xpay: in future, don't MPP to pay bolt12 invoices unless invoice explicitly says so.

We don't want to enable this yet, since we only just fixed CLN this release!

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

doc/developers-guide/deprecations.md

@@ -37,6 +37,7 @@ hidden: false
 | close.tx                             | Field              | v24.11           | v25.11         | Use txs array instead                                                                                                                  |
 | close.txid                           | Field              | v24.11           | v25.11         | Use txids array instead                                                                                                                |
 | experimental-offers                  | Config             | v24.11           | v25.05         | Now the default                                                                                                                        |
+| xpay.ignore_bolt12_mpp               | Field              | v25.05           | v25.11         | Try MPP even if the BOLT12 invoice doesn't explicitly allow it (CLN didn't until 25.02)                                                |
 
 Inevitably there are features which need to change: either to be generalized, or removed when they can no longer be supported.
 

plugins/xpay/xpay.c

@@ -1555,7 +1555,10 @@ static struct command_result *json_xpay_core(struct command *cmd,
 			if (payment->payinfos[i]->cltv_expiry_delta > payment->final_cltv)
 				payment->final_cltv = payment->payinfos[i]->cltv_expiry_delta;
 		}
-		payment->disable_mpp = false;
+		/* We will start honoring this flag in future */
+		payment->disable_mpp = !feature_offered(b12inv->invoice_features, OPT_BASIC_MPP);
+		if (payment->disable_mpp && command_deprecated_in_ok(cmd, "ignore_bolt12_mpp", "v25.05", "v25.11"))
+			payment->disable_mpp = false;
 	} else {
 		struct bolt11 *b11
 			= bolt11_decode(tmpctx, payment->invstring,

tests/test_xpay.py

@@ -649,3 +649,27 @@ def test_xpay_no_mpp(node_factory, chainparams):
     assert ret['successful_parts'] == 1
     assert ret['amount_msat'] == AMOUNT
     assert ret['amount_sent_msat'] == AMOUNT + AMOUNT // 100000 + 1
+
+
+def test_xpay_bolt12_no_mpp(node_factory, chainparams):
+    """We should not (yet!) avoid mpp if BOLT12 invoice doesn't say we should"""
+    l1, l2, l3, l4 = node_factory.get_nodes(4)
+    node_factory.join_nodes([l1, l2, l3], wait_for_announce=True)
+    node_factory.join_nodes([l1, l4, l3], wait_for_announce=True)
+
+    # Amount needs to be enought that it bothers splitting, but not
+    # so much that it can't pay without mpp.
+    AMOUNT = 500000000
+
+    # We create a version of this which doesn't advertize support for MPP
+    # (I grabbed this from a real instance)
+    inv = l3.rpc.createinvoice(invstring="lni1qqg00rlfqualt5wvj9meezlx202qwq3qqc3xu3s3rg94nj40zfsy866mhu5vxne6tcej5878k2mneuvgjy8sspqae4jsqzshw3jhxazl0pcxz72lvfhkcap3xf0kum6ld4c8q93pqdwjkyvjm7apxnssu4qgwhfkd67ghs6n6k48v6uqczgt88p6tky965pqqc3xu3s3rg94nj40zfsy866mhu5vxne6tcej5878k2mneuvgjy84sggrv4a2zv5mfvds5pz9ehe40uws375pc4u6ausees3xuezw7d4kssx6gpr854lme2pqavazfzxk4pgv4pvnamxv7e26787e3zm53kpek3qzmpjx50jc6t625pqae4jspvppqdwjkyvjm7apxnssu4qgwhfkd67ghs6n6k48v6uqczgt88p6tky96",
+                               preimage="637f5b64ae540986826288ed24db980e756b2eb680b6db17f1072643c12ae531",
+                               label="b53c2f55f53a0f979df864d36fff53bbf10454130df9bbf478f1727386f6ef93-03657aa1329b4b1b0a0445cdf357f1d08fa81c579aef219cc226e644ef36b6840d-0")['bolt12']
+
+    # This will MPP ANYWAY, even though MPP is not specified!
+    ret = l1.rpc.xpay(inv)
+    assert ret['failed_parts'] == 0
+    assert ret['successful_parts'] == 2
+    assert ret['amount_msat'] == AMOUNT
+    assert ret['amount_sent_msat'] == AMOUNT + AMOUNT // 100000 + 1