netsync: add TestStartSyncBlockFallback for block-only sync path

Verify that startSync skips header download and directly requests
blocks when the header chain is already caught up to the peer's
height but the block chain lags behind.

netsync/manager_test.go

@@ -1099,3 +1099,45 @@ func syncStalledHeaderRecovery(t *testing.T, sm *SyncManager,
 	replacementState := sm.peerStates[replacementPeer]
 	require.Equal(t, wantRequested, replacementState.requestedBlocks)
 }
+
+// TestStartSyncBlockFallback verifies the startSync fallback path where
+// headers are already caught up but the block chain lags behind.  In this
+// case startSync should skip header download and directly request blocks.
+func TestStartSyncBlockFallback(t *testing.T) {
+	t.Parallel()
+
+	params := chaincfg.RegressionNetParams
+	params.Checkpoints = nil
+
+	sm, tearDown := makeMockSyncManager(t, &params)
+	defer tearDown()
+
+	// Process headers so the header chain is at numBlocks while the
+	// block chain stays at genesis.
+	const numBlocks = 11
+	blocks := generateTestBlocks(t, &params, numBlocks)
+	for _, block := range blocks {
+		_, err := sm.chain.ProcessBlockHeader(
+			&block.MsgBlock().Header, blockchain.BFNone, false)
+		require.NoError(t, err)
+	}
+
+	// Add a peer whose height equals the header height.
+	// fetchHigherPeers(bestHeaderHeight) returns nothing because
+	// the peer is not strictly higher than our headers.
+	// fetchHigherPeers(bestBlockHeight=0) returns the peer.
+	syncPeer := peer.NewInboundPeer(&peer.Config{})
+	syncPeer.UpdateLastBlockHeight(int32(numBlocks))
+	sm.peerStates[syncPeer] = &peerSyncState{
+		syncCandidate:   true,
+		requestedTxns:   make(map[chainhash.Hash]struct{}),
+		requestedBlocks: make(map[chainhash.Hash]struct{}),
+	}
+
+	sm.startSync()
+
+	require.NotNil(t, sm.syncPeer,
+		"sync peer should be set for block download")
+	require.NotEmpty(t, sm.requestedBlocks,
+		"blocks should be requested via fetchHeaderBlocks")
+}