Merge bitcoin#24062: refactor: replace RecursiveMutex m_most_recent_block_mutex with Mutex

83003ff refactor: replace RecursiveMutex `m_most_recent_block_mutex` with Mutex (Sebastian Falbesoner)
8edd0d3 refactor: reduce scope of lock `m_most_recent_block_mutex` (Sebastian Falbesoner)

Pull request description:

  This PR is related to bitcoin#19303 and gets rid of the RecursiveMutex `m_most_recent_block_mutex`. All of the critical sections (5 in total) only directly access the guarded elements, i.e. it is not possible that within one section another one is called, and we can use a regular Mutex:

  https://github.com/bitcoin/bitcoin/blob/b019cdc036343a437fd7ced85467bd95f48d84c4/src/net_processing.cpp#L1650-L1655

  https://github.com/bitcoin/bitcoin/blob/b019cdc036343a437fd7ced85467bd95f48d84c4/src/net_processing.cpp#L1861-L1865

  https://github.com/bitcoin/bitcoin/blob/b019cdc036343a437fd7ced85467bd95f48d84c4/src/net_processing.cpp#L3149-L3152

  https://github.com/bitcoin/bitcoin/blob/b019cdc036343a437fd7ced85467bd95f48d84c4/src/net_processing.cpp#L3201-L3206

  https://github.com/bitcoin/bitcoin/blob/b019cdc036343a437fd7ced85467bd95f48d84c4/src/net_processing.cpp#L4763-L4769

  The scope of the last critical section is reduced in the first commit, in order to avoid calling the non-trivial method `CConnman::PushMessage` while the lock is held.

ACKs for top commit:
  furszy:
    Code ACK 83003ff with a small comment.
  hebasto:
    ACK 83003ff
  w0xlt:
    ACK bitcoin@83003ff

Tree-SHA512: 3df290cafd2f6c4d40afb9f14e822a77d9c1828e66f5e2233f3ac1deccc2b0a8290bc5fb8eb992f49d39e887b50bc0e9aad63e05db2d870791a8d409fb95695f

Author: MacroFake

src/net_processing.cpp

@@ -687,7 +687,7 @@ class PeerManagerImpl final : public PeerManager
 
 
     // All of the following cache a recent block, and are protected by m_most_recent_block_mutex
-    RecursiveMutex m_most_recent_block_mutex;
+    Mutex m_most_recent_block_mutex;
     std::shared_ptr<const CBlock> m_most_recent_block GUARDED_BY(m_most_recent_block_mutex);
     std::shared_ptr<const CBlockHeaderAndShortTxIDs> m_most_recent_compact_block GUARDED_BY(m_most_recent_block_mutex);
     uint256 m_most_recent_block_hash GUARDED_BY(m_most_recent_block_mutex);
@@ -4759,15 +4759,16 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
                     LogPrint(BCLog::NET, "%s sending header-and-ids %s to peer=%d\n", __func__,
                             vHeaders.front().GetHash().ToString(), pto->GetId());
 
-                    bool fGotBlockFromCache = false;
+                    std::optional<CSerializedNetMsg> cached_cmpctblock_msg;
                     {
                         LOCK(m_most_recent_block_mutex);
                         if (m_most_recent_block_hash == pBestIndex->GetBlockHash()) {
-                            m_connman.PushMessage(pto, msgMaker.Make(NetMsgType::CMPCTBLOCK, *m_most_recent_compact_block));
-                            fGotBlockFromCache = true;
+                            cached_cmpctblock_msg = msgMaker.Make(NetMsgType::CMPCTBLOCK, *m_most_recent_compact_block);
                         }
                     }
-                    if (!fGotBlockFromCache) {
+                    if (cached_cmpctblock_msg.has_value()) {
+                        m_connman.PushMessage(pto, std::move(cached_cmpctblock_msg.value()));
+                    } else {
                         CBlock block;
                         bool ret = ReadBlockFromDisk(block, pBestIndex, consensusParams);
                         assert(ret);