lightningd: honor `payment-fronting-node` when making bolt12 offers.

diff --git a/plugins/offers.c b/plugins/offers.c
index 26686920..79b13065 100644
--- a/plugins/offers.c
+++ b/plugins/offers.c
@@ -78,6 +78,10 @@ bool we_want_blinded_path(struct plugin *plugin, bool for_payment)
 	u8 rgb_color[3], alias[32];
 	struct tlv_node_ann_tlvs *na_tlvs;
 
+	/* If we're fronting, we always want a blinded path */
+	if (od->fronting_nodes)
+		return true;
+
 	node_id_from_pubkey(&local_nodeid, &od->id);
 
 	node = gossmap_find_node(gossmap, &local_nodeid);
@@ -1614,6 +1618,25 @@ static struct command_result *json_decode(struct command *cmd,
 	return command_finished(cmd, response);
 }
 
+static struct pubkey *json_to_pubkeys(const tal_t *ctx,
+				      const char *buffer,
+				      const jsmntok_t *tok)
+{
+	size_t i;
+	const jsmntok_t *t;
+	struct pubkey *arr;
+
+	if (tok->type != JSMN_ARRAY)
+		return NULL;
+
+	arr = tal_arr(ctx, struct pubkey, tok->size);
+	json_for_each_arr(i, t, tok) {
+		if (!json_to_pubkey(buffer, t, &arr[i]))
+			return tal_free(arr);
+	}
+	return arr;
+}
+
 static const char *init(struct command *init_cmd,
 			const char *buf UNUSED,
 			const jsmntok_t *config UNUSED)
@@ -1631,8 +1654,13 @@ static const char *init(struct command *init_cmd,
 	rpc_scan(init_cmd, "listconfigs",
 		 take(json_out_obj(NULL, NULL, NULL)),
 		 "{configs:"
-		 "{cltv-final:{value_int:%}}}",
-		 JSON_SCAN(json_to_u16, &od->cltv_final));
+		 "{cltv-final:{value_int:%},"
+		 "payment-fronting-node?:{values_str:%}}}",
+		 JSON_SCAN(json_to_u16, &od->cltv_final),
+		 JSON_SCAN_TAL(od, json_to_pubkeys, &od->fronting_nodes));
+	/* Keep it simple if no fronting nodes */
+	if (tal_count(od->fronting_nodes) == 0)
+		od->fronting_nodes = tal_free(od->fronting_nodes);
 
 	rpc_scan(init_cmd, "makesecret",
 		 take(json_out_obj(NULL, "string", BOLT12_ID_BASE_STRING)),
diff --git a/plugins/offers.h b/plugins/offers.h
index fef29c60..13968a0e 100644
--- a/plugins/offers.h
+++ b/plugins/offers.h
@@ -23,6 +23,8 @@ struct offers_data {
 	struct secret offerblinding_base;
 	/* Base for node aliases for invoice requests */
 	struct secret nodealias_base;
+	/* Any --payment-fronting-node specified */
+	struct pubkey *fronting_nodes;
 	/* --dev-invoice-bpath-scid */
 	bool dev_invoice_bpath_scid;
 	/* --dev-invoice-internal-scid */
diff --git a/plugins/offers_invreq_hook.c b/plugins/offers_invreq_hook.c
index 3e6b0338..38de98a6 100644
--- a/plugins/offers_invreq_hook.c
+++ b/plugins/offers_invreq_hook.c
@@ -250,6 +250,10 @@ static struct command_result *create_invoicereq(struct command *cmd,
 	return send_outreq(req);
 }
 
+/* FIXME: Allow multihop! */
+/* FIXME: And add padding! */
+
+
 /* FIXME: This is naive:
  * - Only creates if we have no public channels.
  * - Always creates a path from direct neighbor.
diff --git a/plugins/offers_offer.c b/plugins/offers_offer.c
index c6c2acbd..6729c1f8 100644
--- a/plugins/offers_offer.c
+++ b/plugins/offers_offer.c
@@ -251,6 +251,50 @@ static struct command_result *create_offer(struct command *cmd,
 	return send_outreq(req);
 }
 
+/* Create num_node_ids paths from these node_ids to us (one hop each) */
+static struct blinded_path **offer_onehop_paths(const tal_t *ctx,
+						const struct offers_data *od,
+						const struct tlv_offer *offer,
+						const struct pubkey *neighbors,
+						size_t num_neighbors)
+{
+	struct pubkey *ids = tal_arr(tmpctx, struct pubkey, 2);
+	struct secret blinding_path_secret;
+	struct sha256 offer_id;
+	struct blinded_path **offer_paths;
+
+	/* Note: "id" of offer minus paths */
+	assert(!offer->offer_paths);
+	offer_offer_id(offer, &offer_id);
+
+	offer_paths = tal_arr(ctx, struct blinded_path *, num_neighbors);
+	for (size_t i = 0; i < num_neighbors; i++) {
+		ids[0] = neighbors[i];
+		ids[1] = od->id;
+
+		/* So we recognize this */
+		/* We can check this when they try to take up offer. */
+		bolt12_path_secret(&od->offerblinding_base, &offer_id,
+				   &blinding_path_secret);
+
+		offer_paths[i]
+			= incoming_message_blinded_path(offer_paths,
+							ids,
+							NULL,
+							&blinding_path_secret);
+	}
+	return offer_paths;
+}
+
+/* Common case of making a single path */
+static struct blinded_path **offer_onehop_path(const tal_t *ctx,
+					       const struct offers_data *od,
+					       const struct tlv_offer *offer,
+					       const struct pubkey *neighbor)
+{
+	return offer_onehop_paths(ctx, od, offer, neighbor, 1);
+}
+
 static struct command_result *found_best_peer(struct command *cmd,
 					      const struct chaninfo *best,
 					      struct offer_info *offinfo)
@@ -267,29 +311,9 @@ static struct command_result *found_best_peer(struct command *cmd,
 		plugin_log(cmd->plugin, LOG_UNUSUAL,
 			   "No incoming channel to public peer, so no blinded path");
 	} else {
-		struct pubkey *ids;
-		struct secret blinding_path_secret;
-		struct sha256 offer_id;
-
-		/* Note: "id" of offer minus paths */
-		offer_offer_id(offinfo->offer, &offer_id);
-
-		/* Make a small 1-hop path to us */
-		ids = tal_arr(tmpctx, struct pubkey, 2);
-		ids[0] = best->id;
-		ids[1] = od->id;
-
-		/* So we recognize this */
-		/* We can check this when they try to take up offer. */
-		bolt12_path_secret(&od->offerblinding_base, &offer_id,
-				   &blinding_path_secret);
-
-		offinfo->offer->offer_paths = tal_arr(offinfo->offer, struct blinded_path *, 1);
-		offinfo->offer->offer_paths[0]
-			= incoming_message_blinded_path(offinfo->offer->offer_paths,
-							ids,
-							NULL,
-							&blinding_path_secret);
+		offinfo->offer->offer_paths
+			= offer_onehop_path(offinfo->offer, od,
+					    offinfo->offer, &best->id);
 	}
 
 	return create_offer(cmd, offinfo);
@@ -298,16 +322,31 @@ static struct command_result *found_best_peer(struct command *cmd,
 static struct command_result *maybe_add_path(struct command *cmd,
 					     struct offer_info *offinfo)
 {
-	/* BOLT #12:
-	 *   - if it is connected only by private channels:
-	 *     - MUST include `offer_paths` containing one or more paths to the node from
-	 *       publicly reachable nodes.
-	 */
+	const struct offers_data *od = get_offers_data(cmd->plugin);
+
+	/* Populate paths assuming not already set by dev_paths */
 	if (!offinfo->offer->offer_paths) {
-		if (we_want_blinded_path(cmd->plugin, false))
-			return find_best_peer(cmd, 1ULL << OPT_ONION_MESSAGES,
-					      found_best_peer, offinfo);
+		/* BOLT #12:
+		 *   - if it is connected only by private channels:
+		 *     - MUST include `offer_paths` containing one or more paths to the node from
+		 *       publicly reachable nodes.
+		 */
+		if (we_want_blinded_path(cmd->plugin, false)) {
+			/* We use *all* fronting nodes (not just "best" one)
+			 * for offers */
+			if (od->fronting_nodes) {
+				offinfo->offer->offer_paths
+					= offer_onehop_paths(offinfo->offer, od,
+							     offinfo->offer,
+							     od->fronting_nodes,
+							     tal_count(od->fronting_nodes));
+			} else {
+				return find_best_peer(cmd, 1ULL << OPT_ONION_MESSAGES,
+						      found_best_peer, offinfo);
+			}
+		}
 	}
+
 	return create_offer(cmd, offinfo);
 }
 
diff --git a/tests/test_invoices.py b/tests/test_invoices.py
index f174af55..1bb47d46 100644
--- a/tests/test_invoices.py
+++ b/tests/test_invoices.py
@@ -943,9 +943,12 @@ def test_invoice_botched_migration(node_factory, chainparams):
 
 
 def test_payment_fronting(node_factory):
-    l1, l2 = node_factory.get_nodes(2)
-    l3, l4 = node_factory.get_nodes(2, opts=[{'payment-fronting-node': l1.info['id']},
-                                             {'payment-fronting-node': [l1.info['id'], l2.info['id']]}])
+    # Nodes will not front for offers if they don't have an advertized address, so allow localhost.
+    l1, l2 = node_factory.get_nodes(2, opts={'dev-allow-localhost': None})
+    l3, l4 = node_factory.get_nodes(2, opts=[{'payment-fronting-node': l1.info['id'],
+                                              'dev-allow-localhost': None},
+                                             {'payment-fronting-node': [l1.info['id'], l2.info['id']],
+                                              'dev-allow-localhost': None}])
 
     assert l3.rpc.listconfigs('payment-fronting-node') == {'configs': {'payment-fronting-node': {'sources': ['cmdline'], 'values_str': [l1.info['id']]}}}
     assert l4.rpc.listconfigs('payment-fronting-node') == {'configs': {'payment-fronting-node': {'sources': ['cmdline', 'cmdline'], 'values_str': [l1.info['id'], l2.info['id']]}}}
@@ -966,6 +969,16 @@ def test_payment_fronting(node_factory):
     l1.rpc.xpay(l3inv)
     l1.rpc.xpay(l4inv)
 
+    # Now test offers.
+    l3offer = l3.rpc.offer(1000, 'l3offer', 'l3offer')['bolt12']
+    assert only_one(l3.rpc.decode(l3offer)['offer_paths'])['first_node_id'] == l1.info['id']
+
+    l4offer = l4.rpc.offer(1000, 'l4offer', 'l4offer')['bolt12']
+    assert [r['first_node_id'] for r in l4.rpc.decode(l4offer)['offer_paths']] == [l1.info['id'], l2.info['id']]
+
+    l1.rpc.fetchinvoice(l3offer)['invoice']
+    l1.rpc.fetchinvoice(l4offer)['invoice']
+
 
 def test_invoice_maxdesc(node_factory, chainparams):
     l1, l2 = node_factory.line_graph(2)