Merge #9375: Relay compact block messages prior to full block connection

02ee4eb Make most_recent_compact_block a pointer to a const (Matt Corallo)
73666ad Add comment to describe callers to ActivateBestChain (Matt Corallo)
962f7f0 Call ActivateBestChain without cs_main/with most_recent_block (Matt Corallo)
0df777d Use a temp pindex to avoid a const_cast in ProcessNewBlockHeaders (Matt Corallo)
c1ae4fc Avoid holding cs_most_recent_block while calling ReadBlockFromDisk (Matt Corallo)
9eb67f5 Ensure we meet the BIP 152 old-relay-types response requirements (Matt Corallo)
5749a85 Cache most-recently-connected compact block (Matt Corallo)
9eaec08 Cache most-recently-announced block's shared_ptr (Matt Corallo)
c802092 Relay compact block messages prior to full block connection (Matt Corallo)
6987219 Add a CValidationInterface::NewPoWValidBlock callback (Matt Corallo)
180586f Call AcceptBlock with the block's shared_ptr instead of CBlock& (Matt Corallo)
8baaba6 [qa] Avoid race in preciousblock test. (Matt Corallo)
9a0b2f4 [qa] Make compact blocks test construction using fetch methods (Matt Corallo)
8017547 Make CBlockIndex*es in net_processing const (Matt Corallo)

8 files changed, 220 insertions, 60 deletions

diff --git a/qa/rpc-tests/p2p-compactblocks.py b/qa/rpc-tests/p2p-compactblocks.py
index fc1f16c6d2..9151ecf5de 100755
--- a/qa/rpc-tests/p2p-compactblocks.py
+++ b/qa/rpc-tests/p2p-compactblocks.py
@@ -310,6 +310,9 @@ class CompactBlocksTest(BitcoinTestFramework):
         tip = int(node.getbestblockhash(), 16)
         assert(test_node.wait_for_block_announcement(tip))
 
+        # Make sure we will receive a fast-announce compact block
+        self.request_cb_announcements(test_node, node, version)
+
         # Now mine a block, and look at the resulting compact block.
         test_node.clear_block_announcement()
         block_hash = int(node.generate(1)[0], 16)
@@ -319,27 +322,36 @@ class CompactBlocksTest(BitcoinTestFramework):
         [tx.calc_sha256() for tx in block.vtx]
         block.rehash()
 
-        # Don't care which type of announcement came back for this test; just
-        # request the compact block if we didn't get one yet.
+        # Wait until the block was announced (via compact blocks)
         wait_until(test_node.received_block_announcement, timeout=30)
         assert(test_node.received_block_announcement())
 
+        # Now fetch and check the compact block
+        header_and_shortids = None
+        with mininode_lock:
+            assert(test_node.last_cmpctblock is not None)
+            # Convert the on-the-wire representation to absolute indexes
+            header_and_shortids = HeaderAndShortIDs(test_node.last_cmpctblock.header_and_shortids)
+        self.check_compactblock_construction_from_block(version, header_and_shortids, block_hash, block)
+
+        # Now fetch the compact block using a normal non-announce getdata
         with mininode_lock:
-            if test_node.last_cmpctblock is None:
-                test_node.clear_block_announcement()
-                inv = CInv(4, block_hash)  # 4 == "CompactBlock"
-                test_node.send_message(msg_getdata([inv]))
+            test_node.clear_block_announcement()
+            inv = CInv(4, block_hash)  # 4 == "CompactBlock"
+            test_node.send_message(msg_getdata([inv]))
 
         wait_until(test_node.received_block_announcement, timeout=30)
         assert(test_node.received_block_announcement())
 
-        # Now we should have the compactblock
+        # Now fetch and check the compact block
         header_and_shortids = None
         with mininode_lock:
             assert(test_node.last_cmpctblock is not None)
             # Convert the on-the-wire representation to absolute indexes
             header_and_shortids = HeaderAndShortIDs(test_node.last_cmpctblock.header_and_shortids)
+        self.check_compactblock_construction_from_block(version, header_and_shortids, block_hash, block)
 
+    def check_compactblock_construction_from_block(self, version, header_and_shortids, block_hash, block):
         # Check that we got the right block!
         header_and_shortids.header.calc_sha256()
         assert_equal(header_and_shortids.header.sha256, block_hash)
diff --git a/qa/rpc-tests/preciousblock.py b/qa/rpc-tests/preciousblock.py
index f43160e19a..a806294648 100755
--- a/qa/rpc-tests/preciousblock.py
+++ b/qa/rpc-tests/preciousblock.py
@@ -102,7 +102,7 @@ class PreciousTest(BitcoinTestFramework):
         assert_equal(self.nodes[2].getblockcount(), 6)
         hashL = self.nodes[2].getbestblockhash()
         print("Connect nodes and check no reorg occurs")
-        node_sync_via_rpc(self.nodes[0:3])
+        node_sync_via_rpc(self.nodes[1:3])
         connect_nodes_bi(self.nodes,1,2)
         connect_nodes_bi(self.nodes,0,2)
         assert_equal(self.nodes[0].getbestblockhash(), hashH)
diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index ee52d1a9af..59c1d1bf8f 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -101,7 +101,7 @@ namespace {
     /** Blocks that are in flight, and that are in the queue to be downloaded. Protected by cs_main. */
     struct QueuedBlock {
         uint256 hash;
-        CBlockIndex* pindex;                                     //!< Optional.
+        const CBlockIndex* pindex;                               //!< Optional.
         bool fValidatedHeaders;                                  //!< Whether this block has validated headers at the time of request.
         std::unique_ptr<PartiallyDownloadedBlock> partialBlock;  //!< Optional, used for CMPCTBLOCK downloads
     };
@@ -156,13 +156,13 @@ struct CNodeState {
     //! List of asynchronously-determined block rejections to notify this peer about.
     std::vector<CBlockReject> rejects;
     //! The best known block we know this peer has announced.
-    CBlockIndex *pindexBestKnownBlock;
+    const CBlockIndex *pindexBestKnownBlock;
     //! The hash of the last unknown block this peer has announced.
     uint256 hashLastUnknownBlock;
     //! The last full block we both have.
-    CBlockIndex *pindexLastCommonBlock;
+    const CBlockIndex *pindexLastCommonBlock;
     //! The best header we have sent our peer.
-    CBlockIndex *pindexBestHeaderSent;
+    const CBlockIndex *pindexBestHeaderSent;
     //! Length of current-streak of unconnecting headers announcements
     int nUnconnectingHeaders;
     //! Whether we've started headers synchronization with this peer.
@@ -331,7 +331,7 @@ bool MarkBlockAsReceived(const uint256& hash) {
 // Requires cs_main.
 // returns false, still setting pit, if the block was already in flight from the same peer
 // pit will only be valid as long as the same cs_main lock is being held
-bool MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, const Consensus::Params& consensusParams, CBlockIndex *pindex = NULL, list<QueuedBlock>::iterator **pit = NULL) {
+bool MarkBlockAsInFlight(NodeId nodeid, const uint256& hash, const Consensus::Params& consensusParams, const CBlockIndex *pindex = NULL, list<QueuedBlock>::iterator **pit = NULL) {
     CNodeState *state = State(nodeid);
     assert(state != NULL);
 
@@ -432,7 +432,7 @@ bool CanDirectFetch(const Consensus::Params &consensusParams)
 }
 
 // Requires cs_main
-bool PeerHasHeader(CNodeState *state, CBlockIndex *pindex)
+bool PeerHasHeader(CNodeState *state, const CBlockIndex *pindex)
 {
     if (state->pindexBestKnownBlock && pindex == state->pindexBestKnownBlock->GetAncestor(pindex->nHeight))
         return true;
@@ -443,7 +443,7 @@ bool PeerHasHeader(CNodeState *state, CBlockIndex *pindex)
 
 /** Find the last common ancestor two blocks have.
  *  Both pa and pb must be non-NULL. */
-CBlockIndex* LastCommonAncestor(CBlockIndex* pa, CBlockIndex* pb) {
+const CBlockIndex* LastCommonAncestor(const CBlockIndex* pa, const CBlockIndex* pb) {
     if (pa->nHeight > pb->nHeight) {
         pa = pa->GetAncestor(pb->nHeight);
     } else if (pb->nHeight > pa->nHeight) {
@@ -462,7 +462,7 @@ CBlockIndex* LastCommonAncestor(CBlockIndex* pa, CBlockIndex* pb) {
 
 /** Update pindexLastCommonBlock and add not-in-flight missing successors to vBlocks, until it has
  *  at most count entries. */
-void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBlockIndex*>& vBlocks, NodeId& nodeStaller, const Consensus::Params& consensusParams) {
+void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<const CBlockIndex*>& vBlocks, NodeId& nodeStaller, const Consensus::Params& consensusParams) {
     if (count == 0)
         return;
 
@@ -490,8 +490,8 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBl
     if (state->pindexLastCommonBlock == state->pindexBestKnownBlock)
         return;
 
-    std::vector<CBlockIndex*> vToFetch;
-    CBlockIndex *pindexWalk = state->pindexLastCommonBlock;
+    std::vector<const CBlockIndex*> vToFetch;
+    const CBlockIndex *pindexWalk = state->pindexLastCommonBlock;
     // Never fetch further than the best block we know the peer has, or more than BLOCK_DOWNLOAD_WINDOW + 1 beyond the last
     // linked block we have in common with this peer. The +1 is so we can detect stalling, namely if we would be able to
     // download that next block if the window were 1 larger.
@@ -514,7 +514,7 @@ void FindNextBlocksToDownload(NodeId nodeid, unsigned int count, std::vector<CBl
         // are not yet downloaded and not in flight to vBlocks. In the mean time, update
         // pindexLastCommonBlock as long as all ancestors are already downloaded, or if it's
         // already part of our chain (and therefore don't need it even if pruned).
-        BOOST_FOREACH(CBlockIndex* pindex, vToFetch) {
+        BOOST_FOREACH(const CBlockIndex* pindex, vToFetch) {
             if (!pindex->IsValid(BLOCK_VALID_TREE)) {
                 // We consider the chain that this peer is on invalid.
                 return;
@@ -752,6 +752,51 @@ void PeerLogicValidation::SyncTransaction(const CTransaction& tx, const CBlockIn
     }
 }
 
+static CCriticalSection cs_most_recent_block;
+static std::shared_ptr<const CBlock> most_recent_block;
+static std::shared_ptr<const CBlockHeaderAndShortTxIDs> most_recent_compact_block;
+static uint256 most_recent_block_hash;
+
+void PeerLogicValidation::NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& pblock) {
+    std::shared_ptr<const CBlockHeaderAndShortTxIDs> pcmpctblock = std::make_shared<const CBlockHeaderAndShortTxIDs> (*pblock, true);
+    CNetMsgMaker msgMaker(PROTOCOL_VERSION);
+
+    LOCK(cs_main);
+
+    static int nHighestFastAnnounce = 0;
+    if (pindex->nHeight <= nHighestFastAnnounce)
+        return;
+    nHighestFastAnnounce = pindex->nHeight;
+
+    bool fWitnessEnabled = IsWitnessEnabled(pindex->pprev, Params().GetConsensus());
+    uint256 hashBlock(pblock->GetHash());
+
+    {
+        LOCK(cs_most_recent_block);
+        most_recent_block_hash = hashBlock;
+        most_recent_block = pblock;
+        most_recent_compact_block = pcmpctblock;
+    }
+
+    connman->ForEachNode([this, &pcmpctblock, pindex, &msgMaker, fWitnessEnabled, &hashBlock](CNode* pnode) {
+        // TODO: Avoid the repeated-serialization here
+        if (pnode->nVersion < INVALID_CB_NO_BAN_VERSION || pnode->fDisconnect)
+            return;
+        ProcessBlockAvailability(pnode->GetId());
+        CNodeState &state = *State(pnode->GetId());
+        // If the peer has, or we announced to them the previous block already,
+        // but we don't think they have this one, go ahead and announce it
+        if (state.fPreferHeaderAndIDs && (!fWitnessEnabled || state.fWantsCmpctWitness) &&
+                !PeerHasHeader(&state, pindex) && PeerHasHeader(&state, pindex->pprev)) {
+
+            LogPrint("net", "%s sending header-and-ids %s to peer %d\n", "PeerLogicValidation::NewPoWValidBlock",
+                    hashBlock.ToString(), pnode->id);
+            connman->PushMessage(pnode, msgMaker.Make(NetMsgType::CMPCTBLOCK, *pcmpctblock));
+            state.pindexBestHeaderSent = pindex;
+        }
+    });
+}
+
 void PeerLogicValidation::UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload) {
     const int nNewHeight = pindexNew->nHeight;
     connman->SetBestHeight(nNewHeight);
@@ -911,6 +956,21 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
                 BlockMap::iterator mi = mapBlockIndex.find(inv.hash);
                 if (mi != mapBlockIndex.end())
                 {
+                    if (mi->second->nChainTx && !mi->second->IsValid(BLOCK_VALID_SCRIPTS) &&
+                            mi->second->IsValid(BLOCK_VALID_TREE)) {
+                        // If we have the block and all of its parents, but have not yet validated it,
+                        // we might be in the middle of connecting it (ie in the unlock of cs_main
+                        // before ActivateBestChain but after AcceptBlock).
+                        // In this case, we need to run ActivateBestChain prior to checking the relay
+                        // conditions below.
+                        std::shared_ptr<const CBlock> a_recent_block;
+                        {
+                            LOCK(cs_most_recent_block);
+                            a_recent_block = most_recent_block;
+                        }
+                        CValidationState dummy;
+                        ActivateBestChain(dummy, Params(), a_recent_block);
+                    }
                     if (chainActive.Contains(mi->second)) {
                         send = true;
                     } else {
@@ -1048,7 +1108,7 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
     }
 }
 
-uint32_t GetFetchFlags(CNode* pfrom, CBlockIndex* pprev, const Consensus::Params& chainparams) {
+uint32_t GetFetchFlags(CNode* pfrom, const CBlockIndex* pprev, const Consensus::Params& chainparams) {
     uint32_t nFetchFlags = 0;
     if ((pfrom->GetLocalServices() & NODE_WITNESS) && State(pfrom->GetId())->fHaveWitness) {
         nFetchFlags |= MSG_WITNESS_FLAG;
@@ -1056,6 +1116,23 @@ uint32_t GetFetchFlags(CNode* pfrom, CBlockIndex* pprev, const Consensus::Params
     return nFetchFlags;
 }
 
+inline void static SendBlockTransactions(const CBlock& block, const BlockTransactionsRequest& req, CNode* pfrom, CConnman& connman) {
+    BlockTransactions resp(req);
+    for (size_t i = 0; i < req.indexes.size(); i++) {
+        if (req.indexes[i] >= block.vtx.size()) {
+            LOCK(cs_main);
+            Misbehaving(pfrom->GetId(), 100);
+            LogPrintf("Peer %d sent us a getblocktxn with out-of-bounds tx indices", pfrom->id);
+            return;
+        }
+        resp.txn[i] = block.vtx[req.indexes[i]];
+    }
+    LOCK(cs_main);
+    CNetMsgMaker msgMaker(pfrom->GetSendVersion());
+    int nSendFlags = State(pfrom->GetId())->fWantsCmpctWitness ? 0 : SERIALIZE_TRANSACTION_NO_WITNESS;
+    connman.PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::BLOCKTXN, resp));
+}
+
 bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv, int64_t nTimeReceived, const CChainParams& chainparams, CConnman& connman, std::atomic<bool>& interruptMsgProc)
 {
     LogPrint("net", "received: %s (%u bytes) peer=%d\n", SanitizeString(strCommand), vRecv.size(), pfrom->id);
@@ -1445,10 +1522,27 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         uint256 hashStop;
         vRecv >> locator >> hashStop;
 
+        // We might have announced the currently-being-connected tip using a
+        // compact block, which resulted in the peer sending a getblocks
+        // request, which we would otherwise respond to without the new block.
+        // To avoid this situation we simply verify that we are on our best
+        // known chain now. This is super overkill, but we handle it better
+        // for getheaders requests, and there are no known nodes which support
+        // compact blocks but still use getblocks to request blocks.
+        {
+            std::shared_ptr<const CBlock> a_recent_block;
+            {
+                LOCK(cs_most_recent_block);
+                a_recent_block = most_recent_block;
+            }
+            CValidationState dummy;
+            ActivateBestChain(dummy, Params(), a_recent_block);
+        }
+
         LOCK(cs_main);
 
         // Find the last block the caller has in the main chain
-        CBlockIndex* pindex = FindForkInGlobalIndex(chainActive, locator);
+        const CBlockIndex* pindex = FindForkInGlobalIndex(chainActive, locator);
 
         // Send the rest of the chain
         if (pindex)
@@ -1488,6 +1582,18 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         BlockTransactionsRequest req;
         vRecv >> req;
 
+        std::shared_ptr<const CBlock> recent_block;
+        {
+            LOCK(cs_most_recent_block);
+            if (most_recent_block_hash == req.blockhash)
+                recent_block = most_recent_block;
+            // Unlock cs_most_recent_block to avoid cs_main lock inversion
+        }
+        if (recent_block) {
+            SendBlockTransactions(*recent_block, req, pfrom, connman);
+            return true;
+        }
+
         LOCK(cs_main);
 
         BlockMap::iterator it = mapBlockIndex.find(req.blockhash);
@@ -1517,17 +1623,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         bool ret = ReadBlockFromDisk(block, it->second, chainparams.GetConsensus());
         assert(ret);
 
-        BlockTransactions resp(req);
-        for (size_t i = 0; i < req.indexes.size(); i++) {
-            if (req.indexes[i] >= block.vtx.size()) {
-                Misbehaving(pfrom->GetId(), 100);
-                LogPrintf("Peer %d sent us a getblocktxn with out-of-bounds tx indices", pfrom->id);
-                return true;
-            }
-            resp.txn[i] = block.vtx[req.indexes[i]];
-        }
-        int nSendFlags = State(pfrom->GetId())->fWantsCmpctWitness ? 0 : SERIALIZE_TRANSACTION_NO_WITNESS;
-        connman.PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::BLOCKTXN, resp));
+        SendBlockTransactions(block, req, pfrom, connman);
     }
 
 
@@ -1544,7 +1640,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         }
 
         CNodeState *nodestate = State(pfrom->GetId());
-        CBlockIndex* pindex = NULL;
+        const CBlockIndex* pindex = NULL;
         if (locator.IsNull())
         {
             // If locator is null, return the hashStop block
@@ -1575,6 +1671,14 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         // if our peer has chainActive.Tip() (and thus we are sending an empty
         // headers message). In both cases it's safe to update
         // pindexBestHeaderSent to be our tip.
+        //
+        // It is important that we simply reset the BestHeaderSent value here,
+        // and not max(BestHeaderSent, newHeaderSent). We might have announced
+        // the currently-being-connected tip using a compact block, which
+        // resulted in the peer sending a headers request, which we respond to
+        // without the new block. By resetting the BestHeaderSent, we ensure we
+        // will re-announce the new block via headers (or compact blocks again)
+        // in the SendMessages logic.
         nodestate->pindexBestHeaderSent = pindex ? pindex : chainActive.Tip();
         connman.PushMessage(pfrom, msgMaker.Make(NetMsgType::HEADERS, vHeaders));
     }
@@ -1770,7 +1874,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         }
         }
 
-        CBlockIndex *pindex = NULL;
+        const CBlockIndex *pindex = NULL;
         CValidationState state;
         if (!ProcessNewBlockHeaders({cmpctblock.header}, state, chainparams, &pindex)) {
             int nDoS;
@@ -2046,7 +2150,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             return true;
         }
 
-        CBlockIndex *pindexLast = NULL;
+        const CBlockIndex *pindexLast = NULL;
         {
         LOCK(cs_main);
         CNodeState *nodestate = State(pfrom->GetId());
@@ -2123,8 +2227,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         // If this set of headers is valid and ends in a block with at least as
         // much work as our tip, download as much as possible.
         if (fCanDirectFetch && pindexLast->IsValid(BLOCK_VALID_TREE) && chainActive.Tip()->nChainWork <= pindexLast->nChainWork) {
-            vector<CBlockIndex *> vToFetch;
-            CBlockIndex *pindexWalk = pindexLast;
+            vector<const CBlockIndex *> vToFetch;
+            const CBlockIndex *pindexWalk = pindexLast;
             // Calculate all the blocks we'd need to switch to pindexLast, up to a limit.
             while (pindexWalk && !chainActive.Contains(pindexWalk) && vToFetch.size() <= MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
                 if (!(pindexWalk->nStatus & BLOCK_HAVE_DATA) &&
@@ -2146,7 +2250,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             } else {
                 vector<CInv> vGetData;
                 // Download as much as possible, from earliest to latest.
-                BOOST_REVERSE_FOREACH(CBlockIndex *pindex, vToFetch) {
+                BOOST_REVERSE_FOREACH(const CBlockIndex *pindex, vToFetch) {
                     if (nodestate->nBlocksInFlight >= MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
                         // Can't download any more from this peer
                         break;
@@ -2727,7 +2831,7 @@ bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsg
             bool fRevertToInv = ((!state.fPreferHeaders &&
                                  (!state.fPreferHeaderAndIDs || pto->vBlockHashesToAnnounce.size() > 1)) ||
                                 pto->vBlockHashesToAnnounce.size() > MAX_BLOCKS_TO_ANNOUNCE);
-            CBlockIndex *pBestIndex = NULL; // last header queued for delivery
+            const CBlockIndex *pBestIndex = NULL; // last header queued for delivery
             ProcessBlockAvailability(pto->id); // ensure pindexBestKnownBlock is up-to-date
 
             if (!fRevertToInv) {
@@ -2738,7 +2842,7 @@ bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsg
                 BOOST_FOREACH(const uint256 &hash, pto->vBlockHashesToAnnounce) {
                     BlockMap::iterator mi = mapBlockIndex.find(hash);
                     assert(mi != mapBlockIndex.end());
-                    CBlockIndex *pindex = mi->second;
+                    const CBlockIndex *pindex = mi->second;
                     if (chainActive[pindex->nHeight] != pindex) {
                         // Bail out if we reorged away from this block
                         fRevertToInv = true;
@@ -2784,13 +2888,29 @@ bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsg
                     // probably means we're doing an initial-ish-sync or they're slow
                     LogPrint("net", "%s sending header-and-ids %s to peer %d\n", __func__,
                             vHeaders.front().GetHash().ToString(), pto->id);
-                    //TODO: Shouldn't need to reload block from disk, but requires refactor
-                    CBlock block;
-                    bool ret = ReadBlockFromDisk(block, pBestIndex, consensusParams);
-                    assert(ret);
-                    CBlockHeaderAndShortTxIDs cmpctblock(block, state.fWantsCmpctWitness);
+
                     int nSendFlags = state.fWantsCmpctWitness ? 0 : SERIALIZE_TRANSACTION_NO_WITNESS;
-                    connman.PushMessage(pto, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, cmpctblock));
+
+                    bool fGotBlockFromCache = false;
+                    {
+                        LOCK(cs_most_recent_block);
+                        if (most_recent_block_hash == pBestIndex->GetBlockHash()) {
+                            if (state.fWantsCmpctWitness)
+                                connman.PushMessage(pto, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, *most_recent_compact_block));
+                            else {
+                                CBlockHeaderAndShortTxIDs cmpctblock(*most_recent_block, state.fWantsCmpctWitness);
+                                connman.PushMessage(pto, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, cmpctblock));
+                            }
+                            fGotBlockFromCache = true;
+                        }
+                    }
+                    if (!fGotBlockFromCache) {
+                        CBlock block;
+                        bool ret = ReadBlockFromDisk(block, pBestIndex, consensusParams);
+                        assert(ret);
+                        CBlockHeaderAndShortTxIDs cmpctblock(block, state.fWantsCmpctWitness);
+                        connman.PushMessage(pto, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, cmpctblock));
+                    }
                     state.pindexBestHeaderSent = pBestIndex;
                 } else if (state.fPreferHeaders) {
                     if (vHeaders.size() > 1) {
@@ -2815,7 +2935,7 @@ bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsg
                     const uint256 &hashToAnnounce = pto->vBlockHashesToAnnounce.back();
                     BlockMap::iterator mi = mapBlockIndex.find(hashToAnnounce);
                     assert(mi != mapBlockIndex.end());
-                    CBlockIndex *pindex = mi->second;
+                    const CBlockIndex *pindex = mi->second;
 
                     // Warn if we're announcing a block that is not on the main chain.
                     // This should be very rare and could be optimized out.
@@ -3000,10 +3120,10 @@ bool SendMessages(CNode* pto, CConnman& connman, std::atomic<bool>& interruptMsg
         //
         vector<CInv> vGetData;
         if (!pto->fClient && (fFetch || !IsInitialBlockDownload()) && state.nBlocksInFlight < MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
-            vector<CBlockIndex*> vToDownload;
+            vector<const CBlockIndex*> vToDownload;
             NodeId staller = -1;
             FindNextBlocksToDownload(pto->GetId(), MAX_BLOCKS_IN_TRANSIT_PER_PEER - state.nBlocksInFlight, vToDownload, staller, consensusParams);
-            BOOST_FOREACH(CBlockIndex *pindex, vToDownload) {
+            BOOST_FOREACH(const CBlockIndex *pindex, vToDownload) {
                 uint32_t nFetchFlags = GetFetchFlags(pto, pindex->pprev, consensusParams);
                 vGetData.push_back(CInv(MSG_BLOCK | nFetchFlags, pindex->GetBlockHash()));
                 MarkBlockAsInFlight(pto->GetId(), pindex->GetBlockHash(), consensusParams, pindex);
diff --git a/src/net_processing.h b/src/net_processing.h
index 1f33def1f7..eaa0305136 100644
--- a/src/net_processing.h
+++ b/src/net_processing.h
@@ -24,6 +24,7 @@ public:
     virtual void SyncTransaction(const CTransaction& tx, const CBlockIndex* pindex, int nPosInBlock);
     virtual void UpdatedBlockTip(const CBlockIndex *pindexNew, const CBlockIndex *pindexFork, bool fInitialDownload);
     virtual void BlockChecked(const CBlock& block, const CValidationState& state);
+    virtual void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& pblock);
 };
 
 struct CNodeStateStats {
diff --git a/src/validation.cpp b/src/validation.cpp
index 3faa1bf005..426b71b6eb 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2414,6 +2414,11 @@ static void NotifyHeaderTip() {
  * that is already loaded (to avoid loading it again from disk).
  */
 bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams, std::shared_ptr<const CBlock> pblock) {
+    // Note that while we're often called here from ProcessNewBlock, this is
+    // far from a guarantee. Things in the P2P/RPC will often end up calling
+    // us in the middle of ProcessNewBlock - do not assume pblock is set
+    // sanely for performance or correctness!
+
     CBlockIndex *pindexMostWork = NULL;
     CBlockIndex *pindexNewTip = NULL;
     do {
@@ -3056,14 +3061,18 @@ static bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state
 }
 
 // Exposed wrapper for AcceptBlockHeader
-bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& headers, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex)
+bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& headers, CValidationState& state, const CChainParams& chainparams, const CBlockIndex** ppindex)
 {
     {
         LOCK(cs_main);
         for (const CBlockHeader& header : headers) {
-            if (!AcceptBlockHeader(header, state, chainparams, ppindex)) {
+            CBlockIndex *pindex = NULL; // Use a temp pindex instead of ppindex to avoid a const_cast
+            if (!AcceptBlockHeader(header, state, chainparams, &pindex)) {
                 return false;
             }
+            if (ppindex) {
+                *ppindex = pindex;
+            }
         }
     }
     NotifyHeaderTip();
@@ -3071,8 +3080,10 @@ bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& headers, CValidatio
 }
 
 /** Store block on disk. If dbp is non-NULL, the file is known to already reside on disk */
-static bool AcceptBlock(const CBlock& block, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock)
+static bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock)
 {
+    const CBlock& block = *pblock;
+
     if (fNewBlock) *fNewBlock = false;
     AssertLockHeld(cs_main);
 
@@ -3118,6 +3129,11 @@ static bool AcceptBlock(const CBlock& block, CValidationState& state, const CCha
         return error("%s: %s", __func__, FormatStateMessage(state));
     }
 
+    // Header is valid/has work, merkle tree and segwit merkle tree are good...RELAY NOW
+    // (but if it does not build on our best tip, let the SendMessages loop relay it)
+    if (!IsInitialBlockDownload() && chainActive.Tip() == pindex->pprev)
+        GetMainSignals().NewPoWValidBlock(pindex, pblock);
+
     int nHeight = pindex->nHeight;
 
     // Write block to history file
@@ -3152,7 +3168,7 @@ bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr<cons
         CBlockIndex *pindex = NULL;
         if (fNewBlock) *fNewBlock = false;
         CValidationState state;
-        bool ret = AcceptBlock(*pblock, state, chainparams, &pindex, fForceProcessing, NULL, fNewBlock);
+        bool ret = AcceptBlock(pblock, state, chainparams, &pindex, fForceProcessing, NULL, fNewBlock);
         CheckBlockIndex(chainparams.GetConsensus());
         if (!ret) {
             GetMainSignals().BlockChecked(*pblock, state);
@@ -3808,7 +3824,8 @@ bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskB
                     dbp->nPos = nBlockPos;
                 blkdat.SetLimit(nBlockPos + nSize);
                 blkdat.SetPos(nBlockPos);
-                CBlock block;
+                std::shared_ptr<CBlock> pblock = std::make_shared<CBlock>();
+                CBlock& block = *pblock;
                 blkdat >> block;
                 nRewind = blkdat.GetPos();
 
@@ -3826,7 +3843,7 @@ bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskB
                 if (mapBlockIndex.count(hash) == 0 || (mapBlockIndex[hash]->nStatus & BLOCK_HAVE_DATA) == 0) {
                     LOCK(cs_main);
                     CValidationState state;
-                    if (AcceptBlock(block, state, chainparams, NULL, true, dbp, NULL))
+                    if (AcceptBlock(pblock, state, chainparams, NULL, true, dbp, NULL))
                         nLoaded++;
                     if (state.IsError())
                         break;
@@ -3853,16 +3870,17 @@ bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskB
                     std::pair<std::multimap<uint256, CDiskBlockPos>::iterator, std::multimap<uint256, CDiskBlockPos>::iterator> range = mapBlocksUnknownParent.equal_range(head);
                     while (range.first != range.second) {
                         std::multimap<uint256, CDiskBlockPos>::iterator it = range.first;
-                        if (ReadBlockFromDisk(block, it->second, chainparams.GetConsensus()))
+                        std::shared_ptr<CBlock> pblockrecursive = std::make_shared<CBlock>();
+                        if (ReadBlockFromDisk(*pblockrecursive, it->second, chainparams.GetConsensus()))
                         {
-                            LogPrint("reindex", "%s: Processing out of order child %s of %s\n", __func__, block.GetHash().ToString(),
+                            LogPrint("reindex", "%s: Processing out of order child %s of %s\n", __func__, pblockrecursive->GetHash().ToString(),
                                     head.ToString());
                             LOCK(cs_main);
                             CValidationState dummy;
-                            if (AcceptBlock(block, dummy, chainparams, NULL, true, &it->second, NULL))
+                            if (AcceptBlock(pblockrecursive, dummy, chainparams, NULL, true, &it->second, NULL))
                             {
                                 nLoaded++;
-                                queue.push_back(block.GetHash());
+                                queue.push_back(pblockrecursive->GetHash());
                             }
                         }
                         range.first++;
diff --git a/src/validation.h b/src/validation.h
index 40e8907724..43a37e46e5 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -246,7 +246,7 @@ bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr<cons
  * @param[in]  chainparams The params for the chain we want to connect to
  * @param[out] ppindex If set, the pointer will be set to point to the last new block index object for the given headers
  */
-bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& block, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex=NULL);
+bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& block, CValidationState& state, const CChainParams& chainparams, const CBlockIndex** ppindex=NULL);
 
 /** Check whether enough disk space is available for an incoming block */
 bool CheckDiskSpace(uint64_t nAdditionalBytes = 0);
diff --git a/src/validationinterface.cpp b/src/validationinterface.cpp
index 215c342dea..d4121a28bc 100644
--- a/src/validationinterface.cpp
+++ b/src/validationinterface.cpp
@@ -22,6 +22,7 @@ void RegisterValidationInterface(CValidationInterface* pwalletIn) {
     g_signals.BlockChecked.connect(boost::bind(&CValidationInterface::BlockChecked, pwalletIn, _1, _2));
     g_signals.ScriptForMining.connect(boost::bind(&CValidationInterface::GetScriptForMining, pwalletIn, _1));
     g_signals.BlockFound.connect(boost::bind(&CValidationInterface::ResetRequestCount, pwalletIn, _1));
+    g_signals.NewPoWValidBlock.connect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
 }
 
 void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
@@ -34,6 +35,7 @@ void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
     g_signals.UpdatedTransaction.disconnect(boost::bind(&CValidationInterface::UpdatedTransaction, pwalletIn, _1));
     g_signals.SyncTransaction.disconnect(boost::bind(&CValidationInterface::SyncTransaction, pwalletIn, _1, _2, _3));
     g_signals.UpdatedBlockTip.disconnect(boost::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, _1, _2, _3));
+    g_signals.NewPoWValidBlock.disconnect(boost::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, _1, _2));
 }
 
 void UnregisterAllValidationInterfaces() {
@@ -46,4 +48,5 @@ void UnregisterAllValidationInterfaces() {
     g_signals.UpdatedTransaction.disconnect_all_slots();
     g_signals.SyncTransaction.disconnect_all_slots();
     g_signals.UpdatedBlockTip.disconnect_all_slots();
+    g_signals.NewPoWValidBlock.disconnect_all_slots();
 }
diff --git a/src/validationinterface.h b/src/validationinterface.h
index 717026389c..594072719c 100644
--- a/src/validationinterface.h
+++ b/src/validationinterface.h
@@ -8,6 +8,7 @@
 
 #include <boost/signals2/signal.hpp>
 #include <boost/shared_ptr.hpp>
+#include <memory>
 
 class CBlock;
 class CBlockIndex;
@@ -40,6 +41,7 @@ protected:
     virtual void BlockChecked(const CBlock&, const CValidationState&) {}
     virtual void GetScriptForMining(boost::shared_ptr<CReserveScript>&) {};
     virtual void ResetRequestCount(const uint256 &hash) {};
+    virtual void NewPoWValidBlock(const CBlockIndex *pindex, const std::shared_ptr<const CBlock>& block) {};
     friend void ::RegisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterValidationInterface(CValidationInterface*);
     friend void ::UnregisterAllValidationInterfaces();
@@ -66,6 +68,10 @@ struct CMainSignals {
     boost::signals2::signal<void (boost::shared_ptr<CReserveScript>&)> ScriptForMining;
     /** Notifies listeners that a block has been successfully mined */
     boost::signals2::signal<void (const uint256 &)> BlockFound;
+    /**
+     * Notifies listeners that a block which builds directly on our current tip
+     * has been received and connected to the headers tree, though not validated yet */
+    boost::signals2::signal<void (const CBlockIndex *, const std::shared_ptr<const CBlock>&)> NewPoWValidBlock;
 };
 
 CMainSignals& GetMainSignals();