net: store best block tip time inside PeerManager

And implement 'ApproximateBestBlockDepth()' to estimate
the distance, in blocks, between the best-known block
and the network chain tip. Utilizing the best-block time
and the chainparams blocks spacing to approximate it.

src/init.cpp

@@ -1754,13 +1754,15 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // ********************************************************* Step 12: start node
 
     //// debug print
+    int64_t best_block_time{};
     {
         LOCK(cs_main);
         LogPrintf("block tree size = %u\n", chainman.BlockIndex().size());
         chain_active_height = chainman.ActiveChain().Height();
+        best_block_time = chainman.ActiveChain().Tip() ? chainman.ActiveChain().Tip()->GetBlockTime() : chainman.GetParams().GenesisBlock().GetBlockTime();
         if (tip_info) {
             tip_info->block_height = chain_active_height;
-            tip_info->block_time = chainman.ActiveChain().Tip() ? chainman.ActiveChain().Tip()->GetBlockTime() : chainman.GetParams().GenesisBlock().GetBlockTime();
+            tip_info->block_time = best_block_time;
             tip_info->verification_progress = GuessVerificationProgress(chainman.GetParams().TxData(), chainman.ActiveChain().Tip());
         }
         if (tip_info && chainman.m_best_header) {
@@ -1769,7 +1771,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
         }
     }
     LogPrintf("nBestHeight = %d\n", chain_active_height);
-    if (node.peerman) node.peerman->SetBestHeight(chain_active_height);
+    if (node.peerman) node.peerman->SetBestBlock(chain_active_height, std::chrono::seconds{best_block_time});
 
     // Map ports with UPnP or NAT-PMP.
     StartMapPort(args.GetBoolArg("-upnp", DEFAULT_UPNP), args.GetBoolArg("-natpmp", DEFAULT_NATPMP));

src/net_processing.cpp

@@ -513,7 +513,11 @@ public:
     bool IgnoresIncomingTxs() override { return m_opts.ignore_incoming_txs; }
     void SendPings() override EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex);
     void RelayTransaction(const uint256& txid, const uint256& wtxid) override EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex);
-    void SetBestHeight(int height) override { m_best_height = height; };
+    void SetBestBlock(int height, std::chrono::seconds time) override
+    {
+        m_best_height = height;
+        m_best_block_time = time;
+    };
     void UnitTestMisbehaving(NodeId peer_id, int howmuch) override EXCLUSIVE_LOCKS_REQUIRED(!m_peer_mutex) { Misbehaving(*Assert(GetPeerRef(peer_id)), howmuch, ""); };
     void ProcessMessage(CNode& pfrom, const std::string& msg_type, DataStream& vRecv,
                         const std::chrono::microseconds time_received, const std::atomic<bool>& interruptMsgProc) override
@@ -721,6 +725,8 @@ private:
 
     /** The height of the best chain */
     std::atomic<int> m_best_height{-1};
+    /** The time of the best chain tip block */
+    std::atomic<std::chrono::seconds> m_best_block_time{0s};
 
     /** Next time to check for stale tip */
     std::chrono::seconds m_stale_tip_check_time GUARDED_BY(cs_main){0s};
@@ -992,6 +998,12 @@ private:
     void UpdateBlockAvailability(NodeId nodeid, const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
     bool CanDirectFetch() EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
+    /**
+     * Estimates the distance, in blocks, between the best-known block and the network chain tip.
+     * Utilizes the best-block time and the chainparams blocks spacing to approximate it.
+     */
+    int64_t ApproximateBestBlockDepth() const;
+
     /**
      * To prevent fingerprinting attacks, only send blocks/headers outside of
      * the active chain if they are no more than a month older (both in time,
@@ -1311,6 +1323,11 @@ bool PeerManagerImpl::TipMayBeStale()
     return m_last_tip_update.load() < GetTime<std::chrono::seconds>() - std::chrono::seconds{consensusParams.nPowTargetSpacing * 3} && mapBlocksInFlight.empty();
 }
 
+int64_t PeerManagerImpl::ApproximateBestBlockDepth() const
+{
+    return (GetTime<std::chrono::seconds>() - m_best_block_time.load()).count() / m_chainparams.GetConsensus().nPowTargetSpacing;
+}
+
 bool PeerManagerImpl::CanDirectFetch()
 {
     return m_chainman.ActiveChain().Tip()->Time() > GetAdjustedTime() - m_chainparams.GetConsensus().PowTargetSpacing() * 20;
@@ -2047,7 +2064,7 @@ void PeerManagerImpl::NewPoWValidBlock(const CBlockIndex *pindex, const std::sha
  */
 void PeerManagerImpl::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload)
 {
-    SetBestHeight(pindexNew->nHeight);
+    SetBestBlock(pindexNew->nHeight, std::chrono::seconds{pindexNew->GetBlockTime()});
     SetServiceFlagsIBDCache(!fInitialDownload);
 
     // Don't relay inventory during initial block download.

src/net_processing.h

@@ -92,8 +92,8 @@ public:
     /** Send ping message to all peers */
     virtual void SendPings() = 0;
 
-    /** Set the best height */
+    /** Set the height of the best block and its time (seconds since epoch). */
-    virtual void SetBestHeight(int height) = 0;
+    virtual void SetBestBlock(int height, std::chrono::seconds time) = 0;
 
     /* Public for unit testing. */
     virtual void UnitTestMisbehaving(NodeId peer_id, int howmuch) = 0;