p2p: Allow block downloads from peers without snapshot block after assumeutxo validation

src/net_processing.cpp

@@ -1388,7 +1388,7 @@ void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int co
     // We can't reorg to this chain due to missing undo data until the background sync has finished,
     // so downloading blocks from it would be futile.
     const CBlockIndex* snap_base{m_chainman.GetSnapshotBaseBlock()};
-    if (snap_base && state->pindexBestKnownBlock->GetAncestor(snap_base->nHeight) != snap_base) {
+    if (snap_base && !m_chainman.IsSnapshotValidated() && state->pindexBestKnownBlock->GetAncestor(snap_base->nHeight) != snap_base) {
         LogDebug(BCLog::NET, "Not downloading blocks from peer=%d, which doesn't have the snapshot block in its best chain.\n", peer.m_id);
         return;
     }

test/functional/feature_assumeutxo.py

@@ -347,6 +347,29 @@ def test_sync_from_assumeutxo_node(self, snapshot):
         assert 'NETWORK' in ibd_node.getpeerinfo()[0]['servicesnames']
         self.sync_blocks(nodes=(ibd_node, snapshot_node))
 
+    def test_sync_to_most_work_chain_after_background_validation(self):
+        """
+        After background validation completes, node should be able
+        to download and process blocks from peers without the snapshot block in their chain.
+        """
+        self.log.info("Testing sync to the most-work chain without the snapshot block after background validation")
+
+        miner = self.nodes[0]
+        snapshot_node = self.nodes[2]  # Has already completed background validation
+
+        self.log.info("Miner switches to an alternative chain that forks one block before the snapshot block")
+        fork_point = SNAPSHOT_BASE_HEIGHT - 1
+        miner_old_height = miner.getblockcount()
+        miner_old_chainwork = int(miner.getblockchaininfo()['chainwork'], 16)
+        miner.invalidateblock(miner.getblockhash(fork_point + 1))
+
+        self.log.info("Mine one more block than original chain to make the new chain have most work")
+        self.generate(miner, nblocks=(miner_old_height - fork_point) + 1, sync_fun=self.no_op)
+        assert int(miner.getblockchaininfo()['chainwork'], 16) > miner_old_chainwork
+
+        self.log.info("Snapshot node should reorg to the most-work chain without the snapshot block")
+        self.sync_blocks(nodes=(snapshot_node, miner))
+
     def assert_only_network_limited_service(self, node):
         node_services = node.getnetworkinfo()['localservicesnames']
         assert 'NETWORK' not in node_services
@@ -775,6 +798,8 @@ def check_tx_counts(final: bool) -> None:
         # The following test cleans node2 and node3 chain directories.
         self.test_sync_from_assumeutxo_node(snapshot=dump_output)
 
+        self.test_sync_to_most_work_chain_after_background_validation()
+
 @dataclass
 class Block:
     hash: str