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

c5b5843d8f test: avoid requesting blocks beyond limited peer threshold (furszy)
2f6a05512f p2p: sync from limited peer, only request blocks below threshold (furszy)
73127722a2 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 c5b5843d8f
  vasild:
    ACK c5b5843d8f
  mzumsande:
    Code Review ACK c5b5843d8f
  pinheadmz:
    ACK c5b5843d8f

Tree-SHA512: 9e550698bc6e63cc587b2b988a87d0ab555a8fa188c91c3f33287f8201d77c28b373331845356ad86f17bb21c15950b6466bc1cafd0ce8139d70364cb71c2ad2

test/functional/p2p_node_network_limited.py

@@ -21,8 +21,12 @@ from test_framework.p2p import P2PInterface
 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 @@ class NodeNetworkLimitedTest(BitcoinTestFramework):
         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 @@ class NodeNetworkLimitedTest(BitcoinTestFramework):
         # 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()