pay: Skip the presplitter mod if it'd exhaust our HTLC budget

Changelog-Fixed: pay: The presplitter mod will no longer exhaust the HTLC budget.

plugins/libplugin-pay.c

@@ -3476,14 +3476,20 @@ static void presplit_cb(struct presplit_mod_data *d, struct payment *p)
 		if (htlcs >= PRESPLIT_MAX_HTLC_SHARE)
 			htlcs /= PRESPLIT_MAX_HTLC_SHARE;
 
+		int targethtlcs =
+		    p->amount.millisatoshis / target_amount; /* Raw: division */
 		if (htlcs == 0) {
 			p->abort = true;
 			return payment_fail(
 			    p, "Cannot attempt payment, we have no channel to "
 			       "which we can add an HTLC");
-		} else if (p->amount.millisatoshis / target_amount > htlcs) /* Raw: division */
-			target = amount_msat_div(p->amount, htlcs);
-		else
+		} else if (targethtlcs > htlcs) {
+			paymod_log(p, LOG_INFORM,
+				   "Number of pre-split HTLCs (%d) exceeds our "
+				   "HTLC budget (%d), skipping pre-splitter",
+				   targethtlcs, htlcs);
+			return payment_continue(p);
+		} else
 			target = amount_msat(target_amount);
 
 		/* If we are already below the target size don't split it

tests/test_pay.py

@@ -4294,11 +4294,13 @@ def test_routehint_tous(node_factory, bitcoind):
         l2.rpc.pay(inv)
 
 
-@pytest.mark.xfail(strict=True)
 def test_pay_low_max_htlcs(node_factory):
     """Test we can pay if *any* HTLC slots are available"""
 
     l1, l2, l3 = node_factory.line_graph(3,
                                          opts={'max-concurrent-htlcs': 1},
                                          wait_for_announce=True)
     l1.rpc.pay(l3.rpc.invoice(FUNDAMOUNT * 50, "test", "test")['bolt11'])
+    l1.daemon.wait_for_log(
+        r'Number of pre-split HTLCs \([0-9]+\) exceeds our HTLC budget \([0-9]+\), skipping pre-splitter'
+    )