Merge bitcoin/bitcoin#25720: p2p: Reduce bandwidth during initial headers sync when a block is found

f6a916683d75ed5489666dbfbd711f000ad0707f Add functional test for block announcements during initial headers sync (Suhas Daftuar)
05f7f31598b8bb06acb12e1e2a3ccf324b035ea8 Reduce bandwidth during initial headers sync when a block is found (Suhas Daftuar)

Pull request description:

  On startup, if our headers chain is more than a day behind current time, we'll pick one peer to sync headers with until our best headers chain is caught up (at that point, we'll try to sync headers with all peers).

  However, if an INV for a block is received before our headers chain is caught up, we'll then start to sync headers from each peer announcing the block.  This can result in doing a big headers sync with many (if not all) of our peers simultaneously, which wastes bandwidth.

  This PR would reduce that overhead by picking (at most) one new peer to try syncing headers with whenever a new block is announced, prior to our headers chain being caught up.

ACKs for top commit:
  LarryRuane:
    ACK f6a916683d75ed5489666dbfbd711f000ad0707f
  ajtowns:
    ACK f6a916683d75ed5489666dbfbd711f000ad0707f
  mzumsande:
    ACK f6a916683d75ed5489666dbfbd711f000ad0707f
  dergoegge:
    Code review ACK f6a916683d75ed5489666dbfbd711f000ad0707f
  achow101:
    ACK f6a916683d75ed5489666dbfbd711f000ad0707f

Tree-SHA512: 0662000bd68db146f55981de4adc2e2b07cbfda222b1176569d61c22055e5556752ffd648426f69687ed1cc203105515e7304c12b915d6270df8e41a4a0e1eaa

2 files changed, 106 insertions, 0 deletions

diff --git a/test/functional/p2p_initial_headers_sync.py b/test/functional/p2p_initial_headers_sync.py
new file mode 100755
index 0000000000..e67c384da7
--- /dev/null
+++ b/test/functional/p2p_initial_headers_sync.py
@@ -0,0 +1,105 @@
+#!/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 initial headers download
+
+Test that we only try to initially sync headers from one peer (until our chain
+is close to caught up), and that each block announcement results in only one
+additional peer receiving a getheaders message.
+"""
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.messages import (
+    CInv,
+    MSG_BLOCK,
+    msg_headers,
+    msg_inv,
+)
+from test_framework.p2p import (
+    p2p_lock,
+    P2PInterface,
+)
+from test_framework.util import (
+    assert_equal,
+)
+import random
+
+class HeadersSyncTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 1
+
+    def announce_random_block(self, peers):
+        new_block_announcement = msg_inv(inv=[CInv(MSG_BLOCK, random.randrange(1<<256))])
+        for p in peers:
+            p.send_and_ping(new_block_announcement)
+
+    def run_test(self):
+        self.log.info("Adding a peer to node0")
+        peer1 = self.nodes[0].add_p2p_connection(P2PInterface())
+
+        # Wait for peer1 to receive a getheaders
+        peer1.wait_for_getheaders()
+        # An empty reply will clear the outstanding getheaders request,
+        # allowing additional getheaders requests to be sent to this peer in
+        # the future.
+        peer1.send_message(msg_headers())
+
+        self.log.info("Connecting two more peers to node0")
+        # Connect 2 more peers; they should not receive a getheaders yet
+        peer2 = self.nodes[0].add_p2p_connection(P2PInterface())
+        peer3 = self.nodes[0].add_p2p_connection(P2PInterface())
+
+        all_peers = [peer1, peer2, peer3]
+
+        self.log.info("Verify that peer2 and peer3 don't receive a getheaders after connecting")
+        for p in all_peers:
+            p.sync_with_ping()
+        with p2p_lock:
+            assert "getheaders" not in peer2.last_message
+            assert "getheaders" not in peer3.last_message
+
+        with p2p_lock:
+            peer1.last_message.pop("getheaders", None)
+
+        self.log.info("Have all peers announce a new block")
+        self.announce_random_block(all_peers)
+
+        self.log.info("Check that peer1 receives a getheaders in response")
+        peer1.wait_for_getheaders()
+        peer1.send_message(msg_headers()) # Send empty response, see above
+        with p2p_lock:
+            peer1.last_message.pop("getheaders", None)
+
+        self.log.info("Check that exactly 1 of {peer2, peer3} received a getheaders in response")
+        count = 0
+        peer_receiving_getheaders = None
+        for p in [peer2, peer3]:
+            with p2p_lock:
+                if "getheaders" in p.last_message:
+                    count += 1
+                    peer_receiving_getheaders = p
+                    p.last_message.pop("getheaders", None)
+                    p.send_message(msg_headers()) # Send empty response, see above
+
+        assert_equal(count, 1)
+
+        self.log.info("Announce another new block, from all peers")
+        self.announce_random_block(all_peers)
+
+        self.log.info("Check that peer1 receives a getheaders in response")
+        peer1.wait_for_getheaders()
+
+        self.log.info("Check that the remaining peer received a getheaders as well")
+        expected_peer = peer2
+        if peer2 == peer_receiving_getheaders:
+            expected_peer = peer3
+
+        expected_peer.wait_for_getheaders()
+
+        self.log.info("Success!")
+
+if __name__ == '__main__':
+    HeadersSyncTest().main()
+
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 3676b698f0..37d0c4f87e 100755
--- a/test/functional/test_runner.py
+++ b/test/functional/test_runner.py
@@ -247,6 +247,7 @@ BASE_SCRIPTS = [
     'rpc_generate.py',
     'wallet_balance.py --legacy-wallet',
     'wallet_balance.py --descriptors',
+    'p2p_initial_headers_sync.py',
     'feature_nulldummy.py',
     'mempool_accept.py',
     'mempool_expiry.py',