pay: fix infinite loop when CLTV budget exceeded

When p->route is NULL (no route found), the adaptive splitter
was still trying to split payments, causing thousands of attempts
all failing for the same reason.

Fixes: #8167
Changelog-Fixed: pay: no longer loops infinitely when CLTV budget exceeded

Author: wqxoxo

plugins/libplugin-pay.c

@@ -3692,6 +3692,9 @@ static struct command_result *adaptive_splitter_cb(struct adaptive_split_mod_dat
 			    fields, root->payment_secret,
 			    root->final_amount.millisatoshis); /* Raw: onion payload */
 	} else if (p->step == PAYMENT_STEP_FAILED && !p->abort) {
+		/* No route means splitting won't help. */
+		if (p->route == NULL)
+			return payment_continue(p);
 		if (amount_msat_greater(p->our_amount, MPP_ADAPTIVE_LOWER_LIMIT)) {
 			struct payment *a, *b;
 			/* Random number in the range [90%, 110%] */

tests/test_pay.py

@@ -150,6 +150,34 @@ def test_pay_limits(node_factory):
     assert status[0]['strategy'] == "Initial attempt"
 
 
+def test_pay_cltv_budget_exceeded_no_infinite_loop(node_factory):
+    """Test that CLTV budget exceeded doesn't cause infinite splitting loop.
+
+    Reproduces issue #8167: When CLTV delay exceeds budget, the adaptive
+    splitter would keep splitting the payment into thousands of sub-payments,
+    even though splitting cannot help (CLTV requirements don't depend on
+    payment amount).
+    """
+    l1, l2, l3 = node_factory.line_graph(3, wait_for_announce=True)
+
+    # Use amount > 100k msat to enable MPP adaptive splitting
+    amt = 1000000
+
+    inv = l3.rpc.invoice(amt, "test_cltv_no_infinite_loop", 'description')['bolt11']
+
+    # Set maxdelay very low to trigger CLTV budget exceeded
+    PAY_STOPPED_RETRYING = 210
+    failmsg = r'CLTV delay exceeds our CLTV budget'
+
+    with pytest.raises(RpcError, match=failmsg) as err:
+        l1.rpc.call('pay', {'bolt11': inv, 'maxdelay': 5})
+
+    assert err.value.error['code'] == PAY_STOPPED_RETRYING
+
+    status = l1.rpc.call('paystatus', {'bolt11': inv})['pay'][0]['attempts']
+    assert len(status) < 10, f"Too many attempts ({len(status)}), possible infinite loop bug"
+
+
 def test_pay_exclude_node(node_factory, bitcoind):
     """Test excluding the node if there's the NODE-level error in the failure_code
     """