merge bitcoin#23880: Serialize cmpctblock at most once in NewPoWValid…

…Block

this commit will not work with `--enable-c++20` as c++20 does away with
aggregate initialization when constructors are declared. a partial
backport of bitcoin#24169 will sort that out.

src/net_processing.cpp

@@ -36,6 +36,7 @@
 #include <algorithm>
 #include <atomic>
 #include <chrono>
+#include <future>
 #include <list>
 #include <memory>
 #include <optional>
@@ -2042,6 +2043,8 @@ void PeerManagerImpl::NewPoWValidBlock(const CBlockIndex *pindex, const std::sha
     nHighestFastAnnounce = pindex->nHeight;
 
     uint256 hashBlock(pblock->GetHash());
+    const std::shared_future<CSerializedNetMsg> lazy_ser{
+        std::async(std::launch::deferred, [&] { return msgMaker.Make(NetMsgType::CMPCTBLOCK, *pcmpctblock); })};
 
     {
         LOCK(cs_most_recent_block);
@@ -2050,7 +2053,7 @@ void PeerManagerImpl::NewPoWValidBlock(const CBlockIndex *pindex, const std::sha
         most_recent_compact_block = pcmpctblock;
     }
 
-    m_connman.ForEachNode([this, &pcmpctblock, pindex, &msgMaker, &hashBlock](CNode* pnode) {
+    m_connman.ForEachNode([this, pindex, &lazy_ser, &hashBlock](CNode* pnode) {
         LockAssertion lock(::cs_main);
         // TODO: Avoid the repeated-serialization here
         if (pnode->fDisconnect)
@@ -2062,7 +2065,9 @@ void PeerManagerImpl::NewPoWValidBlock(const CBlockIndex *pindex, const std::sha
         if (state.m_requested_hb_cmpctblocks && !PeerHasHeader(&state, pindex) && PeerHasHeader(&state, pindex->pprev)) {
             LogPrint(BCLog::NET, "%s sending header-and-ids %s to peer=%d\n", "PeerManager::NewPoWValidBlock",
                     hashBlock.ToString(), pnode->GetId());
-            m_connman.PushMessage(pnode, msgMaker.Make(NetMsgType::CMPCTBLOCK, *pcmpctblock));
+
+            const CSerializedNetMsg& ser_cmpctblock{lazy_ser.get()};
+            m_connman.PushMessage(pnode, CSerializedNetMsg{ser_cmpctblock.data, ser_cmpctblock.m_type});
             state.pindexBestHeaderSent = pindex;
         }
     });