net_processing: Request assumeutxo background chain blocks

Add new PeerManagerImpl::TryDownloadingHistoricalBlocks method and use it to
request background chain blocks in addition to blocks normally requested by
FindNextBlocksToDownload.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>
Co-authored-by: James O'Beirne <james.obeirne@gmail.com>

3 files changed, 105 insertions, 8 deletions

diff --git a/src/net_processing.cpp b/src/net_processing.cpp
index b046b3ac16..4675942366 100644
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -892,6 +892,38 @@ private:
      */
     void FindNextBlocksToDownload(const Peer& peer, unsigned int count, std::vector<const CBlockIndex*>& vBlocks, NodeId& nodeStaller) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
+    /** Request blocks for the background chainstate, if one is in use. */
+    void TryDownloadingHistoricalBlocks(const Peer& peer, unsigned int count, std::vector<const CBlockIndex*>& vBlocks, const CBlockIndex* from_tip, const CBlockIndex* target_block) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+
+    /**
+    * \brief Find next blocks to download from a peer after a starting block.
+    *
+    * \param vBlocks      Vector of blocks to download which will be appended to.
+    * \param peer         Peer which blocks will be downloaded from.
+    * \param state        Pointer to the state of the peer.
+    * \param pindexWalk   Pointer to the starting block to add to vBlocks.
+    * \param count        Maximum number of blocks to allow in vBlocks. No more
+    *                     blocks will be added if it reaches this size.
+    * \param nWindowEnd   Maximum height of blocks to allow in vBlocks. No
+    *                     blocks will be added above this height.
+    * \param activeChain  Optional pointer to a chain to compare against. If
+    *                     provided, any next blocks which are already contained
+    *                     in this chain will not be appended to vBlocks, but
+    *                     instead will be used to update the
+    *                     state->pindexLastCommonBlock pointer.
+    * \param nodeStaller  Optional pointer to a NodeId variable that will receive
+    *                     the ID of another peer that might be causing this peer
+    *                     to stall. This is set to the ID of the peer which
+    *                     first requested the first in-flight block in the
+    *                     download window. It is only set if vBlocks is empty at
+    *                     the end of this function call and if increasing
+    *                     nWindowEnd by 1 would cause it to be non-empty (which
+    *                     indicates the download might be stalled because every
+    *                     block in the window is in flight and no other peer is
+    *                     trying to download the next block).
+    */
+    void FindNextBlocks(std::vector<const CBlockIndex*>& vBlocks, const Peer& peer, CNodeState *state, const CBlockIndex *pindexWalk, unsigned int count, int nWindowEnd, const CChain* activeChain=nullptr, NodeId* nodeStaller=nullptr) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+
     /* Multimap used to preserve insertion order */
     typedef std::multimap<uint256, std::pair<NodeId, std::list<QueuedBlock>::iterator>> BlockDownloadMap;
     BlockDownloadMap mapBlocksInFlight GUARDED_BY(cs_main);
@@ -1312,6 +1344,7 @@ void PeerManagerImpl::UpdateBlockAvailability(NodeId nodeid, const uint256 &hash
     }
 }
 
+// Logic for calculating which blocks to download from a given peer, given our current tip.
 void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int count, std::vector<const CBlockIndex*>& vBlocks, NodeId& nodeStaller)
 {
     if (count == 0)
@@ -1341,12 +1374,47 @@ void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int co
     if (state->pindexLastCommonBlock == state->pindexBestKnownBlock)
         return;
 
-    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.
     int nWindowEnd = state->pindexLastCommonBlock->nHeight + BLOCK_DOWNLOAD_WINDOW;
+
+    FindNextBlocks(vBlocks, peer, state, pindexWalk, count, nWindowEnd, &m_chainman.ActiveChain(), &nodeStaller);
+}
+
+void PeerManagerImpl::TryDownloadingHistoricalBlocks(const Peer& peer, unsigned int count, std::vector<const CBlockIndex*>& vBlocks, const CBlockIndex *from_tip, const CBlockIndex* target_block)
+{
+    Assert(from_tip);
+    Assert(target_block);
+
+    if (vBlocks.size() >= count) {
+        return;
+    }
+
+    vBlocks.reserve(count);
+    CNodeState *state = Assert(State(peer.m_id));
+
+    if (state->pindexBestKnownBlock == nullptr || state->pindexBestKnownBlock->GetAncestor(target_block->nHeight) != target_block) {
+        // This peer can't provide us the complete series of blocks leading up to the
+        // assumeutxo snapshot base.
+        //
+        // Presumably this peer's chain has less work than our ActiveChain()'s tip, or else we
+        // will eventually crash when we try to reorg to it. Let other logic
+        // deal with whether we disconnect this peer.
+        //
+        // TODO at some point in the future, we might choose to request what blocks
+        // this peer does have from the historical chain, despite it not having a
+        // complete history beneath the snapshot base.
+        return;
+    }
+
+    FindNextBlocks(vBlocks, peer, state, from_tip, count, std::min<int>(from_tip->nHeight + BLOCK_DOWNLOAD_WINDOW, target_block->nHeight));
+}
+
+void PeerManagerImpl::FindNextBlocks(std::vector<const CBlockIndex*>& vBlocks, const Peer& peer, CNodeState *state, const CBlockIndex *pindexWalk, unsigned int count, int nWindowEnd, const CChain* activeChain, NodeId* nodeStaller)
+{
+    std::vector<const CBlockIndex*> vToFetch;
     int nMaxHeight = std::min<int>(state->pindexBestKnownBlock->nHeight, nWindowEnd + 1);
     NodeId waitingfor = -1;
     while (pindexWalk->nHeight < nMaxHeight) {
@@ -1374,8 +1442,8 @@ void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int co
                 // We wouldn't download this block or its descendants from this peer.
                 return;
             }
-            if (pindex->nStatus & BLOCK_HAVE_DATA || m_chainman.ActiveChain().Contains(pindex)) {
-                if (pindex->HaveTxsDownloaded())
+            if (pindex->nStatus & BLOCK_HAVE_DATA || (activeChain && activeChain->Contains(pindex))) {
+                if (activeChain && pindex->HaveTxsDownloaded())
                     state->pindexLastCommonBlock = pindex;
             } else if (!IsBlockRequested(pindex->GetBlockHash())) {
                 // The block is not already downloaded, and not yet in flight.
@@ -1383,7 +1451,7 @@ void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int co
                     // We reached the end of the window.
                     if (vBlocks.size() == 0 && waitingfor != peer.m_id) {
                         // We aren't able to fetch anything, but we would be if the download window was one larger.
-                        nodeStaller = waitingfor;
+                        if (nodeStaller) *nodeStaller = waitingfor;
                     }
                     return;
                 }
@@ -5847,7 +5915,20 @@ bool PeerManagerImpl::SendMessages(CNode* pto)
         if (CanServeBlocks(*peer) && ((sync_blocks_and_headers_from_peer && !IsLimitedPeer(*peer)) || !m_chainman.IsInitialBlockDownload()) && state.vBlocksInFlight.size() < MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
             std::vector<const CBlockIndex*> vToDownload;
             NodeId staller = -1;
-            FindNextBlocksToDownload(*peer, MAX_BLOCKS_IN_TRANSIT_PER_PEER - state.vBlocksInFlight.size(), vToDownload, staller);
+            auto get_inflight_budget = [&state]() {
+                return std::max(0, MAX_BLOCKS_IN_TRANSIT_PER_PEER - static_cast<int>(state.vBlocksInFlight.size()));
+            };
+
+            // If a snapshot chainstate is in use, we want to find its next blocks
+            // before the background chainstate to prioritize getting to network tip.
+            FindNextBlocksToDownload(*peer, get_inflight_budget(), vToDownload, staller);
+            if (m_chainman.BackgroundSyncInProgress() && !IsLimitedPeer(*peer)) {
+                TryDownloadingHistoricalBlocks(
+                    *peer,
+                    get_inflight_budget(),
+                    vToDownload, m_chainman.GetBackgroundSyncTip(),
+                    Assert(m_chainman.GetSnapshotBaseBlock()));
+            }
             for (const CBlockIndex *pindex : vToDownload) {
                 uint32_t nFetchFlags = GetFetchFlags(*peer);
                 vGetData.push_back(CInv(MSG_BLOCK | nFetchFlags, pindex->GetBlockHash()));
diff --git a/src/validation.cpp b/src/validation.cpp
index 357b4d422d..a12f121dc3 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -4148,6 +4148,12 @@ bool ChainstateManager::ProcessNewBlock(const std::shared_ptr<const CBlock>& blo
         return error("%s: ActivateBestChain failed (%s)", __func__, state.ToString());
     }
 
+    Chainstate* bg_chain{WITH_LOCK(cs_main, return BackgroundSyncInProgress() ? m_ibd_chainstate.get() : nullptr)};
+    BlockValidationState bg_state;
+    if (bg_chain && !bg_chain->ActivateBestChain(bg_state, block)) {
+        return error("%s: [background] ActivateBestChain failed (%s)", __func__, bg_state.ToString());
+     }
+
     return true;
 }
 
diff --git a/src/validation.h b/src/validation.h
index 3f0a2312b5..319e40447b 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -881,9 +881,6 @@ private:
     /** Most recent headers presync progress update, for rate-limiting. */
     std::chrono::time_point<std::chrono::steady_clock> m_last_presync_update GUARDED_BY(::cs_main) {};
 
-    //! Returns nullptr if no snapshot has been loaded.
-    const CBlockIndex* GetSnapshotBaseBlock() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
-
     //! Return the height of the base block of the snapshot in use, if one exists, else
     //! nullopt.
     std::optional<int> GetSnapshotBaseHeight() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
@@ -1034,12 +1031,25 @@ public:
     //! Otherwise, revert to using the ibd chainstate and shutdown.
     SnapshotCompletionResult MaybeCompleteSnapshotValidation() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
+    //! Returns nullptr if no snapshot has been loaded.
+    const CBlockIndex* GetSnapshotBaseBlock() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+
     //! The most-work chain.
     Chainstate& ActiveChainstate() const;
     CChain& ActiveChain() const EXCLUSIVE_LOCKS_REQUIRED(GetMutex()) { return ActiveChainstate().m_chain; }
     int ActiveHeight() const EXCLUSIVE_LOCKS_REQUIRED(GetMutex()) { return ActiveChain().Height(); }
     CBlockIndex* ActiveTip() const EXCLUSIVE_LOCKS_REQUIRED(GetMutex()) { return ActiveChain().Tip(); }
 
+    //! The state of a background sync (for net processing)
+    bool BackgroundSyncInProgress() const EXCLUSIVE_LOCKS_REQUIRED(GetMutex()) {
+        return IsUsable(m_snapshot_chainstate.get()) && IsUsable(m_ibd_chainstate.get());
+    }
+
+    //! The tip of the background sync chain
+    const CBlockIndex* GetBackgroundSyncTip() const EXCLUSIVE_LOCKS_REQUIRED(GetMutex()) {
+        return BackgroundSyncInProgress() ? m_ibd_chainstate->m_chain.Tip() : nullptr;
+    }
+
     node::BlockMap& BlockIndex() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
     {
         AssertLockHeld(::cs_main);