db/Interface: remove IsPlugin(), use `dynamic_cast` instead

src/Main.cxx

@@ -216,17 +216,17 @@ glue_db_init_and_load(const ConfigData &config)
 		std::throw_with_nested(std::runtime_error("Failed to open database plugin"));
 	}
 
-	if (!instance->database->IsPlugin(simple_db_plugin))
+	auto *db = dynamic_cast<SimpleDatabase *>(instance->database);
+	if (db == nullptr)
 		return true;
 
-	SimpleDatabase &db = *(SimpleDatabase *)instance->database;
 	instance->update = new UpdateService(config,
-					     instance->event_loop, db,
+					     instance->event_loop, *db,
 					     static_cast<CompositeStorage &>(*instance->storage),
 					     *instance);
 
 	/* run database update after daemonization? */
-	return db.FileExists();
+	return db->FileExists();
 }
 
 static bool

src/command/StorageCommands.cxx

@@ -209,12 +209,9 @@ handle_mount(Client &client, Request args, Response &r)
 	instance.EmitIdle(IDLE_MOUNT);
 
 #ifdef ENABLE_DATABASE
-	Database *_db = instance.database;
-	if (_db != nullptr && _db->IsPlugin(simple_db_plugin)) {
-		SimpleDatabase &db = *(SimpleDatabase *)_db;
-
+	if (auto *db = dynamic_cast<SimpleDatabase *>(instance.database)) {
 		try {
-			db.Mount(local_uri, remote_uri);
+			db->Mount(local_uri, remote_uri);
 		} catch (...) {
 			composite.Unmount(local_uri);
 			throw;
@@ -256,11 +253,8 @@ handle_unmount(Client &client, Request args, Response &r)
 		   destroy here */
 		instance.update->CancelMount(local_uri);
 
-	Database *_db = instance.database;
-	if (_db != nullptr && _db->IsPlugin(simple_db_plugin)) {
-		SimpleDatabase &db = *(SimpleDatabase *)_db;
-
-		if (db.Unmount(local_uri))
+	if (auto *db = dynamic_cast<SimpleDatabase *>(instance.database)) {
+		if (db->Unmount(local_uri))
 			// TODO: call Instance::OnDatabaseModified()?
 			instance.EmitIdle(IDLE_DATABASE);
 	}

src/db/Interface.hxx

@@ -52,10 +52,6 @@ public:
 		return plugin;
 	}
 
-	bool IsPlugin(const DatabasePlugin &other) const noexcept {
-		return &plugin == &other;
-	}
-
 	/**
          * Open the database.  Read it into memory if applicable.
 	 *

src/db/update/Service.cxx

@@ -94,11 +94,9 @@ UpdateService::CancelMount(const char *uri)
 		cancel_current = next.IsDefined() && next.storage == storage2;
 	}
 
-	Database &_db2 = *lr.directory->mounted_database;
-	if (_db2.IsPlugin(simple_db_plugin)) {
-		SimpleDatabase &db2 = static_cast<SimpleDatabase &>(_db2);
-		queue.Erase(db2);
-		cancel_current |= next.IsDefined() && next.db == &db2;
+	if (auto *db2 = dynamic_cast<SimpleDatabase *>(lr.directory->mounted_database)) {
+		queue.Erase(*db2);
+		cancel_current |= next.IsDefined() && next.db == db2;
 	}
 
 	if (cancel_current && walk != nullptr) {
@@ -190,12 +188,10 @@ UpdateService::Enqueue(const char *path, bool discard)
 		/* follow the mountpoint, update the mounted
 		   database */
 
-		Database &_db2 = *lr.directory->mounted_database;
-		if (!_db2.IsPlugin(simple_db_plugin))
+		db2 = dynamic_cast<SimpleDatabase *>(lr.directory->mounted_database);
+		if (db2 == nullptr)
 			throw std::runtime_error("Cannot update this type of database");
 
-		db2 = static_cast<SimpleDatabase *>(&_db2);
-
 		if (lr.uri == nullptr) {
 			storage2 = storage.GetMount(path);
 			path = "";

src/storage/StorageState.cxx

@@ -106,10 +106,9 @@ storage_state_restore(const char *line, TextFile &file, Instance &instance)
 		return true;
 	}
 
-	Database *db = instance.database;
-	if (db != nullptr && db->IsPlugin(simple_db_plugin)) {
+	if (auto *db = dynamic_cast<SimpleDatabase *>(instance.database)) {
 		try {
-			((SimpleDatabase *)db)->Mount(uri.c_str(), url.c_str());
+			db->Mount(uri.c_str(), url.c_str());
 		} catch (...) {
 			FormatError(std::current_exception(),
 				    "Failed to restore mount to %s",