Bitcoin Core  24.99.0
P2P Digital Currency
headerssync.cpp
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1 // Copyright (c) 2022 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 <headerssync.h>
6 #include <logging.h>
7 #include <pow.h>
8 #include <timedata.h>
9 #include <util/check.h>
10 
11 // The two constants below are computed using the simulation script on
12 // https://gist.github.com/sipa/016ae445c132cdf65a2791534dfb7ae1
13 
15 constexpr size_t HEADER_COMMITMENT_PERIOD{584};
16 
19 constexpr size_t REDOWNLOAD_BUFFER_SIZE{13959}; // 13959/584 = ~23.9 commitments
20 
21 // Our memory analysis assumes 48 bytes for a CompressedHeader (so we should
22 // re-calculate parameters if we compress further)
23 static_assert(sizeof(CompressedHeader) == 48);
24 
26  const CBlockIndex* chain_start, const arith_uint256& minimum_required_work) :
27  m_id(id), m_consensus_params(consensus_params),
28  m_chain_start(chain_start),
29  m_minimum_required_work(minimum_required_work),
30  m_current_chain_work(chain_start->nChainWork),
31  m_commit_offset(GetRand<unsigned>(HEADER_COMMITMENT_PERIOD)),
32  m_last_header_received(m_chain_start->GetBlockHeader()),
33  m_current_height(chain_start->nHeight)
34 {
35  // Estimate the number of blocks that could possibly exist on the peer's
36  // chain *right now* using 6 blocks/second (fastest blockrate given the MTP
37  // rule) times the number of seconds from the last allowed block until
38  // today. This serves as a memory bound on how many commitments we might
39  // store from this peer, and we can safely give up syncing if the peer
40  // exceeds this bound, because it's not possible for a consensus-valid
41  // chain to be longer than this (at the current time -- in the future we
42  // could try again, if necessary, to sync a longer chain).
43  m_max_commitments = 6*(Ticks<std::chrono::seconds>(GetAdjustedTime() - NodeSeconds{std::chrono::seconds{chain_start->GetMedianTimePast()}}) + MAX_FUTURE_BLOCK_TIME) / HEADER_COMMITMENT_PERIOD;
44 
45  LogPrint(BCLog::NET, "Initial headers sync started with peer=%d: height=%i, max_commitments=%i, min_work=%s\n", m_id, m_current_height, m_max_commitments, m_minimum_required_work.ToString());
46 }
47 
52 {
60  m_current_height = 0;
61 
63 }
64 
69  std::vector<CBlockHeader>& received_headers, const bool full_headers_message)
70 {
72 
73  Assume(!received_headers.empty());
74  if (received_headers.empty()) return ret;
75 
77  if (m_download_state == State::FINAL) return ret;
78 
80  // During PRESYNC, we minimally validate block headers and
81  // occasionally add commitments to them, until we reach our work
82  // threshold (at which point m_download_state is updated to REDOWNLOAD).
83  ret.success = ValidateAndStoreHeadersCommitments(received_headers);
84  if (ret.success) {
85  if (full_headers_message || m_download_state == State::REDOWNLOAD) {
86  // A full headers message means the peer may have more to give us;
87  // also if we just switched to REDOWNLOAD then we need to re-request
88  // headers from the beginning.
89  ret.request_more = true;
90  } else {
92  // If we're in PRESYNC and we get a non-full headers
93  // message, then the peer's chain has ended and definitely doesn't
94  // have enough work, so we can stop our sync.
95  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (presync phase)\n", m_id, m_current_height);
96  }
97  }
98  } else if (m_download_state == State::REDOWNLOAD) {
99  // During REDOWNLOAD, we compare our stored commitments to what we
100  // receive, and add headers to our redownload buffer. When the buffer
101  // gets big enough (meaning that we've checked enough commitments),
102  // we'll return a batch of headers to the caller for processing.
103  ret.success = true;
104  for (const auto& hdr : received_headers) {
106  // Something went wrong -- the peer gave us an unexpected chain.
107  // We could consider looking at the reason for failure and
108  // punishing the peer, but for now just give up on sync.
109  ret.success = false;
110  break;
111  }
112  }
113 
114  if (ret.success) {
115  // Return any headers that are ready for acceptance.
116  ret.pow_validated_headers = PopHeadersReadyForAcceptance();
117 
118  // If we hit our target blockhash, then all remaining headers will be
119  // returned and we can clear any leftover internal state.
121  LogPrint(BCLog::NET, "Initial headers sync complete with peer=%d: releasing all at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height);
122  } else if (full_headers_message) {
123  // If the headers message is full, we need to request more.
124  ret.request_more = true;
125  } else {
126  // For some reason our peer gave us a high-work chain, but is now
127  // declining to serve us that full chain again. Give up.
128  // Note that there's no more processing to be done with these
129  // headers, so we can still return success.
130  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height);
131  }
132  }
133  }
134 
135  if (!(ret.success && ret.request_more)) Finalize();
136  return ret;
137 }
138 
139 bool HeadersSyncState::ValidateAndStoreHeadersCommitments(const std::vector<CBlockHeader>& headers)
140 {
141  // The caller should not give us an empty set of headers.
142  Assume(headers.size() > 0);
143  if (headers.size() == 0) return true;
144 
146  if (m_download_state != State::PRESYNC) return false;
147 
148  if (headers[0].hashPrevBlock != m_last_header_received.GetHash()) {
149  // Somehow our peer gave us a header that doesn't connect.
150  // This might be benign -- perhaps our peer reorged away from the chain
151  // they were on. Give up on this sync for now (likely we will start a
152  // new sync with a new starting point).
153  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (presync phase)\n", m_id, m_current_height);
154  return false;
155  }
156 
157  // If it does connect, (minimally) validate and occasionally store
158  // commitments.
159  for (const auto& hdr : headers) {
160  if (!ValidateAndProcessSingleHeader(hdr)) {
161  return false;
162  }
163  }
164 
166  m_redownloaded_headers.clear();
172  LogPrint(BCLog::NET, "Initial headers sync transition with peer=%d: reached sufficient work at height=%i, redownloading from height=%i\n", m_id, m_current_height, m_redownload_buffer_last_height);
173  }
174  return true;
175 }
176 
178 {
180  if (m_download_state != State::PRESYNC) return false;
181 
182  int next_height = m_current_height + 1;
183 
184  // Verify that the difficulty isn't growing too fast; an adversary with
185  // limited hashing capability has a greater chance of producing a high
186  // work chain if they compress the work into as few blocks as possible,
187  // so don't let anyone give a chain that would violate the difficulty
188  // adjustment maximum.
190  m_last_header_received.nBits, current.nBits)) {
191  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (presync phase)\n", m_id, next_height);
192  return false;
193  }
194 
195  if (next_height % HEADER_COMMITMENT_PERIOD == m_commit_offset) {
196  // Add a commitment.
199  // The peer's chain is too long; give up.
200  // It's possible the chain grew since we started the sync; so
201  // potentially we could succeed in syncing the peer's chain if we
202  // try again later.
203  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: exceeded max commitments at height=%i (presync phase)\n", m_id, next_height);
204  return false;
205  }
206  }
207 
209  m_last_header_received = current;
210  m_current_height = next_height;
211 
212  return true;
213 }
214 
216 {
218  if (m_download_state != State::REDOWNLOAD) return false;
219 
220  int64_t next_height = m_redownload_buffer_last_height + 1;
221 
222  // Ensure that we're working on a header that connects to the chain we're
223  // downloading.
225  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (redownload phase)\n", m_id, next_height);
226  return false;
227  }
228 
229  // Check that the difficulty adjustments are within our tolerance:
230  uint32_t previous_nBits{0};
231  if (!m_redownloaded_headers.empty()) {
232  previous_nBits = m_redownloaded_headers.back().nBits;
233  } else {
234  previous_nBits = m_chain_start->nBits;
235  }
236 
238  previous_nBits, header.nBits)) {
239  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (redownload phase)\n", m_id, next_height);
240  return false;
241  }
242 
243  // Track work on the redownloaded chain
245 
248  }
249 
250  // If we're at a header for which we previously stored a commitment, verify
251  // it is correct. Failure will result in aborting download.
252  // Also, don't check commitments once we've gotten to our target blockhash;
253  // it's possible our peer has extended its chain between our first sync and
254  // our second, and we don't want to return failure after we've seen our
255  // target blockhash just because we ran out of commitments.
257  if (m_header_commitments.size() == 0) {
258  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: commitment overrun at height=%i (redownload phase)\n", m_id, next_height);
259  // Somehow our peer managed to feed us a different chain and
260  // we've run out of commitments.
261  return false;
262  }
263  bool commitment = m_hasher(header.GetHash()) & 1;
264  bool expected_commitment = m_header_commitments.front();
266  if (commitment != expected_commitment) {
267  LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: commitment mismatch at height=%i (redownload phase)\n", m_id, next_height);
268  return false;
269  }
270  }
271 
272  // Store this header for later processing.
273  m_redownloaded_headers.push_back(header);
274  m_redownload_buffer_last_height = next_height;
276 
277  return true;
278 }
279 
281 {
282  std::vector<CBlockHeader> ret;
283 
285  if (m_download_state != State::REDOWNLOAD) return ret;
286 
289  ret.emplace_back(m_redownloaded_headers.front().GetFullHeader(m_redownload_buffer_first_prev_hash));
290  m_redownloaded_headers.pop_front();
291  m_redownload_buffer_first_prev_hash = ret.back().GetHash();
292  }
293  return ret;
294 }
295 
297 {
299  if (m_download_state == State::FINAL) return {};
300 
301  auto chain_start_locator = LocatorEntries(m_chain_start);
302  std::vector<uint256> locator;
303 
305  // During pre-synchronization, we continue from the last header received.
306  locator.push_back(m_last_header_received.GetHash());
307  }
308 
310  // During redownload, we will download from the last received header that we stored.
311  locator.push_back(m_redownload_buffer_last_hash);
312  }
313 
314  locator.insert(locator.end(), chain_start_locator.begin(), chain_start_locator.end());
315 
316  return CBlockLocator{std::move(locator)};
317 }
int ret
arith_uint256 GetBlockProof(const CBlockIndex &block)
Definition: chain.cpp:131
std::vector< uint256 > LocatorEntries(const CBlockIndex *index)
Construct a list of hash entries to put in a locator.
Definition: chain.cpp:31
static constexpr int64_t MAX_FUTURE_BLOCK_TIME
Maximum amount of time that a block timestamp is allowed to exceed the current network-adjusted time ...
Definition: chain.h:23
#define Assume(val)
Assume is the identity function.
Definition: check.h:86
Nodes collect new transactions into a block, hash them into a hash tree, and scan through nonce value...
Definition: block.h:22
uint32_t nBits
Definition: block.h:29
uint256 hashPrevBlock
Definition: block.h:26
void SetNull()
Definition: block.h:39
uint256 GetHash() const
Definition: block.cpp:11
The block chain is a tree shaped structure starting with the genesis block at the root,...
Definition: chain.h:152
arith_uint256 nChainWork
(memory only) Total amount of work (expected number of hashes) in the chain up to and including this ...
Definition: chain.h:176
uint256 GetBlockHash() const
Definition: chain.h:264
int64_t GetMedianTimePast() const
Definition: chain.h:296
uint32_t nBits
Definition: chain.h:207
int nHeight
height of the entry in the chain. The genesis block has height 0
Definition: chain.h:164
const SaltedTxidHasher m_hasher
m_hasher is a salted hasher for making our 1-bit commitments to headers we've seen.
Definition: headerssync.h:220
uint64_t m_max_commitments
m_max_commitments is a bound we calculate on how long an honest peer's chain could be,...
Definition: headerssync.h:236
HeadersSyncState(NodeId id, const Consensus::Params &consensus_params, const CBlockIndex *chain_start, const arith_uint256 &minimum_required_work)
Construct a HeadersSyncState object representing a headers sync via this download-twice mechanism).
Definition: headerssync.cpp:25
arith_uint256 m_redownload_chain_work
The accumulated work on the redownloaded chain.
Definition: headerssync.h:265
@ FINAL
We're done syncing with this peer and can discard any remaining state.
@ PRESYNC
PRESYNC means the peer has not yet demonstrated their chain has sufficient work and we're only buildi...
@ REDOWNLOAD
REDOWNLOAD means the peer has given us a high-enough-work chain, and now we're redownloading the head...
CBlockHeader m_last_header_received
Store the latest header received while in PRESYNC (initialized to m_chain_start)
Definition: headerssync.h:239
arith_uint256 m_current_chain_work
Work that we've seen so far on the peer's chain.
Definition: headerssync.h:217
int64_t m_current_height
Height of m_last_header_received.
Definition: headerssync.h:242
const unsigned m_commit_offset
The (secret) offset on the heights for which to create commitments.
Definition: headerssync.h:229
const arith_uint256 m_minimum_required_work
Minimum work that we're looking for on this chain.
Definition: headerssync.h:214
std::vector< CBlockHeader > PopHeadersReadyForAcceptance()
Return a set of headers that satisfy our proof-of-work threshold.
bool ValidateAndStoreHeadersCommitments(const std::vector< CBlockHeader > &headers)
Only called in PRESYNC.
const Consensus::Params & m_consensus_params
We use the consensus params in our anti-DoS calculations.
Definition: headerssync.h:208
bool ValidateAndProcessSingleHeader(const CBlockHeader &current)
In PRESYNC, process and update state for a single header.
State m_download_state
Current state of our headers sync.
Definition: headerssync.h:274
bool ValidateAndStoreRedownloadedHeader(const CBlockHeader &header)
In REDOWNLOAD, check a header's commitment (if applicable) and add to buffer for later processing.
bitdeque m_header_commitments
A queue of commitment bits, created during the 1st phase, and verified during the 2nd.
Definition: headerssync.h:223
const NodeId m_id
NodeId of the peer (used for log messages)
Definition: headerssync.h:205
int64_t m_redownload_buffer_last_height
Height of last header in m_redownloaded_headers.
Definition: headerssync.h:250
std::deque< CompressedHeader > m_redownloaded_headers
During phase 2 (REDOWNLOAD), we buffer redownloaded headers in memory until enough commitments have b...
Definition: headerssync.h:247
ProcessingResult ProcessNextHeaders(const std::vector< CBlockHeader > &received_headers, bool full_headers_message)
Process a batch of headers, once a sync via this mechanism has started.
Definition: headerssync.cpp:68
bool m_process_all_remaining_headers
Set this to true once we encounter the target blockheader during phase 2 (REDOWNLOAD).
Definition: headerssync.h:271
void Finalize()
Clear out all download state that might be in progress (freeing any used memory), and mark this objec...
Definition: headerssync.cpp:51
uint256 m_redownload_buffer_last_hash
Hash of last header in m_redownloaded_headers (initialized to m_chain_start).
Definition: headerssync.h:256
uint256 m_redownload_buffer_first_prev_hash
The hashPrevBlock entry for the first header in m_redownloaded_headers We need this to reconstruct th...
Definition: headerssync.h:262
const CBlockIndex * m_chain_start
Store the last block in our block index that the peer's chain builds from.
Definition: headerssync.h:211
CBlockLocator NextHeadersRequestLocator() const
Issue the next GETHEADERS message to our peer.
256-bit unsigned big integer.
void SetNull()
Definition: uint256.h:42
std::string ToString() const
void push_back(bool val)
Definition: bitdeque.h:352
reference front()
Definition: bitdeque.h:333
void pop_front()
Definition: bitdeque.h:386
size_type size() const noexcept
Count the number of bits in the container.
Definition: bitdeque.h:263
constexpr size_t REDOWNLOAD_BUFFER_SIZE
Only feed headers to validation once this many headers on top have been received and validated agains...
Definition: headerssync.cpp:19
constexpr size_t HEADER_COMMITMENT_PERIOD
Store a commitment to a header every HEADER_COMMITMENT_PERIOD blocks.
Definition: headerssync.cpp:15
#define LogPrint(category,...)
Definition: logging.h:243
unsigned int nHeight
@ NET
Definition: logging.h:40
int64_t NodeId
Definition: net.h:92
bool PermittedDifficultyTransition(const Consensus::Params &params, int64_t height, uint32_t old_nbits, uint32_t new_nbits)
Return false if the proof-of-work requirement specified by new_nbits at a given height is not possibl...
Definition: pow.cpp:76
T GetRand(T nMax=std::numeric_limits< T >::max()) noexcept
Generate a uniform random integer of type T in the range [0..nMax) nMax defaults to std::numeric_limi...
Definition: random.h:79
Describes a place in the block chain to another node such that if the other node doesn't have the sam...
Definition: block.h:121
Parameters that influence chain consensus.
Definition: params.h:73
Result data structure for ProcessNextHeaders.
Definition: headerssync.h:142
std::chrono::time_point< NodeClock, std::chrono::seconds > NodeSeconds
Definition: time.h:25
NodeClock::time_point GetAdjustedTime()
Definition: timedata.cpp:35