client/BackgroundCommand: infrastructure for commands running in background

meson.build

@@ -220,6 +220,7 @@ sources = [
   'src/client/Subscribe.cxx',
   'src/client/File.cxx',
   'src/client/Response.cxx',
+  'src/client/ThreadBackgroundCommand.cxx',
   'src/Listen.cxx',
   'src/LogInit.cxx',
   'src/LogBackend.cxx',

src/client/BackgroundCommand.hxx

+/*
+ * 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_BACKGROUND_COMMAND_HXX
+#define MPD_BACKGROUND_COMMAND_HXX
+
+/**
+ * A command running in background.  It can take some time to finish,
+ * and will then call Client::OnBackgroundCommandFinished() from
+ * inside the client's #EventLoop thread.  The important point is that
+ * sucha long-running command does not block MPD's main loop, and
+ * other clients can still be handled meanwhile.
+ *
+ * (Note: "idle" is not a "background command" by this definition; it
+ * is a special case.)
+ *
+ * @see ThreadBackgroundCommand
+ */
+class BackgroundCommand {
+public:
+	virtual ~BackgroundCommand() = default;
+
+	/**
+	 * Cancel command execution.  After this method returns, the
+	 * object will be deleted.  It will be called from the
+	 * #Client's #EventLoop thread.
+	 */
+	virtual void Cancel() noexcept = 0;
+};
+
+#endif

src/client/Client.cxx

@@ -18,8 +18,10 @@
  */
 
 #include "Client.hxx"
+#include "Config.hxx"
 #include "Partition.hxx"
 #include "Instance.hxx"
+#include "BackgroundCommand.hxx"
 #include "util/Domain.hxx"
 #include "config.h"
 
@@ -27,6 +29,37 @@ Client::~Client() noexcept
 {
 	if (FullyBufferedSocket::IsDefined())
 		FullyBufferedSocket::Close();
+
+	if (background_command) {
+		background_command->Cancel();
+		background_command.reset();
+	}
+}
+
+void
+Client::SetBackgroundCommand(std::unique_ptr<BackgroundCommand> _bc) noexcept
+{
+	assert(!background_command);
+	assert(_bc);
+
+	background_command = std::move(_bc);
+
+	/* disable timeouts while in "idle" */
+	timeout_event.Cancel();
+}
+
+void
+Client::OnBackgroundCommandFinished() noexcept
+{
+	assert(background_command);
+
+	background_command.reset();
+
+	/* just in case OnSocketInput() has returned
+	   InputResult::PAUSE meanwhile */
+	ResumeInput();
+
+	timeout_event.Schedule(client_timeout);
 }
 
 Instance &

src/client/Client.hxx

@@ -34,6 +34,7 @@
 #include <set>
 #include <string>
 #include <list>
+#include <memory>
 
 #include <stddef.h>
 
@@ -47,6 +48,7 @@ class PlayerControl;
 struct playlist;
 class Database;
 class Storage;
+class BackgroundCommand;
 
 class Client final
 	: FullyBufferedSocket,
@@ -102,6 +104,14 @@ private:
 	 */
 	std::list<ClientMessage> messages;
 
+	/**
+	 * The command currently running in background.  If this is
+	 * set, then the client is occupied and will not process any
+	 * new input.  If the connection gets closed, the
+	 * #BackgroundCommand will be cancelled.
+	 */
+	std::unique_ptr<BackgroundCommand> background_command;
+
 public:
 	Client(EventLoop &loop, Partition &partition,
 	       UniqueSocketDescriptor fd, int uid,
@@ -152,6 +162,19 @@ public:
 	void IdleAdd(unsigned flags) noexcept;
 	bool IdleWait(unsigned flags) noexcept;
 
+	/**
+	 * Called by a command handler to defer execution to a
+	 * #BackgroundCommand.
+	 */
+	void SetBackgroundCommand(std::unique_ptr<BackgroundCommand> _bc) noexcept;
+
+	/**
+	 * Called by the current #BackgroundCommand when it has
+	 * finished, after sending the response.  This method then
+	 * deletes the #BackgroundCommand.
+	 */
+	void OnBackgroundCommandFinished() noexcept;
+
 	enum class SubscribeResult {
 		/** success */
 		OK,

src/client/Expire.cxx

@@ -25,6 +25,7 @@ void
 Client::OnTimeout() noexcept
 {
 	assert(!idle_waiting);
+	assert(!background_command);
 
 	FormatDebug(client_domain, "[%u] timeout", num);
 

src/client/New.cxx

@@ -21,6 +21,7 @@
 #include "Config.hxx"
 #include "Domain.hxx"
 #include "List.hxx"
+#include "BackgroundCommand.hxx"
 #include "Partition.hxx"
 #include "Instance.hxx"
 #include "net/UniqueSocketDescriptor.hxx"

src/client/Process.cxx

@@ -54,6 +54,8 @@ Client::ProcessCommandList(bool list_ok,
 CommandResult
 Client::ProcessLine(char *line) noexcept
 {
+	assert(!background_command);
+
 	if (!IsLowerAlphaASCII(*line)) {
 		/* all valid MPD commands begin with a lower case
 		   letter; this could be a badly routed HTTP

src/client/Read.cxx

@@ -29,6 +29,9 @@
 BufferedSocket::InputResult
 Client::OnSocketInput(void *data, size_t length) noexcept
 {
+	if (background_command)
+		return InputResult::PAUSE;
+
 	char *p = (char *)data;
 	char *newline = (char *)memchr(p, '\n', length);
 	if (newline == nullptr)
@@ -48,6 +51,7 @@ Client::OnSocketInput(void *data, size_t length) noexcept
 	switch (result) {
 	case CommandResult::OK:
 	case CommandResult::IDLE:
+	case CommandResult::BACKGROUND:
 	case CommandResult::ERROR:
 		break;
 

src/client/ThreadBackgroundCommand.cxx

+/*
+ * 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.
+ */
+
+#include "ThreadBackgroundCommand.hxx"
+#include "Client.hxx"
+#include "Response.hxx"
+#include "command/CommandError.hxx"
+#include "protocol/Result.hxx"
+
+ThreadBackgroundCommand::ThreadBackgroundCommand(Client &_client) noexcept
+	:thread(BIND_THIS_METHOD(_Run)),
+	 defer_finish(_client.GetEventLoop(), BIND_THIS_METHOD(DeferredFinish)),
+	 client(_client)
+{
+}
+
+void
+ThreadBackgroundCommand::_Run() noexcept
+{
+	assert(!error);
+
+	try {
+		Run();
+	} catch (...) {
+		error = std::current_exception();
+	}
+
+	defer_finish.Schedule();
+}
+
+void
+ThreadBackgroundCommand::DeferredFinish() noexcept
+{
+	/* free the Thread */
+	thread.Join();
+
+	/* send the response */
+	Response response(client, 0);
+
+	if (!error) {
+		PrintError(response, std::move(error));
+	} else {
+		SendResponse(response);
+		command_success(client);
+	}
+
+	/* delete this object */
+	client.OnBackgroundCommandFinished();
+}
+
+void
+ThreadBackgroundCommand::Cancel() noexcept
+{
+	CancelThread();
+	thread.Join();
+
+	/* cancel the DeferEvent, just in case the Thread has
+	   meanwhile finished execution */
+	defer_finish.Cancel();
+}

src/client/ThreadBackgroundCommand.hxx

+/*
+ * 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

src/command/CommandResult.hxx

@@ -42,6 +42,11 @@ enum class CommandResult {
 	IDLE,
 
 	/**
+	 * A #BackgroundCommand has been installed.
+	 */
+	BACKGROUND,
+
+	/**
 	 * There was an error.  The "ACK" response was sent to the
 	 * client.
 	 */