sendpay: insist that partid be an exact duplicate if in progress.

The test had part 1 and 2 backward, but still worked.  When I copied that to
*after* the test had succeeded, it complained.  It should always complain,
to catch bugs.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/pay.c

@@ -837,8 +837,29 @@ send_payment_core(struct lightningd *ld,
 						    "Already have %s payment in progress",
 						    payments[i]->partid ? "parallel" : "non-parallel");
 			}
-			if (payments[i]->partid == partid)
+			if (payments[i]->partid == partid) {
+				/* You can't change details while it's pending */
+				if (!amount_msat_eq(payments[i]->msatoshi, msat)) {
+					return command_fail(cmd, PAY_RHASH_ALREADY_USED,
+						    "Already pending "
+						    "with amount %s (not %s)",
+						    type_to_string(tmpctx,
+								   struct amount_msat,
+								   &payments[i]->msatoshi),
+						    type_to_string(tmpctx,
+								   struct amount_msat, &msat));
+				}
+				if (payments[i]->destination && destination
+				    && !node_id_eq(payments[i]->destination,
+						   destination)) {
+					return command_fail(cmd, PAY_RHASH_ALREADY_USED,
+							    "Already pending to %s",
+							    type_to_string(tmpctx,
+									   struct node_id,
+									   payments[i]->destination));
+				}
 				return json_sendpay_in_progress(cmd, payments[i]);
+			}
 			/* You shouldn't change your mind about amount being
 			 * sent, since we'll use it in onion! */
 			else if (!amount_msat_eq(payments[i]->total_msat,

tests/test_pay.py

@@ -2609,9 +2609,12 @@ def test_partial_payment(node_factory, bitcoind, executor):
         l1.rpc.sendpay(route=r124, payment_hash=inv['payment_hash'],
                        msatoshi=1000, bolt11=inv['bolt11'], payment_secret=paysecret, partid=3)
 
-    # But repeat is a NOOP.
+    # But repeat is a NOOP, as long as they're exactly the same!
+    with pytest.raises(RpcError, match=r'Already pending with amount 501msat \(not 499msat\)'):
         l1.rpc.sendpay(route=r124, payment_hash=inv['payment_hash'], msatoshi=1000, bolt11=inv['bolt11'], payment_secret=paysecret, partid=1)
-    l1.rpc.sendpay(route=r134, payment_hash=inv['payment_hash'], msatoshi=1000, bolt11=inv['bolt11'], payment_secret=paysecret, partid=2)
+
+    l1.rpc.sendpay(route=r134, payment_hash=inv['payment_hash'], msatoshi=1000, bolt11=inv['bolt11'], payment_secret=paysecret, partid=1)
+    l1.rpc.sendpay(route=r124, payment_hash=inv['payment_hash'], msatoshi=1000, bolt11=inv['bolt11'], payment_secret=paysecret, partid=2)
 
     # Make sure they've done the suppress-commitment thing before we unsuppress
     l2.daemon.wait_for_log(r'dev_disconnect')