xpay: don't crash on circular routehints.

We earlier fixed the cases of gossipd inserting a same-node channel,
but didn't prevent it for local modifications:

```
cln-askrene: common/gossmap.c:52: nodeidx_htable_add: Assertion `!nodeidx_htable_getmatch_(ht, k, h, v, &i)' failed.
cln-askrene: FATAL SIGNAL 6 (version v26.06rc2-5-gd389c3f-modded)
0x5c50e80dd5cb send_backtrace
        common/daemon.c:38
0x5c50e80dd685 crashdump
        common/daemon.c:83
0x70a5b1e4532f ???
        ./signal/../sysdeps/unix/sysv/linux/x86_64/libc_sigaction.c:0
0x70a5b1e9eb2c __pthread_kill_implementation
        ./nptl/pthread_kill.c:44
0x70a5b1e9eb2c __pthread_kill_internal
        ./nptl/pthread_kill.c:78
0x70a5b1e9eb2c __GI___pthread_kill
        ./nptl/pthread_kill.c:89
0x70a5b1e4527d __GI_raise
        ../sysdeps/posix/raise.c:26
0x70a5b1e288fe __GI_abort
        ./stdlib/abort.c:79
0x70a5b1e2881a __assert_fail_base
        ./assert/assert.c:96
0x70a5b1e3b516 __assert_fail
        ./assert/assert.c:105
0x5c50e80dfb44 nodeidx_htable_add
        common/gossmap.c:52
0x5c50e80e1066 add_channel
        common/gossmap.c:515
0x5c50e80e327c gossmap_apply_localmods
        common/gossmap.c:1239
0x5c50e80bed55 do_getroutes
        plugins/askrene/askrene.c:620
0x5c50e80bf919 listpeerchannels_done
```

Reported-by: Won Hoi Kim and Ahmad Elmoursi
Changelog-Fixed: Plugins: xpay no longer crashes on circular bolt11 routehints.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

common/gossmap.c

@@ -1056,6 +1056,8 @@ bool gossmap_local_addchan(struct gossmap_localmods *localmods,
 	u64 off;
 	struct localmod mod;
 
+	assert(!node_id_eq(n1, n2));
+
 	/* Don't create duplicate channels. */
 	if (find_localmod(localmods, scid))
 		return false;

common/gossmap.h

@@ -89,6 +89,7 @@ struct gossmap_localmods *gossmap_localmods_new(const tal_t *ctx);
 /* Create a local-only channel; if this conflicts with a real channel when added,
  * that will be used instead.
  * Returns false (and does nothing) if scid was already in localmods.
+ * n1 must be different from n2.
  */
 bool gossmap_local_addchan(struct gossmap_localmods *localmods,
 			   const struct node_id *n1,

contrib/msggen/msggen/schema.json

@@ -652,7 +652,7 @@
           "destination": {
             "type": "pubkey",
             "description": [
-              "The destination node id for the channel."
+              "The destination node id for the channel (must be different from source)."
             ]
           },
           "short_channel_id": {

doc/schemas/askrene-create-channel.json

@@ -32,7 +32,7 @@
       "destination": {
         "type": "pubkey",
         "description": [
-          "The destination node id for the channel."
+          "The destination node id for the channel (must be different from source)."
         ]
       },
       "short_channel_id": {

plugins/askrene/askrene.c

@@ -1066,6 +1066,10 @@ static struct command_result *json_askrene_create_channel(struct command *cmd,
 	plugin_log(cmd->plugin, LOG_TRACE, "%s called: %.*s", __func__,
 		   json_tok_full_len(params), json_tok_full(buffer, params));
 
+	if (node_id_cmp(src, dst) == 0)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "source and destination must be different");
+
 	if (layer_find_local_channel(layer, *scid)) {
 		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 				    "channel already exists");

plugins/askrene/layer.c

@@ -234,6 +234,7 @@ static struct local_channel *add_local_channel(struct layer *layer,
 		lc->n1 = *n1;
 		lc->n2 = *n2;
 	} else {
+		assert(!node_id_eq(n1, n2));
 		lc->n1 = *n2;
 		lc->n2 = *n1;
 	}

plugins/askrene/layer.h

@@ -52,7 +52,7 @@ bool layer_check_local_channel(const struct local_channel *lc,
 			       const struct node_id *n2,
 			       struct amount_msat capacity);
 
-/* Add a local channel to a layer! */
+/* Add a local channel to a layer: src must not be equal to dst!*/
 void layer_add_local_channel(struct layer *layer,
 			     const struct node_id *src,
 			     const struct node_id *dst,

plugins/xpay/xpay.c

@@ -1922,6 +1922,16 @@ static void add_fake_channel(struct command *aux_cmd,
 	struct out_req *req;
 	struct short_channel_id_dir scidd;
 
+	/* We're not allowed to send these to askrene-create-channel,
+	 * so catch them now */
+	if (node_id_eq(src, dst)) {
+		payment_log(payment, LOG_UNUSUAL,
+			    "Invoice gave bad self-node route %s->%s",
+			    fmt_node_id(tmpctx, src),
+			    fmt_node_id(tmpctx, dst));
+		return;
+	}
+
 	scidd.scid = scid;
 	scidd.dir = node_id_idx(src, dst);
 	payment_log(payment, LOG_DBG,

tests/test_xpay.py

@@ -975,7 +975,6 @@ def test_xpay_offer(node_factory):
     l1.rpc.xpay(offer2, 5000)
 
 
-@pytest.mark.xfail(strict=True)
 def test_xpay_circular_routehint(node_factory):
     """Test that xpay gracefully skips a circular bolt11 routehint (src == dst)."""
     l1, l2 = node_factory.line_graph(2)