output/Interface: add "attributes" map

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

src/output/Control.cxx

@@ -76,6 +76,18 @@ AudioOutputControl::GetMixer() const noexcept
 	return output->mixer;
 }
 
+const std::map<std::string, std::string>
+AudioOutputControl::GetAttributes() const noexcept
+{
+	return output->GetAttributes();
+}
+
+void
+AudioOutputControl::SetAttribute(std::string &&name, std::string &&value)
+{
+	output->SetAttribute(std::move(name), std::move(value));
+}
+
 bool
 AudioOutputControl::LockSetEnabled(bool new_value) noexcept
 {

src/output/Control.hxx

@@ -30,6 +30,8 @@
 
 #include <utility>
 #include <exception>
+#include <string>
+#include <map>
 
 #ifndef NDEBUG
 #include <assert.h>
@@ -324,6 +326,9 @@ public:
 	void BeginDestroy() noexcept;
 	void FinishDestroy() noexcept;
 
+	const std::map<std::string, std::string> GetAttributes() const noexcept;
+	void SetAttribute(std::string &&name, std::string &&value);
+
 	/**
 	 * Enables the device, but don't wait for completion.
 	 *

src/output/Filtered.cxx

@@ -40,6 +40,18 @@ FilteredAudioOutput::SupportsPause() const noexcept
 	return output->SupportsPause();
 }
 
+const std::map<std::string, std::string>
+FilteredAudioOutput::GetAttributes() const noexcept
+{
+	return output->GetAttributes();
+}
+
+void
+FilteredAudioOutput::SetAttribute(std::string &&_name, std::string &&_value)
+{
+	output->SetAttribute(std::move(_name), std::move(_value));
+}
+
 void
 FilteredAudioOutput::Enable()
 {

src/output/Filtered.hxx

@@ -25,6 +25,7 @@
 
 #include <memory>
 #include <string>
+#include <map>
 #include <chrono>
 
 class PreparedFilter;
@@ -167,6 +168,9 @@ public:
 	gcc_pure
 	bool SupportsPause() const noexcept;
 
+	const std::map<std::string, std::string> GetAttributes() const noexcept;
+	void SetAttribute(std::string &&name, std::string &&value);
+
 	/**
 	 * Throws #std::runtime_error on error.
 	 */

src/output/Interface.cxx

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2003-2017 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 "Interface.hxx"
+#include "Compiler.h"
+
+#include <stdexcept>
+
+void
+AudioOutput::SetAttribute(gcc_unused std::string &&name,
+			  gcc_unused std::string &&value)
+{
+	throw std::invalid_argument("Unsupported attribute");
+}

src/output/Interface.hxx

@@ -20,7 +20,10 @@
 #ifndef MPD_AUDIO_OUTPUT_INTERFACE_HXX
 #define MPD_AUDIO_OUTPUT_INTERFACE_HXX
 
+#include <map>
+#include <string>
 #include <chrono>
+#include <stdexcept>
 
 struct AudioFormat;
 struct Tag;
@@ -57,6 +60,22 @@ public:
 		return flags & FLAG_NEED_FULLY_DEFINED_AUDIO_FORMAT;
 	}
 
+	/**
+	 * Returns a map of runtime attributes.
+	 *
+	 * This method must be thread-safe.
+	 */
+	virtual const std::map<std::string, std::string> GetAttributes() const noexcept {
+		return {};
+	}
+
+	/**
+	 * Manipulate a runtime attribute on client request.
+	 *
+	 * This method must be thread-safe.
+	 */
+	virtual void SetAttribute(std::string &&name, std::string &&value);
+
 	/**
 	 * Enable the device.  This may allocate resources, preparing
 	 * for the device to be opened.

src/output/Print.cxx

@@ -41,5 +41,9 @@ printAudioDevices(Response &r, const MultipleOutputs &outputs)
 			 i,
 			 ao.GetName(), ao.GetPluginName(),
 			 ao.IsEnabled());
+
+		for (const auto &a : ao.GetAttributes())
+			r.Format("attribute: %s=%s\n",
+				 a.first.c_str(), a.second.c_str());
 	}
 }