plugin: Introduce plugin type to allow singleton and chaining

The newly introduced type is used to determine what the call semantics of the
hook are. We have `single` corresponding to the old behavior, as well as
`chain` which allows multiple plugins to register for the hook, and they are
then called sequentially (if all plugins return `{"result": "continue"}`) or
exit the chain if the hook event was handled.

lightningd/invoice.c

@@ -269,6 +269,7 @@ invoice_payment_hook_cb(struct invoice_payment_hook_payload *payload,
 }
 
 REGISTER_PLUGIN_HOOK(invoice_payment,
+		     PLUGIN_HOOK_SINGLE,
 		     invoice_payment_hook_cb,
 		     struct invoice_payment_hook_payload *,
 		     invoice_payment_serialize,

lightningd/jsonrpc.c

@@ -753,7 +753,8 @@ rpc_command_hook_callback(struct rpc_command_hook_payload *p,
 	                         "Bad response to 'rpc_command' hook."));
 }
 
-REGISTER_PLUGIN_HOOK(rpc_command, rpc_command_hook_callback,
+REGISTER_PLUGIN_HOOK(rpc_command, PLUGIN_HOOK_SINGLE,
+		     rpc_command_hook_callback,
                      struct rpc_command_hook_payload *,
                      rpc_command_hook_serialize,
                      struct rpc_command_hook_payload *);

lightningd/opening_control.c

@@ -821,6 +821,7 @@ static void openchannel_hook_cb(struct openchannel_hook_payload *payload,
 }
 
 REGISTER_PLUGIN_HOOK(openchannel,
+		     PLUGIN_HOOK_SINGLE,
 		     openchannel_hook_cb,
 		     struct openchannel_hook_payload *,
 		     openchannel_hook_serialize,

lightningd/peer_control.c

@@ -908,7 +908,8 @@ send_error:
 	tal_free(payload);
 }
 
-REGISTER_PLUGIN_HOOK(peer_connected, peer_connected_hook_cb,
+REGISTER_PLUGIN_HOOK(peer_connected, PLUGIN_HOOK_SINGLE,
+		     peer_connected_hook_cb,
 		     struct peer_connected_hook_payload *,
 		     peer_connected_serialize,
 		     struct peer_connected_hook_payload *);
@@ -2393,8 +2394,9 @@ static void custommsg_payload_serialize(struct custommsg_payload *payload,
 	json_add_node_id(stream, "peer_id", &payload->peer_id);
 }
 
-REGISTER_PLUGIN_HOOK(custommsg, custommsg_callback, struct custommsg_payload *,
-		     custommsg_payload_serialize, struct custommsg_payload *);
+REGISTER_PLUGIN_HOOK(custommsg, PLUGIN_HOOK_SINGLE, custommsg_callback,
+		     struct custommsg_payload *, custommsg_payload_serialize,
+		     struct custommsg_payload *);
 
 void handle_custommsg_in(struct lightningd *ld, const struct node_id *peer_id,
 			 const u8 *msg)

lightningd/peer_htlcs.c

@@ -873,7 +873,8 @@ htlc_accepted_hook_callback(struct htlc_accepted_hook_payload *request,
 	tal_free(request);
 }
 
-REGISTER_PLUGIN_HOOK(htlc_accepted, htlc_accepted_hook_callback,
+REGISTER_PLUGIN_HOOK(htlc_accepted, PLUGIN_HOOK_SINGLE,
+		     htlc_accepted_hook_callback,
 		     struct htlc_accepted_hook_payload *,
 		     htlc_accepted_hook_serialize,
 		     struct htlc_accepted_hook_payload *);

lightningd/plugin_hook.c

@@ -130,7 +130,8 @@ void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
  * annoying, and to make it clear that it's totally synchronous. */
 
 /* Special synchronous hook for db */
-static struct plugin_hook db_write_hook = { "db_write", NULL, NULL, NULL };
+static struct plugin_hook db_write_hook = {"db_write", PLUGIN_HOOK_SINGLE, NULL,
+					   NULL, NULL};
 AUTODATA(hooks, &db_write_hook);
 
 static void db_hook_response(const char *buffer, const jsmntok_t *toks,

lightningd/plugin_hook.h

@@ -44,8 +44,18 @@
  * and callback have the correct type.
  */
 
+enum plugin_hook_type {
+	PLUGIN_HOOK_SINGLE,
+	PLUGIN_HOOK_CHAIN,
+};
+
 struct plugin_hook {
 	const char *name;
+
+	/* Which type of plugin is this? It'll determine how many plugins can
+	 * register this hook, and how the hooks are called. */
+	enum plugin_hook_type type;
+
 	void (*response_cb)(void *arg, const char *buffer, const jsmntok_t *toks);
 	void (*serialize_payload)(void *src, struct json_stream *dest);
 
@@ -84,13 +94,15 @@ void plugin_hook_call_(struct lightningd *ld, const struct plugin_hook *hook,
  * response_cb function accepts the deserialized response format and
  * an arbitrary extra argument used to maintain context.
  */
-#define REGISTER_PLUGIN_HOOK(name, response_cb, response_cb_arg_type,          \
+#define REGISTER_PLUGIN_HOOK(name, type, response_cb, response_cb_arg_type,    \
 			     serialize_payload, payload_type)                  \
 	struct plugin_hook name##_hook_gen = {                                 \
 	    stringify(name),                                                   \
-	    typesafe_cb_cast(void (*)(void *, const char *, const jsmntok_t *),\
-			     void (*)(response_cb_arg_type,		       \
-				      const char *, const jsmntok_t *),	       \
+	    type,                                                              \
+	    typesafe_cb_cast(                                                  \
+		void (*)(void *, const char *, const jsmntok_t *),             \
+		void (*)(response_cb_arg_type, const char *,                   \
+			 const jsmntok_t *),                                   \
 		response_cb),                                                  \
 	    typesafe_cb_cast(void (*)(void *, struct json_stream *),           \
 			     void (*)(payload_type, struct json_stream *),     \