lightningd/plugin_hook.c: Make `db_write` a chained hook.

Fixes: #4219

Changelog-Changed: Plugins: Multiple plugins can now register `db_write` hooks.

doc/PLUGINS.md

@@ -897,6 +897,14 @@ to error without
 committing to the database!
 This is the expected way to halt and catch fire.
 
+`db_write` is a parallel-chained hook, i.e., multiple plugins can
+register it, and all of them will be invoked simultaneously without
+regard for order of registration.
+The hook is considered handled if all registered plugins return
+`{"result": "continue"}`.
+If any plugin returns anything else, `lightningd` will error without
+committing to the database.
+
 ### `invoice_payment`
 
 This hook is called whenever a valid payment for an unpaid invoice has arrived.

lightningd/plugin_hook.c

@@ -294,20 +294,27 @@ bool 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", PLUGIN_HOOK_SINGLE, NULL,
+static struct plugin_hook db_write_hook = {"db_write", PLUGIN_HOOK_CHAIN, NULL,
 					   NULL, NULL};
 AUTODATA(hooks, &db_write_hook);
 
+/* A `db_write` for one particular plugin hook.  */
+struct db_write_hook_req {
+	struct plugin *plugin;
+	struct plugin_hook_request *ph_req;
+	size_t *num_hooks;
+};
+
 static void db_hook_response(const char *buffer, const jsmntok_t *toks,
 			     const jsmntok_t *idtok,
-			     struct plugin_hook_request *ph_req)
+			     struct db_write_hook_req *dwh_req)
 {
 	const jsmntok_t *resulttok;
 
 	resulttok = json_get_member(buffer, toks, "result");
 	if (!resulttok)
-		fatal("Plugin returned an invalid response to the db_write "
-		      "hook: %s", buffer);
+		fatal("Plugin '%s' returned an invalid response to the "
+		      "db_write hook: %s", dwh_req->plugin->cmd, buffer);
 
 	/* We expect result: { 'result' : 'continue' }.
 	 * Anything else we abort.
@@ -315,13 +322,23 @@ static void db_hook_response(const char *buffer, const jsmntok_t *toks,
 	resulttok = json_get_member(buffer, resulttok, "result");
 	if (resulttok) {
 		if (!json_tok_streq(buffer, resulttok, "continue"))
-			fatal("Plugin returned failed db_write: %s.", buffer);
+			fatal("Plugin '%s' returned failed db_write: %s.",
+			      dwh_req->plugin->cmd,
+			      buffer);
 	} else
-		fatal("Plugin returned an invalid result to the db_write "
-		      "hook: %s", buffer);
+		fatal("Plugin '%s' returned an invalid result to the db_write "
+		      "hook: %s",
+		      dwh_req->plugin->cmd,
+		      buffer);
+
+	assert((*dwh_req->num_hooks) != 0);
+	--(*dwh_req->num_hooks);
+	/* If there are other runners, do not exit yet.  */
+	if ((*dwh_req->num_hooks) != 0)
+		return;
 
 	/* We're done, exit exclusive loop. */
-	io_break(ph_req);
+	io_break(dwh_req->ph_req);
 }
 
 void plugin_hook_db_sync(struct db *db)
@@ -332,27 +349,38 @@ void plugin_hook_db_sync(struct db *db)
 	void *ret;
 	struct plugin **plugins;
 	size_t i;
+	size_t num_hooks;
 
 	const char **changes = db_changes(db);
-	if (tal_count(hook->hooks) == 0)
+	num_hooks = tal_count(hook->hooks);
+	if (num_hooks == 0)
 		return;
 
 	plugins = notleak(tal_arr(NULL, struct plugin *,
-				  tal_count(hook->hooks)));
-	for (i = 0; i < tal_count(hook->hooks); ++i)
+				  num_hooks));
+	for (i = 0; i < num_hooks; ++i)
 		plugins[i] = hook->hooks[i]->plugin;
 
 	ph_req = notleak(tal(hook->hooks, struct plugin_hook_request));
+	ph_req->hook = hook;
+	ph_req->db = db;
+	ph_req->cb_arg = &num_hooks;
+
+	for (i = 0; i < num_hooks; ++i) {
+		/* Create an object for this plugin.  */
+		struct db_write_hook_req *dwh_req;
+		dwh_req = tal(ph_req, struct db_write_hook_req);
+		dwh_req->plugin = plugins[i];
+		dwh_req->ph_req = ph_req;
+		dwh_req->num_hooks = &num_hooks;
+
 		/* FIXME: do IO logging for this! */
 		req = jsonrpc_request_start(NULL, hook->name, NULL, NULL,
 					    db_hook_response,
-				    ph_req);
+					    dwh_req);
 
-	ph_req->hook = hook;
-	ph_req->db = db;
-	ph_req->plugin = hook->hooks[0]->plugin;
-
-	json_add_num(req->stream, "data_version", db_data_version_get(db));
+		json_add_num(req->stream, "data_version",
+			     db_data_version_get(db));
 
 		json_array_start(req->stream, "writes");
 		for (size_t i = 0; i < tal_count(changes); i++)
@@ -360,7 +388,8 @@ void plugin_hook_db_sync(struct db *db)
 		json_array_end(req->stream);
 		jsonrpc_request_end(req);
 
-	plugin_request_send(ph_req->plugin, req);
+		plugin_request_send(plugins[i], req);
+	}
 
 	/* We can be called on way out of an io_loop, which is already breaking.
 	 * That will make this immediately return; save the break value and call
@@ -371,7 +400,9 @@ void plugin_hook_db_sync(struct db *db)
 		assert(ret2 == ph_req);
 		io_break(ret);
 	}
+	assert(num_hooks == 0);
 	tal_free(plugins);
+	tal_free(ph_req);
 }
 
 static void add_deps(const char ***arr,

tests/plugins/dbdummy.py

@@ -0,0 +1,16 @@
+#! /usr/bin/env python3
+'''This plugin is a do-nothing backup plugin which just checks that we
+can handle multiple backup plugins.
+'''
+
+from pyln.client import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook('db_write')
+def db_write(plugin, **kwargs):
+    return {'result': 'continue'}
+
+
+plugin.run()

tests/test_plugin.py

@@ -478,6 +478,32 @@ def test_db_hook(node_factory, executor):
     assert [x for x in db1.iterdump()] == [x for x in db2.iterdump()]
 
 
+@unittest.skipIf(os.getenv('TEST_DB_PROVIDER', 'sqlite3') != 'sqlite3', "Only sqlite3 implements the db_write_hook currently")
+def test_db_hook_multiple(node_factory, executor):
+    """This tests the db hook for multiple-plugin case."""
+    dbfile = os.path.join(node_factory.directory, "dblog.sqlite3")
+    l1 = node_factory.get_node(options={'plugin': os.path.join(os.getcwd(), 'tests/plugins/dblog.py'),
+                                        'important-plugin': os.path.join(os.getcwd(), 'tests/plugins/dbdummy.py'),
+                                        'dblog-file': dbfile})
+
+    # It should see the db being created, and sometime later actually get
+    # initted.
+    # This precedes startup, so needle already past
+    assert l1.daemon.is_in_log(r'plugin-dblog.py: deferring \d+ commands')
+    l1.daemon.logsearch_start = 0
+    l1.daemon.wait_for_log('plugin-dblog.py: replaying pre-init data:')
+    l1.daemon.wait_for_log('plugin-dblog.py: CREATE TABLE version \\(version INTEGER\\)')
+    l1.daemon.wait_for_log("plugin-dblog.py: initialized.* 'startup': True")
+
+    l1.stop()
+
+    # Databases should be identical.
+    db1 = sqlite3.connect(os.path.join(l1.daemon.lightning_dir, TEST_NETWORK, 'lightningd.sqlite3'))
+    db2 = sqlite3.connect(dbfile)
+
+    assert [x for x in db1.iterdump()] == [x for x in db2.iterdump()]
+
+
 def test_utf8_passthrough(node_factory, executor):
     l1 = node_factory.get_node(options={'plugin': os.path.join(os.getcwd(), 'tests/plugins/utf8.py'),
                                         'log-level': 'io'})