Merge bitcoin/bitcoin#32827: mempool: Avoid needless vtx iteration during IBD

249889b orphanage: avoid vtx iteration when no orphans (furszy)
41ad2be mempool: Avoid expensive loop in `removeForBlock` during IBD (Lőrinc)

Pull request description:

  During Initial Block Download, the mempool is usually empty, but `CTxMemPool::removeForBlock` is still called for every connected block where we:
  * iterate over every transaction in the block even though none will be found in the empty `mapTx`, always leaving `txs_removed_for_block` empty...
  * which is pre-allocated regardless with `40 bytes * vtx.size()`, even though it will always remain empty.

  Similarly to bitcoin/bitcoin#32730 (comment), this change introduces a minor performance & memory optimization by only executing the loop if any of the affected mempool maps have any contents. The second commit is cherry-picked from there since it's related to this change as well.

ACKs for top commit:
  optout21:
    ACK 249889b
  glozow:
    ACK 249889b
  ismaelsadeeq:
    reACK 249889b

Tree-SHA512: 80d06ff1515164529cdc3ad21db3041bb5b2a1d4b72ba9e6884cdf40c5f1477fee7479944b8bca32a6f0bf27c4e5501fccd085f6041a2dbb101438629cfb9e4b

Author: glozow

src/node/txorphanage.cpp

@@ -567,6 +567,8 @@ bool TxOrphanageImpl::HaveTxToReconsider(NodeId peer)
 }
 void TxOrphanageImpl::EraseForBlock(const CBlock& block)
 {
+    if (m_orphans.empty()) return;
+
     std::set<Wtxid> wtxids_to_erase;
     for (const CTransactionRef& ptx : block.vtx) {
         const CTransaction& block_tx = *ptx;

src/txmempool.cpp

@@ -661,26 +661,25 @@ void CTxMemPool::removeConflicts(const CTransaction &tx)
     }
 }
 
-/**
- * Called when a block is connected. Removes from mempool.
- */
 void CTxMemPool::removeForBlock(const std::vector<CTransactionRef>& vtx, unsigned int nBlockHeight)
 {
+    // Remove confirmed txs and conflicts when a new block is connected, updating the fee logic
     AssertLockHeld(cs);
     Assume(!m_have_changeset);
     std::vector<RemovedMempoolTransactionInfo> txs_removed_for_block;
-    txs_removed_for_block.reserve(vtx.size());
-    for (const auto& tx : vtx)
-    {
-        txiter it = mapTx.find(tx->GetHash());
-        if (it != mapTx.end()) {
-            setEntries stage;
-            stage.insert(it);
-            txs_removed_for_block.emplace_back(*it);
-            RemoveStaged(stage, true, MemPoolRemovalReason::BLOCK);
+    if (mapTx.size() || mapNextTx.size() || mapDeltas.size()) {
+        txs_removed_for_block.reserve(vtx.size());
+        for (const auto& tx : vtx) {
+            txiter it = mapTx.find(tx->GetHash());
+            if (it != mapTx.end()) {
+                setEntries stage;
+                stage.insert(it);
+                txs_removed_for_block.emplace_back(*it);
+                RemoveStaged(stage, true, MemPoolRemovalReason::BLOCK);
+            }
+            removeConflicts(*tx);
+            ClearPrioritisation(tx->GetHash());
         }
-        removeConflicts(*tx);
-        ClearPrioritisation(tx->GetHash());
     }
     if (m_opts.signals) {
         m_opts.signals->MempoolTransactionsRemovedForBlock(txs_removed_for_block, nBlockHeight);