askrene: don't crash if a layer is removed while getroutes waits

plugins/askrene/askrene.c

@@ -376,15 +376,19 @@ static void add_layer(const struct layer ***layers,
 	layer_clear_overridden_capacities(l, gossmap, capacities);
 }
 
-/* Gather layers, clear capacities where layers contains info */
+/* Gather layers, clear capacities where layers contains info.
+ * Returns NULL (and sets *unknown) if a layer no longer exists: layer
+ * names are only checked when the command arrives, and the layer can be
+ * removed while the request is waiting for a free child. */
 static const struct layer **apply_layers(const tal_t *ctx,
 					 struct askrene *askrene,
 					 struct command *cmd,
 					 const struct node_id *source,
 					 struct gossmap_localmods *localmods,
 					 const char **layernames,
 					 const struct layer *local_layer,
-					 fp16_t *capacities)
+					 fp16_t *capacities,
+					 const char **unknown)
 {
 	const struct layer **layers = tal_arr(ctx, const struct layer *, 0);
 	/* Layers must exist, but might be special ones! */
@@ -402,10 +406,12 @@ static const struct layer **apply_layers(const tal_t *ctx,
 				/* This layer takes effect when converting flows
 				 * into routes. */
 				continue;
-			} else {
-				assert(streq(layernames[i], "auto.sourcefree"));
+			} else if (streq(layernames[i], "auto.sourcefree")) {
 				cmd_log(tmpctx, cmd, LOG_DBG, "Adding auto.sourcefree");
 				l = source_free_layer(layernames, askrene, source, localmods);
+			} else {
+				*unknown = layernames[i];
+				return tal_free(layers);
 			}
 		}
 		add_layer(&layers, l, askrene->gossmap, localmods, capacities);
@@ -553,6 +559,7 @@ static struct command_result *do_getroutes(struct command *cmd,
 	int replyfds[2], logfds[2];
 	struct router_child *child;
 	const struct layer **layers;
+	const char *unknown_layer;
 	s8 *biases;
 	fp16_t *capacities;
 	bool include_next_node_id, include_amount_msat, include_delay;
@@ -600,8 +607,12 @@ static struct command_result *do_getroutes(struct command *cmd,
 	/* apply selected layers to the localmods */
 	layers = apply_layers(cmd, askrene, cmd,
 			      &info->source, localmods,
-			      info->layers, info->local_layer, capacities);
-
+			      info->layers, info->local_layer, capacities,
+			      &unknown_layer);
+	if (!layers)
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "Layer %s was removed before routing began",
+				    unknown_layer);
 
 	/* no parallel payments means we can drop any smaller channels */
 	if (info->maxparts == 1) {

tests/test_askrene.py

@@ -1714,6 +1714,37 @@ def test_real_data(node_factory, bitcoind, executor):
     assert l1.daemon.is_in_log(r"Too many running at once \(4 vs 4\): waiting")
 
 
+def test_layer_removed_while_waiting(node_factory, executor):
+    """A layer removed while getroutes waits for a free child must not crash askrene"""
+    gsfile, nodemap = generate_gossip_store([GenChannel(0, 1),
+                                             GenChannel(1, 2)])
+    l1 = node_factory.get_node(gossip_store_file=gsfile.name)
+
+    def getroutes(layers):
+        return l1.rpc.getroutes(source=nodemap[0],
+                                destination=nodemap[2],
+                                amount_msat=1000,
+                                layers=layers,
+                                maxfee_msat=1000,
+                                final_cltv=99)
+
+    # With no children allowed, the request has to wait.
+    l1.rpc.askrene_create_layer('vanishing')
+    l1.rpc.setconfig('askrene-max-threads', 0)
+    waiting = executor.submit(getroutes, ['vanishing'])
+    l1.daemon.wait_for_log(r"Too many running at once \(0 vs 0\): waiting")
+    l1.rpc.askrene_remove_layer('vanishing')
+
+    # Finishing another request wakes the waiting one.
+    l1.rpc.setconfig('askrene-max-threads', 1)
+    getroutes([])
+    with pytest.raises(RpcError, match="Layer vanishing was removed before routing began"):
+        waiting.result(TIMEOUT)
+
+    # askrene is still alive.
+    getroutes([])
+
+
 @pytest.mark.slow_test
 def test_real_biases(node_factory, bitcoind, executor):
     # Route from Rusty's node to the top 100.