Merge bitcoin/bitcoin#28120: p2p: make block download logic aware of limited peers threshold

c5b5843 test: avoid requesting blocks beyond limited peer threshold (furszy)
2f6a055 p2p: sync from limited peer, only request blocks below threshold (furszy)
7312772 refactor: Make FindNextBlocks friendlier (furszy)

Pull request description:

  Even when the node believes it has IBD completed, need to avoid
  requesting historical blocks from network-limited peers.
  Otherwise, the limited peer will disconnect right away.

  The simplest scenario could be a node that gets synced, drops
  connections, and stays inactive for a while. Then, once it re-connects
  (IBD stays completed), the node tries to fetch all the missing blocks
  from any peer, getting disconnected by the limited ones.

  Note:
  Can verify the behavior by cherry-picking the test commit alone on
  master. It will fail there.

ACKs for top commit:
  achow101:
    ACK c5b5843
  vasild:
    ACK c5b5843
  mzumsande:
    Code Review ACK c5b5843
  pinheadmz:
    ACK c5b5843

Tree-SHA512: 9e550698bc6e63cc587b2b988a87d0ab555a8fa188c91c3f33287f8201d77c28b373331845356ad86f17bb21c15950b6466bc1cafd0ce8139d70364cb71c2ad2

Author: achow101

src/net_processing.cpp

@@ -1451,6 +1451,7 @@ void PeerManagerImpl::FindNextBlocks(std::vector<const CBlockIndex*>& vBlocks, c
 {
     std::vector<const CBlockIndex*> vToFetch;
     int nMaxHeight = std::min<int>(state->pindexBestKnownBlock->nHeight, nWindowEnd + 1);
+    bool is_limited_peer = IsLimitedPeer(peer);
     NodeId waitingfor = -1;
     while (pindexWalk->nHeight < nMaxHeight) {
         // Read up to 128 (or more, if more blocks than that are needed) successors of pindexWalk (towards
@@ -1473,30 +1474,46 @@ void PeerManagerImpl::FindNextBlocks(std::vector<const CBlockIndex*>& vBlocks, c
                 // We consider the chain that this peer is on invalid.
                 return;
             }
+
             if (!CanServeWitnesses(peer) && DeploymentActiveAt(*pindex, m_chainman, Consensus::DEPLOYMENT_SEGWIT)) {
                 // We wouldn't download this block or its descendants from this peer.
                 return;
             }
+
             if (pindex->nStatus & BLOCK_HAVE_DATA || (activeChain && activeChain->Contains(pindex))) {
-                if (activeChain && pindex->HaveNumChainTxs())
+                if (activeChain && pindex->HaveNumChainTxs()) {
                     state->pindexLastCommonBlock = pindex;
-            } else if (!IsBlockRequested(pindex->GetBlockHash())) {
-                // The block is not already downloaded, and not yet in flight.
-                if (pindex->nHeight > nWindowEnd) {
-                    // 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.
-                        if (nodeStaller) *nodeStaller = waitingfor;
-                    }
-                    return;
                 }
-                vBlocks.push_back(pindex);
-                if (vBlocks.size() == count) {
-                    return;
+                continue;
+            }
+
+            // Is block in-flight?
+            if (IsBlockRequested(pindex->GetBlockHash())) {
+                if (waitingfor == -1) {
+                    // This is the first already-in-flight block.
+                    waitingfor = mapBlocksInFlight.lower_bound(pindex->GetBlockHash())->second.first;
                 }
-            } else if (waitingfor == -1) {
-                // This is the first already-in-flight block.
-                waitingfor = mapBlocksInFlight.lower_bound(pindex->GetBlockHash())->second.first;
+                continue;
+            }
+
+            // The block is not already downloaded, and not yet in flight.
+            if (pindex->nHeight > nWindowEnd) {
+                // 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.
+                    if (nodeStaller) *nodeStaller = waitingfor;
+                }
+                return;
+            }
+
+            // Don't request blocks that go further than what limited peers can provide
+            if (is_limited_peer && (state->pindexBestKnownBlock->nHeight - pindex->nHeight >= static_cast<int>(NODE_NETWORK_LIMITED_MIN_BLOCKS) - 2 /* two blocks buffer for possible races */)) {
+                continue;
+            }
+
+            vBlocks.push_back(pindex);
+            if (vBlocks.size() == count) {
+                return;
             }
         }
     }

test/functional/p2p_node_network_limited.py

@@ -21,8 +21,12 @@
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
     assert_equal,
+    assert_raises_rpc_error,
+    try_rpc
 )
 
+# Minimum blocks required to signal NODE_NETWORK_LIMITED #
+NODE_NETWORK_LIMITED_MIN_BLOCKS = 288
 
 class P2PIgnoreInv(P2PInterface):
     firstAddrnServices = 0
@@ -54,6 +58,63 @@ def setup_network(self):
         self.add_nodes(self.num_nodes, self.extra_args)
         self.start_nodes()
 
+    def test_avoid_requesting_historical_blocks(self):
+        self.log.info("Test full node does not request blocks beyond the limited peer threshold")
+        pruned_node = self.nodes[0]
+        miner = self.nodes[1]
+        full_node = self.nodes[2]
+
+        # Connect and generate block to ensure IBD=false
+        self.connect_nodes(1, 0)
+        self.connect_nodes(1, 2)
+        self.generate(miner, 1)
+
+        # Verify peers are out of IBD
+        for node in self.nodes:
+            assert not node.getblockchaininfo()['initialblockdownload']
+
+        # Isolate full_node (the node will remain out of IBD)
+        full_node.setnetworkactive(False)
+        self.wait_until(lambda: len(full_node.getpeerinfo()) == 0)
+
+        # Mine blocks and sync the pruned node. Surpass the NETWORK_NODE_LIMITED threshold.
+        # Blocks deeper than the threshold are considered "historical blocks"
+        num_historial_blocks = 12
+        self.generate(miner, NODE_NETWORK_LIMITED_MIN_BLOCKS + num_historial_blocks, sync_fun=self.no_op)
+        self.sync_blocks([miner, pruned_node])
+
+        # Connect full_node to prune_node and check peers don't disconnect right away.
+        # (they will disconnect if full_node, which is chain-wise behind, request blocks
+        # older than NODE_NETWORK_LIMITED_MIN_BLOCKS)
+        start_height_full_node = full_node.getblockcount()
+        full_node.setnetworkactive(True)
+        self.connect_nodes(2, 0)
+        assert_equal(len(full_node.getpeerinfo()), 1)
+
+        # Wait until the full_node is headers-wise sync
+        best_block_hash = pruned_node.getbestblockhash()
+        self.wait_until(lambda: next(filter(lambda x: x['hash'] == best_block_hash, full_node.getchaintips()))['status'] == "headers-only")
+
+        # Now, since the node aims to download a window of 1024 blocks,
+        # ensure it requests the blocks below the threshold only (with a
+        # 2-block buffer). And also, ensure it does not request any
+        # historical block.
+        tip_height = pruned_node.getblockcount()
+        limit_buffer = 2
+        # Prevent races by waiting for the tip to arrive first
+        self.wait_until(lambda: not try_rpc(-1, "Block not found", full_node.getblock, pruned_node.getbestblockhash()))
+        for height in range(start_height_full_node + 1, tip_height + 1):
+            if height <= tip_height - (NODE_NETWORK_LIMITED_MIN_BLOCKS - limit_buffer):
+                assert_raises_rpc_error(-1, "Block not found on disk", full_node.getblock, pruned_node.getblockhash(height))
+            else:
+                full_node.getblock(pruned_node.getblockhash(height))  # just assert it does not throw an exception
+
+        # Lastly, ensure the full_node is not sync and verify it can get synced by
+        # establishing a connection with another full node capable of providing them.
+        assert_equal(full_node.getblockcount(), start_height_full_node)
+        self.connect_nodes(2, 1)
+        self.sync_blocks([miner, full_node])
+
     def run_test(self):
         node = self.nodes[0].add_p2p_connection(P2PIgnoreInv())
 
@@ -118,5 +179,7 @@ def run_test(self):
         # sync must be possible, node 1 is no longer in IBD and should therefore connect to node 0 (NODE_NETWORK_LIMITED)
         self.sync_blocks([self.nodes[0], self.nodes[1]])
 
+        self.test_avoid_requesting_historical_blocks()
+
 if __name__ == '__main__':
     NodeNetworkLimitedTest().main()