pytest: Add test for the block backfilling trigger by gossip

This is just the test that we use to verify block backfilling below the wallet
birth height is working correctly.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

tests/test_db.py

@@ -1,4 +1,8 @@
 from fixtures import *  # noqa: F401,F403
+from utils import wait_for
+
+
+import pytest
 
 
 def test_db_dangling_peer_fix(node_factory):
@@ -15,3 +19,57 @@ def test_db_dangling_peer_fix(node_factory):
     # Make sure l2 has register connection
     l2.daemon.wait_for_log('Handed peer, entering loop')
     l2.fund_channel(l1, 200000, wait_for_active=True)
+
+
+@pytest.mark.xfail(strict=True)
+def test_block_backfill(node_factory, bitcoind):
+    """Test whether we backfill data from the blockchain correctly.
+
+    For normal operation we will process any block after the initial start
+    height, or rescan height, but for gossip we actually also need to backfill
+    the blocks we skipped initially. We do so on-demand, whenever we see a
+    channel_announcement referencing a blockheight we haven't processed yet,
+    we fetch the entire block, extract P2WSH outputs and ask `bitcoin
+    gettxout` for each of them. We then store the block header in the `blocks`
+    table and the unspent outputs in the `utxoset` table.
+
+    The test consist of two nodes opening a channel at height X, and an
+    unrelated P2WSH transaction being sent at the same height (will be used to
+    check for completeness of the backfill). Then a second node starts at
+    height X+100 and connect to one of the nodes. It should not have the block
+    in its DB before connecting. After connecting it should sync the gossip,
+    triggering a backfill of block X, and all associated P2WSH outputs.
+
+    """
+    # Need to manually open the channels later since otherwise we can't have a
+    # tx in the same block (`line_graph` with `fundchannel=True` generates
+    # blocks).
+    l1, l2 = node_factory.line_graph(2, fundchannel=False)
+
+    # Get some funds to l1
+    addr = l1.rpc.newaddr()['bech32']
+    bitcoind.rpc.sendtoaddress(addr, 1)
+    wait_for(lambda: len(bitcoind.rpc.getrawmempool()) == 1)
+    bitcoind.generate_block(1)
+    l1.daemon.wait_for_log(r'Owning')
+
+    # Now send the needle we will go looking for later:
+    bitcoind.rpc.sendtoaddress('bcrt1qtwxd8wg5eanumk86vfeujvp48hfkgannf77evggzct048wggsrxsum2pmm', 1)
+    l1.rpc.fundchannel(l2.info['id'], 10**6, announce=True)
+    wait_for(lambda: len(bitcoind.rpc.getrawmempool()) == 2)
+
+    # Confirm and get some distance between the funding and the l3 wallet birth date
+    bitcoind.generate_block(100)
+    wait_for(lambda: len(l1.rpc.listnodes()['nodes']) == 2)
+
+    # Start the tester node, and connect it to l1. l0 should sync the gossip
+    # and call out to `bitcoind` to backfill the block.
+    l3 = node_factory.get_node()
+    heights = [r['height'] for r in l3.db_query("SELECT height FROM blocks")]
+    assert(103 not in heights)
+
+    l3.rpc.connect(l1.info['id'], 'localhost', l1.port)
+
+    wait_for(lambda: len(l3.rpc.listnodes()['nodes']) == 2)
+    heights = [r['height'] for r in l3.db_query("SELECT height FROM blocks")]
+    assert(103 in heights)