net: Serve blocks directly from disk when possible

In `ProcessGetBlockData`, send the block data directly from disk if
type MSG_WITNESS_BLOCK is requested. This is a valid shortcut as the
on-disk format matches the network format.

This is expected to increase performance because a deserialization and
subsequent serialization roundtrip is avoided.

3 files changed, 107 insertions, 47 deletions

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index ee4e9e61bc..2091f1da1f 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -1070,12 +1070,13 @@ static void RelayAddress(const CAddress& addr, bool fReachable, CConnman* connma
     connman->ForEachNodeThen(std::move(sortfunc), std::move(pushfunc));
 }
 
-void static ProcessGetBlockData(CNode* pfrom, const Consensus::Params& consensusParams, const CInv& inv, CConnman* connman, const std::atomic<bool>& interruptMsgProc)
+void static ProcessGetBlockData(CNode* pfrom, const CChainParams& chainparams, const CInv& inv, CConnman* connman, const std::atomic<bool>& interruptMsgProc)
 {
     bool send = false;
     std::shared_ptr<const CBlock> a_recent_block;
     std::shared_ptr<const CBlockHeaderAndShortTxIDs> a_recent_compact_block;
     bool fWitnessesPresentInARecentCompactBlock;
+    const Consensus::Params& consensusParams = chainparams.GetConsensus();
     {
         LOCK(cs_most_recent_block);
         a_recent_block = most_recent_block;
@@ -1142,6 +1143,15 @@ void static ProcessGetBlockData(CNode* pfrom, const Consensus::Params& consensus
         std::shared_ptr<const CBlock> pblock;
         if (a_recent_block && a_recent_block->GetHash() == pindex->GetBlockHash()) {
             pblock = a_recent_block;
+        } else if (inv.type == MSG_WITNESS_BLOCK) {
+            // Fast-path: in this case it is possible to serve the block directly from disk,
+            // as the network format matches the format on disk
+            std::vector<uint8_t> block_data;
+            if (!ReadRawBlockFromDisk(block_data, pindex, chainparams.MessageStart())) {
+                assert(!"cannot load block from disk");
+            }
+            connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::BLOCK, MakeSpan(block_data)));
+            // Don't set pblock as we've sent the block
         } else {
             // Send block from disk
             std::shared_ptr<CBlock> pblockRead = std::make_shared<CBlock>();
@@ -1149,53 +1159,55 @@ void static ProcessGetBlockData(CNode* pfrom, const Consensus::Params& consensus
                 assert(!"cannot load block from disk");
             pblock = pblockRead;
         }
-        if (inv.type == MSG_BLOCK)
-            connman->PushMessage(pfrom, msgMaker.Make(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::BLOCK, *pblock));
-        else if (inv.type == MSG_WITNESS_BLOCK)
-            connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::BLOCK, *pblock));
-        else if (inv.type == MSG_FILTERED_BLOCK)
-        {
-            bool sendMerkleBlock = false;
-            CMerkleBlock merkleBlock;
+        if (pblock) {
+            if (inv.type == MSG_BLOCK)
+                connman->PushMessage(pfrom, msgMaker.Make(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::BLOCK, *pblock));
+            else if (inv.type == MSG_WITNESS_BLOCK)
+                connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::BLOCK, *pblock));
+            else if (inv.type == MSG_FILTERED_BLOCK)
             {
-                LOCK(pfrom->cs_filter);
-                if (pfrom->pfilter) {
-                    sendMerkleBlock = true;
-                    merkleBlock = CMerkleBlock(*pblock, *pfrom->pfilter);
+                bool sendMerkleBlock = false;
+                CMerkleBlock merkleBlock;
+                {
+                    LOCK(pfrom->cs_filter);
+                    if (pfrom->pfilter) {
+                        sendMerkleBlock = true;
+                        merkleBlock = CMerkleBlock(*pblock, *pfrom->pfilter);
+                    }
+                }
+                if (sendMerkleBlock) {
+                    connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::MERKLEBLOCK, merkleBlock));
+                    // CMerkleBlock just contains hashes, so also push any transactions in the block the client did not see
+                    // This avoids hurting performance by pointlessly requiring a round-trip
+                    // Note that there is currently no way for a node to request any single transactions we didn't send here -
+                    // they must either disconnect and retry or request the full block.
+                    // Thus, the protocol spec specified allows for us to provide duplicate txn here,
+                    // however we MUST always provide at least what the remote peer needs
+                    typedef std::pair<unsigned int, uint256> PairType;
+                    for (PairType& pair : merkleBlock.vMatchedTxn)
+                        connman->PushMessage(pfrom, msgMaker.Make(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::TX, *pblock->vtx[pair.first]));
                 }
+                // else
+                    // no response
             }
-            if (sendMerkleBlock) {
-                connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::MERKLEBLOCK, merkleBlock));
-                // CMerkleBlock just contains hashes, so also push any transactions in the block the client did not see
-                // This avoids hurting performance by pointlessly requiring a round-trip
-                // Note that there is currently no way for a node to request any single transactions we didn't send here -
-                // they must either disconnect and retry or request the full block.
-                // Thus, the protocol spec specified allows for us to provide duplicate txn here,
-                // however we MUST always provide at least what the remote peer needs
-                typedef std::pair<unsigned int, uint256> PairType;
-                for (PairType& pair : merkleBlock.vMatchedTxn)
-                    connman->PushMessage(pfrom, msgMaker.Make(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::TX, *pblock->vtx[pair.first]));
-            }
-            // else
-                // no response
-        }
-        else if (inv.type == MSG_CMPCT_BLOCK)
-        {
-            // If a peer is asking for old blocks, we're almost guaranteed
-            // they won't have a useful mempool to match against a compact block,
-            // and we don't feel like constructing the object for them, so
-            // instead we respond with the full, non-compact block.
-            bool fPeerWantsWitness = State(pfrom->GetId())->fWantsCmpctWitness;
-            int nSendFlags = fPeerWantsWitness ? 0 : SERIALIZE_TRANSACTION_NO_WITNESS;
-            if (CanDirectFetch(consensusParams) && pindex->nHeight >= chainActive.Height() - MAX_CMPCTBLOCK_DEPTH) {
-                if ((fPeerWantsWitness || !fWitnessesPresentInARecentCompactBlock) && a_recent_compact_block && a_recent_compact_block->header.GetHash() == pindex->GetBlockHash()) {
-                    connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, *a_recent_compact_block));
+            else if (inv.type == MSG_CMPCT_BLOCK)
+            {
+                // If a peer is asking for old blocks, we're almost guaranteed
+                // they won't have a useful mempool to match against a compact block,
+                // and we don't feel like constructing the object for them, so
+                // instead we respond with the full, non-compact block.
+                bool fPeerWantsWitness = State(pfrom->GetId())->fWantsCmpctWitness;
+                int nSendFlags = fPeerWantsWitness ? 0 : SERIALIZE_TRANSACTION_NO_WITNESS;
+                if (CanDirectFetch(consensusParams) && pindex->nHeight >= chainActive.Height() - MAX_CMPCTBLOCK_DEPTH) {
+                    if ((fPeerWantsWitness || !fWitnessesPresentInARecentCompactBlock) && a_recent_compact_block && a_recent_compact_block->header.GetHash() == pindex->GetBlockHash()) {
+                        connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, *a_recent_compact_block));
+                    } else {
+                        CBlockHeaderAndShortTxIDs cmpctblock(*pblock, fPeerWantsWitness);
+                        connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, cmpctblock));
+                    }
                 } else {
-                    CBlockHeaderAndShortTxIDs cmpctblock(*pblock, fPeerWantsWitness);
-                    connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::CMPCTBLOCK, cmpctblock));
+                    connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::BLOCK, *pblock));
                 }
-            } else {
-                connman->PushMessage(pfrom, msgMaker.Make(nSendFlags, NetMsgType::BLOCK, *pblock));
             }
         }
 
@@ -1213,7 +1225,7 @@ void static ProcessGetBlockData(CNode* pfrom, const Consensus::Params& consensus
     }
 }
 
-void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParams, CConnman* connman, const std::atomic<bool>& interruptMsgProc)
+void static ProcessGetData(CNode* pfrom, const CChainParams& chainparams, CConnman* connman, const std::atomic<bool>& interruptMsgProc)
 {
     AssertLockNotHeld(cs_main);
 
@@ -1262,7 +1274,7 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
         const CInv &inv = *it;
         if (inv.type == MSG_BLOCK || inv.type == MSG_FILTERED_BLOCK || inv.type == MSG_CMPCT_BLOCK || inv.type == MSG_WITNESS_BLOCK) {
             it++;
-            ProcessGetBlockData(pfrom, consensusParams, inv, connman, interruptMsgProc);
+            ProcessGetBlockData(pfrom, chainparams, inv, connman, interruptMsgProc);
         }
     }
 
@@ -1971,7 +1983,7 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         }
 
         pfrom->vRecvGetData.insert(pfrom->vRecvGetData.end(), vInv.begin(), vInv.end());
-        ProcessGetData(pfrom, chainparams.GetConsensus(), connman, interruptMsgProc);
+        ProcessGetData(pfrom, chainparams, connman, interruptMsgProc);
     }
 
 
@@ -2941,7 +2953,7 @@ bool PeerLogicValidation::ProcessMessages(CNode* pfrom, std::atomic<bool>& inter
     bool fMoreWork = false;
 
     if (!pfrom->vRecvGetData.empty())
-        ProcessGetData(pfrom, chainparams.GetConsensus(), connman, interruptMsgProc);
+        ProcessGetData(pfrom, chainparams, connman, interruptMsgProc);
 
     if (pfrom->fDisconnect)
         return false;
diff --git a/src/validation.cpp b/src/validation.cpp
index 1b9e982753..69a201d988 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1125,6 +1125,52 @@ bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus
     return true;
 }
 
+bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CDiskBlockPos& pos, const CMessageHeader::MessageStartChars& message_start)
+{
+    CDiskBlockPos hpos = pos;
+    hpos.nPos -= 8; // Seek back 8 bytes for meta header
+    CAutoFile filein(OpenBlockFile(hpos, true), SER_DISK, CLIENT_VERSION);
+    if (filein.IsNull()) {
+        return error("%s: OpenBlockFile failed for %s", __func__, pos.ToString());
+    }
+
+    try {
+        CMessageHeader::MessageStartChars blk_start;
+        unsigned int blk_size;
+
+        filein >> blk_start >> blk_size;
+
+        if (memcmp(blk_start, message_start, CMessageHeader::MESSAGE_START_SIZE)) {
+            return error("%s: Block magic mismatch for %s: %s versus expected %s", __func__, pos.ToString(),
+                    HexStr(blk_start, blk_start + CMessageHeader::MESSAGE_START_SIZE),
+                    HexStr(message_start, message_start + CMessageHeader::MESSAGE_START_SIZE));
+        }
+
+        if (blk_size > MAX_SIZE) {
+            return error("%s: Block data is larger than maximum deserialization size for %s: %s versus %s", __func__, pos.ToString(),
+                    blk_size, MAX_SIZE);
+        }
+
+        block.resize(blk_size); // Zeroing of memory is intentional here
+        filein.read((char*)block.data(), blk_size);
+    } catch(const std::exception& e) {
+        return error("%s: Read from block file failed: %s for %s", __func__, e.what(), pos.ToString());
+    }
+
+    return true;
+}
+
+bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CBlockIndex* pindex, const CMessageHeader::MessageStartChars& message_start)
+{
+    CDiskBlockPos block_pos;
+    {
+        LOCK(cs_main);
+        block_pos = pindex->GetBlockPos();
+    }
+
+    return ReadRawBlockFromDisk(block, block_pos, message_start);
+}
+
 CAmount GetBlockSubsidy(int nHeight, const Consensus::Params& consensusParams)
 {
     int halvings = nHeight / consensusParams.nSubsidyHalvingInterval;
diff --git a/src/validation.h b/src/validation.h
index 9b40100765..b5ab10786a 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -398,6 +398,8 @@ void InitScriptExecutionCache();
 /** Functions for disk access for blocks */
 bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos, const Consensus::Params& consensusParams);
 bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus::Params& consensusParams);
+bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CDiskBlockPos& pos, const CMessageHeader::MessageStartChars& message_start);
+bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CBlockIndex* pindex, const CMessageHeader::MessageStartChars& message_start);
 
 /** Functions for validating blocks and updating the block tree */