lightningd: honor `payment-fronting-node` when making bolt11 invoices.

diff --git a/lightningd/invoice.c b/lightningd/invoice.c
index 6ddecd4d..1dfb31c3 100644
--- a/lightningd/invoice.c
+++ b/lightningd/invoice.c
@@ -660,6 +660,25 @@ static struct route_info **select_inchan_mpp(const tal_t *ctx,
 	return routehints;
 }
 
+static struct route_info **select_inchan_all(const tal_t *ctx,
+					     struct lightningd *ld,
+					     struct routehint_candidate
+					     *candidates)
+{
+	struct route_info **routehints;
+
+	log_debug(ld->log, "Selecting all %zu candidates",
+		  tal_count(candidates));
+
+	routehints = tal_arr(ctx, struct route_info *, tal_count(candidates));
+	for (size_t i = 0; i < tal_count(candidates); i++) {
+		routehints[i] = tal_dup(routehints, struct route_info,
+					candidates[i].r);
+	}
+
+	return routehints;
+}
+
 /* Encapsulating struct while we wait for gossipd to give us incoming channels */
 struct chanhints {
 	bool expose_all_private;
@@ -721,6 +740,10 @@ add_routehints(struct invoice_info *info,
 
 	needed = info->b11->msat ? *info->b11->msat : AMOUNT_MSAT(1);
 
+	/* --payment-fronting-node means use all candidates. */
+	if (tal_count(info->cmd->ld->fronting_nodes))
+		info->b11->routes = select_inchan_all(info->b11, info->cmd->ld, candidates);
+
 	/* If we are not completely unpublished, try with reservoir
 	 * sampling first.
 	 *
@@ -736,7 +759,7 @@ add_routehints(struct invoice_info *info,
 	 * should make an effort to avoid overlapping incoming
 	 * channels, which is done by select_inchan_mpp.
 	 */
-	if (!node_unpublished)
+	else if (!node_unpublished)
 		info->b11->routes = select_inchan(info->b11,
 						  info->cmd->ld,
 						  needed,
diff --git a/lightningd/routehint.c b/lightningd/routehint.c
index 27264f65..b3144f3a 100644
--- a/lightningd/routehint.c
+++ b/lightningd/routehint.c
@@ -13,6 +13,16 @@ static bool scid_in_arr(const struct short_channel_id *scidarr,
 	return false;
 }
 
+static bool is_fronting_node(const struct lightningd *ld,
+			     const struct node_id *node)
+{
+	for (size_t i = 0; i < tal_count(ld->fronting_nodes); i++) {
+		if (node_id_eq(&ld->fronting_nodes[i], node))
+			return true;
+	}
+	return false;
+}
+
 struct routehint_candidate *
 routehint_candidates(const tal_t *ctx,
 		     struct lightningd *ld,
@@ -58,7 +68,7 @@ routehint_candidates(const tal_t *ctx,
 		struct routehint_candidate candidate;
 		struct amount_msat fee_base, htlc_max;
 		struct route_info *r;
-		bool is_public;
+		bool is_public, is_fronting;
 
 		r = tal(tmpctx, struct route_info);
 
@@ -88,6 +98,17 @@ routehint_candidates(const tal_t *ctx,
 			      json_tok_full(buf, toks));
 		}
 
+		/* If they specify fronting nodes, always use them. */
+		if (tal_count(ld->fronting_nodes)) {
+			if (!is_fronting_node(ld, &r->pubkey)) {
+				log_debug(ld->log, "%s: not a fronting node",
+					  fmt_node_id(tmpctx, &r->pubkey));
+				continue;
+			}
+			is_fronting = true;
+		} else
+			is_fronting = false;
+
 		/* Note: listincoming returns real scid or local alias if no real scid. */
 		candidate.c = any_channel_by_scid(ld, r->short_channel_id, true);
 		if (!candidate.c) {
@@ -129,6 +150,10 @@ routehint_candidates(const tal_t *ctx,
 		if (expose_all_private != NULL && *expose_all_private)
 			is_public = true;
 
+		/* Also, consider fronting nodes public */
+		if (is_fronting)
+			is_public = true;
+
 		r->fee_base_msat = fee_base.millisatoshis; /* Raw: route_info */
 		/* Could wrap: if so ignore */
 		if (!amount_msat_eq(amount_msat(r->fee_base_msat), fee_base)) {
@@ -152,7 +177,7 @@ routehint_candidates(const tal_t *ctx,
 				continue;
 			}
 			/* If they give us a hint, we use even if capacity 0 */
-		} else if (amount_msat_is_zero(capacity)) {
+		} else if (!is_fronting && amount_msat_is_zero(capacity)) {
 			log_debug(ld->log, "%s: deadend",
 				  fmt_short_channel_id(tmpctx,
 						       r->short_channel_id));
@@ -162,8 +187,8 @@ routehint_candidates(const tal_t *ctx,
 			continue;
 		}
 
-		/* Is it offline? */
-		if (candidate.c->owner == NULL) {
+		/* Is it offline?  Leave it if it's fronting. */
+		if (!is_fronting && candidate.c->owner == NULL) {
 			log_debug(ld->log, "%s: offline",
 				  fmt_short_channel_id(tmpctx,
 						       r->short_channel_id));
diff --git a/tests/test_invoices.py b/tests/test_invoices.py
index c7d9187f..f174af55 100644
--- a/tests/test_invoices.py
+++ b/tests/test_invoices.py
@@ -942,6 +942,31 @@ def test_invoice_botched_migration(node_factory, chainparams):
     assert l1.rpc.invoice(100, "test", "test")["created_index"] == 3
 
 
+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']]}])
+
+    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']]}}}
+
+    # l1  <----> l3
+    #   \
+    #    \-----> l4 <----> l2
+    node_factory.join_nodes([l1, l3], wait_for_announce=True)
+    node_factory.join_nodes([l1, l4], wait_for_announce=True)
+    node_factory.join_nodes([l2, l4], wait_for_announce=True)
+
+    l3inv = l3.rpc.invoice(1000, 'l3inv', 'l3inv')['bolt11']
+    assert only_one(only_one(l3.rpc.decode(l3inv)['routes']))['pubkey'] == l1.info['id']
+
+    l4inv = l4.rpc.invoice(1000, 'l4inv', 'l4inv')['bolt11']
+    assert [only_one(r)['pubkey'] for r in l4.rpc.decode(l4inv)['routes']] == [l1.info['id'], l2.info['id']]
+
+    l1.rpc.xpay(l3inv)
+    l1.rpc.xpay(l4inv)
+
+
 def test_invoice_maxdesc(node_factory, chainparams):
     l1, l2 = node_factory.line_graph(2)