wallet: Clamp maxheight to positive number for large minconf

Fixes #2518

Signed-off-by: Christian Decker <decker.christian@gmail.com>
Changelog-fixed: `minconf` no longer gets wrapped around for large values, which was causing funds with insufficient confirmations to be selected.

common/wallet_tx.h

@@ -38,6 +38,12 @@ static inline u32 minconf_to_maxheight(u32 minconf, struct lightningd *ld)
 	 * selection */
 	if (minconf == 0)
 		return 0;
+
+	/* Avoid wrapping around and suddenly allowing any confirmed
+	 * outputs. Since we can't have a coinbase output, and 0 is taken for
+	 * the disable case, we can just clamp to 1. */
+	if (minconf >= ld->topology->tip->height)
+		return 1;
 	return ld->topology->tip->height - minconf + 1;
 }
 #endif /* LIGHTNING_COMMON_WALLET_TX_H */

tests/test_misc.py

@@ -482,7 +482,7 @@ def test_withdraw(node_factory, bitcoind):
     with pytest.raises(RpcError, match=r'Cannot afford transaction'):
         l1.rpc.withdraw(waddr, 'all')
 
-@pytest.mark.xfail(strict=True)
+
 def test_minconf_withdraw(node_factory, bitcoind):
     """Issue 2518: ensure that ridiculous confirmation levels don't overflow