, we may need to demote CANDIDATE_BEST and CANDIDATE_READY announcements back
// to CANDIDATE_DELAYED. This is an unusual edge case, and unlikely to matter in production. However,
// it makes it much easier to specify and test TxRequestTracker::Impl's behaviour.
auto it = std::prev(m_index.get<ByTime>().end());
if (it->IsSelectable() && it->m_time > now) {
ChangeAndReselect(m_index.project<ByTxHash>(it), State::CANDIDATE_DELAYED);
} else {
break;
}
}
}
public:
Impl(bool deterministic) :
m_computer(deterministic),
// Explicitly initialize m_index as we need to pass a reference to m_computer to ByTxHashViewExtractor.
m_index(boost::make_tuple(
boost::make_tuple(ByPeerViewExtractor(), std::less<ByPeerView>()),
boost::make_tuple(ByTxHashViewExtractor(m_computer), std::less<ByTxHashView>()),
boost::make_tuple(ByTimeViewExtractor(), std::less<ByTimeView>())
)) {}
// Disable copying and assigning (a default copy won't work due the stateful ByTxHashViewExtractor).
Impl(const Impl&) = delete;
Impl& operator=(const Impl&) = delete;
void DisconnectedPeer(NodeId peer)
{
auto& index = m_index.get<ByPeer>();
auto it = index.lower_bound(ByPeerView{peer, false, uint256::ZERO});
while (it != index.end() && it->m_peer == peer) {
// Check what to continue with after this iteration. 'it' will be deleted in what follows, so we need to
// decide what to continue with afterwards. There are a number of cases to consider:
// - std::next(it) is end() or belongs to a different peer. In that case, this is the last iteration
// of the loop (denote this by setting it_next to end()).
// - 'it' is not the only non-COMPLETED announcement for its txhash. This means it will be deleted, but
// no other Announcement objects will be modified. Continue with std::next(it) if it belongs to the
// same peer, but decide this ahead of time (as 'it' may change position in what follows).
// - 'it' is the only non-COMPLETED announcement for its txhash. This means it will be deleted along
// with all other announcements for the same txhash - which may include std::next(it). However, other
// than 'it', no announcements for the same peer can be affected (due to (peer, txhash) uniqueness).
// In other words, the situation where std::next(it) is deleted can only occur if std::next(it)
// belongs to a different peer but the same txhash as 'it'. This is covered by the first bulletpoint
// already, and we'll have set it_next to end().
auto it_next = (std::next(it) == index.end() || std::next(it)->m_peer != peer) ? index.end() :
std::next(it);
// If the announcement isn't already COMPLETED, first make it COMPLETED (which will mark other
// CANDIDATEs as CANDIDATE_BEST, or delete all of a txhash's announcements if no non-COMPLETED ones are
// left).
if (MakeCompleted(m_index.project<ByTxHash>(it))) {
// Then actually delete the announcement (unless it was already deleted by MakeCompleted).
Erase<ByPeer>(it);
}
it = it_next;
}
}
void ForgetTxHash(const uint256& txhash)
{
auto it = m_index.get<ByTxHash>().lower_bound(ByTxHashView{txhash, State::CANDIDATE_DELAYED, 0});
while (it != m_index.get<ByTxHash>().end() && it->m_txhash == txhash) {
it = Erase<ByTxHash>(it);
}
}
void ReceivedInv(NodeId peer, const GenTxid& gtxid, bool preferred,
std::chrono::microseconds reqtime)
{
// Bail out if we already have a CANDIDATE_BEST announcement for this (txhash, peer) combination. The case
// where there is a non-CANDIDATE_BEST announcement already will be caught by the uniqueness property of the
// ByPeer index when we try to emplace the new object below.
if (m_index.get<ByPeer>().count(ByPeerView{peer, true, gtxid.GetHash()})) return;
// Try creating the announcement with CANDIDATE_DELAYED state (which will fail due to the uniqueness
// of the ByPeer index if a non-CANDIDATE_BEST announcement already exists with the same txhash and peer).
// Bail out in that case.
auto ret = m_index.get<ByPeer>().emplace(gtxid, peer, preferred, reqtime, m_current_sequence);
if (!ret.second) return;
// Update accounting metadata.
++m_peerinfo[peer].m_total;
++m_current_sequence;
}
//! Find the GenTxids to request now from peer.
std::vector<GenTxid> GetRequestable(NodeId peer, std::chrono::microseconds now)
{
// Move time.
SetTimePoint(now);
// Find all CANDIDATE_BEST announcements for this peer.
std::vector<const Announcement*> selected;
auto it_peer = m_index.get<ByPeer>().lower_bound(ByPeerView{peer, true, uint256::ZERO});
while (it_peer != m_index.get<ByPeer>().end() && it_peer->m_peer == peer &&
it_peer->m_state == State::CANDIDATE_BEST) {
selected.emplace_back(&*it_peer);
++it_peer;
}
// Sort by sequence number.
std::sort(selected.begin(), selected.end(), [](const Announcement* a, const Announcement* b) {
return a->m_sequence < b->m_sequence;
});
// Convert to GenTxid and return.
std::vector<GenTxid> ret;
ret.reserve(selected.size());
std::transform(selected.begin(), selected.end(), std::back_inserter(ret), [](const Announcement* ann) {
return GenTxid{ann->m_is_wtxid, ann->m_txhash};
});
return ret;
}
void RequestedTx(NodeId peer, const uint256& txhash, std::chrono::microseconds expiry)
{
auto it = m_index.get<ByPeer>().find(ByPeerView{peer, true, txhash});
if (it == m_index.get<ByPeer>().end()) {
// There is no CANDIDATE_BEST announcement, look for a _READY or _DELAYED instead. If the caller only
// ever invokes RequestedTx with the values returned by GetRequestable, and no other non-const functions
// other than ForgetTxHash and GetRequestable in between, this branch will never execute (as txhashes
// returned by GetRequestable always correspond to CANDIDATE_BEST announcements).
it = m_index.get<ByPeer>().find(ByPeerView{peer, false, txhash});
if (it == m_index.get<ByPeer>().end() || (it->m_state != State::CANDIDATE_DELAYED &&
it->m_state != State::CANDIDATE_READY)) {
// There is no CANDIDATE announcement tracked for this peer, so we have nothing to do. Either this
// txhash wasn't tracked at all (and the caller should have called ReceivedInv), or it was already
// requested and/or completed for other reasons and this is just a superfluous RequestedTx call.
return;
}
// Look for an existing CANDIDATE_BEST or REQUESTED with the same txhash. We only need to do this if the
// found announcement had a different state than CANDIDATE_BEST. If it did, invariants guarantee that no
// other CANDIDATE_BEST or REQUESTED can exist.
auto it_old = m_index.get<ByTxHash>().lower_bound(ByTxHashView{txhash, State::CANDIDATE_BEST, 0});
if (it_old != m_index.get<ByTxHash>().end() && it_old->m_txhash == txhash) {
if (it_old->m_state == State::CANDIDATE_BEST) {
// The data structure's invariants require that there can be at most one CANDIDATE_BEST or one
// REQUESTED announcement per txhash (but not both simultaneously), so we have to convert any
// existing CANDIDATE_BEST to another CANDIDATE_* when constructing another REQUESTED.
// It doesn't matter whether we pick CANDIDATE_READY or _DELAYED here, as SetTimePoint()
// will correct it at GetRequestable() time. If time only goes forward, it will always be
// _READY, so pick that to avoid extra work in SetTimePoint().
Modify<ByTxHash>(it_old, [](Announcement& ann) { ann.m_state = State::CANDIDATE_READY; });
} else if (it_old->m_state == State::REQUESTED) {
// As we're no longer waiting for a response to the previous REQUESTED announcement, convert it
// to COMPLETED. This also helps guaranteeing progress.
Modify<ByTxHash>(it_old, [](Announcement& ann) { ann.m_state = State::COMPLETED; });
}
}
}
Modify<ByPeer>(it, [expiry](Announcement& ann) {
ann.m_state = State::REQUESTED;
ann.m_time = expiry;
});
}
void ReceivedResponse(NodeId peer, const uint256& txhash)
{
// We need to search the ByPeer index for both (peer, false, txhash) and (peer, true, txhash).
auto it = m_index.get<ByPeer>().find(ByPeerView{peer, false, txhash});
if (it == m_index.get<ByPeer>().end()) {
it = m_index.get<ByPeer>().find(ByPeerView{peer, true, txhash});
}
if (it != m_index.get<ByPeer>().end()) MakeCompleted(m_index.project<ByTxHash>(it));
}
size_t CountInFlight(NodeId peer) const
{
auto it = m_peerinfo.find(peer);
if (it != m_peerinfo.end()) return it->second.m_requested;
return 0;
}
size_t CountCandidates(NodeId peer) const
{
auto it = m_peerinfo.find(peer);
if (it != m_peerinfo.end()) return it->second.m_total - it->second.m_requested - it->second.m_completed;
return 0;
}
size_t Count(NodeId peer) const
{
auto it = m_peerinfo.find(peer);
if (it != m_peerinfo.end()) return it->second.m_total;
return 0;
}
//! Count how many announcements are being tracked in total across all peers and transactions.
size_t Size() const { return m_index.size(); }
};
TxRequestTracker::TxRequestTracker(bool deterministic) :
m_impl{MakeUnique<TxRequestTracker::Impl>(deterministic)} {}
TxRequestTracker::~TxRequestTracker() = default;
void TxRequestTracker::ForgetTxHash(const uint256& txhash) { m_impl->ForgetTxHash(txhash); }
void TxRequestTracker::DisconnectedPeer(NodeId peer) { m_impl->DisconnectedPeer(peer); }
size_t TxRequestTracker::CountInFlight(NodeId peer) const { return m_impl->CountInFlight(peer); }
size_t TxRequestTracker::CountCandidates(NodeId peer) const { return m_impl->CountCandidates(peer); }
size_t TxRequestTracker::Count(NodeId peer) const { return m_impl->Count(peer); }
size_t TxRequestTracker::Size() const { return m_impl->Size(); }
void TxRequestTracker::ReceivedInv(NodeId peer, const GenTxid& gtxid, bool preferred,
std::chrono::microseconds reqtime)
{
m_impl->ReceivedInv(peer, gtxid, preferred, reqtime);
}
void TxRequestTracker::RequestedTx(NodeId peer, const uint256& txhash, std::chrono::microseconds expiry)
{
m_impl->RequestedTx(peer, txhash, expiry);
}
void TxRequestTracker::ReceivedResponse(NodeId peer, const uint256& txhash)
{
m_impl->ReceivedResponse(peer, txhash);
}
std::vector<GenTxid> TxRequestTracker::GetRequestable(NodeId peer, std::chrono::microseconds now)
{
return m_impl->GetRequestable(peer, now);
}

197
src/txrequest.h Normal file
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@ -0,0 +1,197 @@
// Copyright (c) 2020 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
#ifndef BITCOIN_TXREQUEST_H
#define BITCOIN_TXREQUEST_H
#include <primitives/transaction.h>
#include <net.h> // For NodeId
#include <uint256.h>
#include <chrono>
#include <vector>
#include <stdint.h>
/** Data structure to keep track of, and schedule, transaction downloads from peers.
*
* === Specification ===
*
* We keep track of which peers have announced which transactions, and use that to determine which requests
* should go to which peer, when, and in what order.
*
* The following information is tracked per peer/tx combination ("announcement"):
* - Which peer announced it (through their NodeId)
* - The txid or wtxid of the transaction (collectively called "txhash" in what follows)
* - Whether it was a tx or wtx announcement (see BIP339).
* - What the earliest permitted time is that that transaction can be requested from that peer (called "reqtime").
* - Whether it's from a "preferred" peer or not. Which announcements get this flag is determined by the caller, but
* this is designed for outbound peers, or other peers that we have a higher level of trust in. Even when the
* peers' preferredness changes, the preferred flag of existing announcements from that peer won't change.
* - Whether or not the transaction was requested already, and if so, when it times out (called "expiry").
* - Whether or not the transaction request failed already (timed out, or invalid transaction or NOTFOUND was
* received).
*
* Transaction requests are then assigned to peers, following these rules:
*
* - No transaction is requested as long as another request for the same txhash is outstanding (it needs to fail
* first by passing expiry, or a NOTFOUND or invalid transaction has to be received for it).
*
* Rationale: to avoid wasting bandwidth on multiple copies of the same transaction. Note that this only works
* per txhash, so if the same transaction is announced both through txid and wtxid, we have no means
* to prevent fetching both (the caller can however mitigate this by delaying one, see further).
*
* - The same transaction is never requested twice from the same peer, unless the announcement was forgotten in
* between, and re-announced. Announcements are forgotten only:
* - If a peer goes offline, all its announcements are forgotten.
* - If a transaction has been successfully received, or is otherwise no longer needed, the caller can call
* ForgetTxHash, which removes all announcements across all peers with the specified txhash.
* - If for a given txhash only already-failed announcements remain, they are all forgotten.
*
* Rationale: giving a peer multiple chances to announce a transaction would allow them to bias requests in their
* favor, worsening transaction censoring attacks. The flip side is that as long as an attacker manages
* to prevent us from receiving a transaction, failed announcements (including those from honest peers)
* will linger longer, increasing memory usage somewhat. The impact of this is limited by imposing a
* cap on the number of tracked announcements per peer. As failed requests in response to announcements
* from honest peers should be rare, this almost solely hinders attackers.
* Transaction censoring attacks can be done by announcing transactions quickly while not answering
* requests for them. See https://allquantor.at/blockchainbib/pdf/miller2015topology.pdf for more
* information.
*
* - Transactions are not requested from a peer until its reqtime has passed.
*
* Rationale: enable the calling code to define a delay for less-than-ideal peers, so that (presumed) better
* peers have a chance to give their announcement first.
*
* - If multiple viable candidate peers exist according to the above rules, pick a peer as follows:
*
* - If any preferred peers are available, non-preferred peers are not considered for what follows.
*
* Rationale: preferred peers are more trusted by us, so are less likely to be under attacker control.
*
* - Pick a uniformly random peer among the candidates.
*
* Rationale: random assignments are hard to influence for attackers.
*
* Together these rules strike a balance between being fast in non-adverserial conditions and minimizing
* susceptibility to censorship attacks. An attacker that races the network:
* - Will be unsuccessful if all preferred connections are honest (and there is at least one preferred connection).
* - If there are P preferred connections of which Ph>=1 are honest, the attacker can delay us from learning
* about a transaction by k expiration periods, where k ~ 1 + NHG(N=P-1,K=P-Ph-1,r=1), which has mean
* P/(Ph+1) (where NHG stands for Negative Hypergeometric distribution). The "1 +" is due to the fact that the
* attacker can be the first to announce through a preferred connection in this scenario, which very likely means
* they get the first request.
* - If all P preferred connections are to the attacker, and there are NP non-preferred connections of which NPh>=1
* are honest, where we assume that the attacker can disconnect and reconnect those connections, the distribution
* becomes k ~ P + NB(p=1-NPh/NP,r=1) (where NB stands for Negative Binomial distribution), which has mean
* P-1+NP/NPh.
*
* Complexity:
* - Memory usage is proportional to the total number of tracked announcements (Size()) plus the number of
* peers with a nonzero number of tracked announcements.
* - CPU usage is generally logarithmic in the total number of tracked announcements, plus the number of
* announcements affected by an operation (amortized O(1) per announcement).
*/
class TxRequestTracker {
// Avoid littering this header file with implementation details.
class Impl;
const std::unique_ptr<Impl> m_impl;
public:
//! Construct a TxRequestTracker.
explicit TxRequestTracker(bool deterministic = false);
~TxRequestTracker();
// Conceptually, the data structure consists of a collection of "announcements", one for each peer/txhash
// combination:
//
// - CANDIDATE announcements represent transactions that were announced by a peer, and that become available for
// download after their reqtime has passed.
//
// - REQUESTED announcements represent transactions that have been requested, and which we're awaiting a
// response for from that peer. Their expiry value determines when the request times out.
//
// - COMPLETED announcements represent transactions that have been requested from a peer, and a NOTFOUND or a
// transaction was received in response (valid or not), or they timed out. They're only kept around to
// prevent requesting them again. If only COMPLETED announcements for a given txhash remain (so no CANDIDATE
// or REQUESTED ones), all of them are deleted (this is an invariant, and maintained by all operations below).
//
// The operations below manipulate the data structure.
/** Adds a new CANDIDATE announcement.
*
* Does nothing if one already exists for that (txhash, peer) combination (whether it's CANDIDATE, REQUESTED, or
* COMPLETED). Note that the txid/wtxid property is ignored for determining uniqueness, so if an announcement
* is added for a wtxid H, while one for txid H from the same peer already exists, it will be ignored. This is
* harmless as the txhashes being equal implies it is a non-segwit transaction, so it doesn't matter how it is
* fetched. The new announcement is given the specified preferred and reqtime values, and takes its is_wtxid
* from the specified gtxid.
*/
void ReceivedInv(NodeId peer, const GenTxid& gtxid, bool preferred,
std::chrono::microseconds reqtime);
/** Deletes all announcements for a given peer.
*
* It should be called when a peer goes offline.
*/
void DisconnectedPeer(NodeId peer);
/** Deletes all announcements for a given txhash (both txid and wtxid ones).
*
* This should be called when a transaction is no longer needed. The caller should ensure that new announcements
* for the same txhash will not trigger new ReceivedInv calls, at least in the short term after this call.
*/
void ForgetTxHash(const uint256& txhash);
/** Find the txids to request now from peer.
*
* It does the following:
* - Convert all REQUESTED announcements (for all txhashes/peers) with (expiry <= now) to COMPLETED ones.
* - Requestable announcements are selected: CANDIDATE announcements from the specified peer with
* (reqtime <= now) for which no existing REQUESTED announcement with the same txhash from a different peer
* exists, and for which the specified peer is the best choice among all (reqtime <= now) CANDIDATE
* announcements with the same txhash (subject to preferredness rules, and tiebreaking using a deterministic
* salted hash of peer and txhash).
* - The selected announcements are converted to GenTxids using their is_wtxid flag, and returned in
* announcement order (even if multiple were added at the same time, or when the clock went backwards while
* they were being added). This is done to minimize disruption from dependent transactions being requested
* out of order: if multiple dependent transactions are announced simultaneously by one peer, and end up
* being requested from them, the requests will happen in announcement order.
*/
std::vector<GenTxid> GetRequestable(NodeId peer, std::chrono::microseconds now);
/** Marks a transaction as requested, with a specified expiry.
*
* If no CANDIDATE announcement for the provided peer and txhash exists, this call has no effect. Otherwise:
* - That announcement is converted to REQUESTED.
* - If any other REQUESTED announcement for the same txhash already existed, it means an unexpected request
* was made (GetRequestable will never advise doing so). In this case it is converted to COMPLETED, as we're
* no longer waiting for a response to it.
*/
void RequestedTx(NodeId peer, const uint256& txhash, std::chrono::microseconds expiry);
/** Converts a CANDIDATE or REQUESTED announcement to a COMPLETED one. If no such announcement exists for the
* provided peer and txhash, nothing happens.
*
* It should be called whenever a transaction or NOTFOUND was received from a peer. When the transaction is
* not needed entirely anymore, ForgetTxhash should be called instead of, or in addition to, this call.
*/
void ReceivedResponse(NodeId peer, const uint256& txhash);
// The operations below inspect the data structure.
/** Count how many REQUESTED announcements a peer has. */
size_t CountInFlight(NodeId peer) const;
/** Count how many CANDIDATE announcements a peer has. */
size_t CountCandidates(NodeId peer) const;
/** Count how many announcements a peer has (REQUESTED, CANDIDATE, and COMPLETED combined). */
size_t Count(NodeId peer) const;
/** Count how many announcements are being tracked in total across all peers and transaction hashes. */
size_t Size() const;
};
#endif // BITCOIN_TXREQUEST_H

1
src/uint256.cpp
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@ -80,4 +80,5 @@ template std::string base_blob<256>::ToString() const;
template void base_blob<256>::SetHex(const char*);
template void base_blob<256>::SetHex(const std::string&);
const uint256 uint256::ZERO(0);
const uint256 uint256::ONE(1);

1
src/uint256.h
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@ -126,6 +126,7 @@ public:
constexpr uint256() {}
constexpr explicit uint256(uint8_t v) : base_blob<256>(v) {}
explicit uint256(const std::vector<unsigned char>& vch) : base_blob<256>(vch) {}
static const uint256 ZERO;
static const uint256 ONE;
};