net: optimize compact block extra tx iteration
What the developer wrote
Authored by Lőrinc
net: optimize compact block extra tx iteration
`vExtraTxnForCompact` was resized to its configured capacity on the first insertion.
Before the ring was filled, compact block reconstruction scanned default `{Wtxid::ZERO, nullptr}` entries created for unused slots.
Reserve the configured capacity and append entries until the cache is full, then keep the same ring-buffer overwrite behavior.
This preserves the configured maximum and replacement order while keeping reconstruction from scanning unused capacity.
What changed, and why it matters
This is a small performance cleanup in Bitcoin Core's network code. A temporary cache used when reconstructing compact blocks used to be fully sized up front, meaning the code would repeatedly scan empty placeholder slots. The change instead grows the cache only as it fills, so reconstruction skips unused slots. There is no direct security vulnerability being fixed; it is an optimization that may slightly reduce CPU work during block reconstruction.
Treat as a routine performance improvement. No urgent security action required. Reviewers may want to confirm that the ring-buffer index vExtraTxnForCompactIt still wraps correctly and that reserve() does not change behavior once the cache is full.
Security signals we found
Performance optimization in compact block reconstruction path
Avoids scanning default/empty entries in vExtraTxnForCompact
No bounds-check, memory-safety, or validation logic changed
Evidence from the diff
In PeerManagerImpl::AddToCompactExtraTransactions(), vExtraTxnForCompact was previously resized to m_opts.max_extra_txs on first insertion, creating default {Wtxid::ZERO, nullptr} entries for all unused slots. Compact block reconstruction would then scan those empty entries. The patch reserves capacity but only appends live entries until the cache is full, preserving the ring-buffer overwrite behavior once full. This avoids scanning default-constructed entries before the cache reaches capacity.
Changed components
src/net_processing.cppPeerManagerImpl::AddToCompactExtraTransactions()Compact block reconstruction / extra transaction cacheInspect captured patch +7 / −5
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index 61b8a4ec..0b6d8fa7 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -1907,11 +1907,13 @@ std::vector<CTransactionRef> PeerManagerImpl::AbortPrivateBroadcast(const uint25
void PeerManagerImpl::AddToCompactExtraTransactions(const CTransactionRef& tx)
{
- if (m_opts.max_extra_txs <= 0)
- return;
- if (!vExtraTxnForCompact.size())
- vExtraTxnForCompact.resize(m_opts.max_extra_txs);
- vExtraTxnForCompact[vExtraTxnForCompactIt] = std::make_pair(tx->GetWitnessHash(), tx);
+ if (m_opts.max_extra_txs == 0) return;
+ if (vExtraTxnForCompact.size() < m_opts.max_extra_txs) {
+ if (vExtraTxnForCompact.empty()) vExtraTxnForCompact.reserve(m_opts.max_extra_txs);
+ vExtraTxnForCompact.emplace_back(tx->GetWitnessHash(), tx);
+ } else {
+ vExtraTxnForCompact[vExtraTxnForCompactIt] = std::make_pair(tx->GetWitnessHash(), tx);
+ }
vExtraTxnForCompactIt = (vExtraTxnForCompactIt + 1) % m_opts.max_extra_txs;
}
Why this scored 16/100
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