trivial: document transaction confirmation safety threshold

Author: kwvg

src/chainlock/chainlock.cpp

@@ -476,8 +476,8 @@ void CChainLocksHandler::Cleanup()
             const auto* pindex = m_chainstate.m_blockman.LookupBlockIndex(hashBlock);
             assert(pindex); // GetTransaction gave us that hashBlock, it should resolve to a valid block index
             if (m_chainstate.m_chain.Tip()->GetAncestor(pindex->nHeight) == pindex &&
-                m_chainstate.m_chain.Height() - pindex->nHeight >= 6) {
-                // tx got confirmed >= 6 times, so we can stop keeping track of it
+                m_chainstate.m_chain.Height() - pindex->nHeight > chainlock::TX_CONFIRM_THRESHOLD) {
+                // tx is sufficiently deep, we can stop tracking it
                 it = txFirstSeenTime.erase(it);
             } else {
                 ++it;

src/chainlock/signing.cpp

@@ -95,10 +95,10 @@ void ChainLockSigner::TrySignChainTip(const llmq::CInstantSendManager& isman)
     if (isman.IsInstantSendEnabled() && isman.RejectConflictingBlocks()) {
         const auto* pindexWalk = pindex;
         while (pindexWalk != nullptr) {
-            if (pindex->nHeight - pindexWalk->nHeight > 5) {
-                // no need to check further down, 6 confs is safe to assume that TXs below this height won't be
+            if (pindex->nHeight - pindexWalk->nHeight > TX_CONFIRM_THRESHOLD) {
+                // no need to check further down, safe to assume that TXs below this height won't be
                 // islocked anymore if they aren't already
-                LogPrint(BCLog::CHAINLOCKS, "%s -- tip and previous 5 blocks all safe\n", __func__);
+                LogPrint(BCLog::CHAINLOCKS, "%s -- tip and previous %d blocks all safe\n", __func__, TX_CONFIRM_THRESHOLD);
                 break;
             }
             if (m_clhandler.HasChainLock(pindexWalk->nHeight, pindexWalk->GetBlockHash())) {

src/chainlock/signing.h

@@ -20,6 +20,9 @@ class CSigSharesManager;
 } // namespace llmq
 
 namespace chainlock {
+//! Depth of block including transactions before it's considered safe
+static constexpr int32_t TX_CONFIRM_THRESHOLD{5};
+
 class ChainLockSignerParent
 {
 public: