input/Plugin: add method scan() with asynchronous callback class

To be used when a URL gets added.

Makefile.am

@@ -1327,6 +1327,7 @@ libinput_a_SOURCES = \
 	src/input/InputStream.cxx src/input/InputStream.hxx \
 	src/input/Ptr.hxx \
 	src/input/InputPlugin.hxx \
+	src/input/RemoteTagScanner.hxx \
 	src/input/Reader.cxx src/input/Reader.hxx \
 	src/input/TextInputStream.cxx src/input/TextInputStream.hxx \
 	src/input/ThreadInputStream.cxx src/input/ThreadInputStream.hxx \

src/input/InputPlugin.hxx

@@ -26,7 +26,8 @@ struct ConfigBlock;
 class Mutex;
 class Cond;
 class EventLoop;
-class InputStream;
+class RemoteTagScanner;
+class RemoteTagHandler;
 
 struct InputPlugin {
 	const char *name;
@@ -52,6 +53,17 @@ struct InputPlugin {
 	 */
 	InputStreamPtr (*open)(const char *uri,
 			       Mutex &mutex, Cond &cond);
+
+	/**
+	 * Prepare a #RemoteTagScanner.  The operation must be started
+	 * using RemoteTagScanner::Start().
+	 *
+	 * Throws on error.
+	 *
+	 * @return nullptr if the given URI is not supported.
+	 */
+	std::unique_ptr<RemoteTagScanner> (*scan_tags)(const char *uri,
+						       RemoteTagHandler &handler) = nullptr;
 };
 
 #endif

src/input/RemoteTagScanner.hxx

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2003-2018 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_REMOTE_TAG_SCANNER_HXX
+#define MPD_REMOTE_TAG_SCANNER_HXX
+
+#include <exception>
+
+struct Tag;
+
+class RemoteTagHandler {
+public:
+	virtual void OnRemoteTag(Tag &&tag) noexcept = 0;
+	virtual void OnRemoteTagError(std::exception_ptr e) noexcept = 0;
+};
+
+class RemoteTagScanner {
+public:
+	virtual ~RemoteTagScanner() noexcept = default;
+	virtual void Start() = 0;
+};
+
+#endif