client/BackgroundCommand: infrastructure for commands running in background

src/client/ThreadBackgroundCommand.hxx

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2003-2019 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef MPD_THREAD_BACKGROUND_COMMAND_HXX
+#define MPD_THREAD_BACKGROUND_COMMAND_HXX
+
+#include "BackgroundCommand.hxx"
+#include "event/DeferEvent.hxx"
+#include "thread/Thread.hxx"
+
+#include <exception>
+
+class Client;
+class Response;
+
+/**
+ * A #BackgroundCommand which defers execution into a new thread.
+ */
+class ThreadBackgroundCommand : public BackgroundCommand {
+	Thread thread;
+	DeferEvent defer_finish;
+	Client &client;
+
+	/**
+	 * The error thrown by Run().
+	 */
+	std::exception_ptr error;
+
+public:
+	explicit ThreadBackgroundCommand(Client &_client) noexcept;
+
+	auto &GetEventLoop() const noexcept {
+		return defer_finish.GetEventLoop();
+	}
+
+	void Start() {
+		thread.Start();
+	}
+
+	void Cancel() noexcept final;
+
+private:
+	void _Run() noexcept;
+	void DeferredFinish() noexcept;
+
+protected:
+	/**
+	 * If this method throws, the exception will be converted to a
+	 * MPD response, and SendResponse() will not be called.
+	 */
+	virtual void Run() = 0;
+
+	/**
+	 * Send the response after Run() has finished.  Note that you
+	 * must not send errors here; if an error occurs, Run() should
+	 * throw an exception instead.
+	 */
+	virtual void SendResponse(Response &response) noexcept = 0;
+
+	virtual void CancelThread() noexcept = 0;
+};
+
+#endif