Merge bitcoin/bitcoin#24171: p2p: Sync chain more readily from inbound peers during IBD

48262a00f5 Add functional test for block sync from inbound peers (Suhas Daftuar)
0569b5c4bb Sync chain more readily from inbound peers during IBD (Suhas Daftuar)

Pull request description:

  When in IBD, if the honest chain is only known by inbound peers, then we must
  eventually sync from them in order to learn it. This change allows us to
  perform initial headers sync and fetch blocks from inbound peers, if we have no
  blocks in flight.

  The restriction on having no blocks in flight means that we will naturally
  throttle our block downloads to any such inbound peers that we may be
  downloading from, until we leave IBD. This is a tradeoff between preferring
  outbound peers for most of our block download, versus making sure we always
  eventually will get blocks we need that are only known by inbound peers even
  during IBD, as otherwise we may be stuck in IBD indefinitely (which could have
  cascading failure on the network, if a large fraction of the network managed to
  get stuck in IBD).

  Note that the test in the second commit fails on master, without the first commit.

ACKs for top commit:
  ajtowns:
    ACK 48262a00f5
  sipa:
    ACK 48262a00f5

Tree-SHA512: ffad3a05fa9a32a92226843c9128f52c275e8d51930fde7368badc340227f2ed680561c4c9f2937b4e3bd722474464849ec9b624f912f5e380ce98d71b55764d

test/functional/p2p_block_sync.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python3
+# Copyright (c) 2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test block download
+
+Ensure that even in IBD, we'll eventually sync chain from inbound peers
+(whether we have only inbound peers or both inbound and outbound peers).
+"""
+
+from test_framework.test_framework import BitcoinTestFramework
+
+class BlockSyncTest(BitcoinTestFramework):
+
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 3
+
+    def setup_network(self):
+        self.setup_nodes()
+        # Construct a network:
+        # node0 -> node1 -> node2
+        # So node1 has both an inbound and outbound peer.
+        # In our test, we will mine a block on node0, and ensure that it makes
+        # to to both node1 and node2.
+        self.connect_nodes(0, 1)
+        self.connect_nodes(1, 2)
+
+    def run_test(self):
+        self.log.info("Setup network: node0->node1->node2")
+        self.log.info("Mining one block on node0 and verify all nodes sync")
+        self.generate(self.nodes[0], 1)
+        self.log.info("Success!")
+
+
+if __name__ == '__main__':
+    BlockSyncTest().main()

test/functional/test_runner.py

@@ -157,6 +157,7 @@ BASE_SCRIPTS = [
     'wallet_avoidreuse.py --descriptors',
     'mempool_reorg.py',
     'mempool_persist.py',
+    'p2p_block_sync.py',
     'wallet_multiwallet.py --legacy-wallet',
     'wallet_multiwallet.py --descriptors',
     'wallet_multiwallet.py --usecli',