input/Stream: remove attribute "cond", replace with handler interface

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

src/input/Handler.hxx

@@ -0,0 +1,53 @@
+/*
+ * 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_INPUT_STREAM_HANDLER_HXX
+#define MPD_INPUT_STREAM_HANDLER_HXX
+
+#include "check.h"
+
+/**
+ * An interface which gets receives events from an #InputStream.  Its
+ * methods will be called from within an arbitrary thread and must not
+ * block.
+ *
+ * A reference to an instance is passed to the #InputStream, but it
+ * remains owned by the caller.
+ */
+class InputStreamHandler {
+public:
+	/**
+	 * Called when InputStream::IsReady() becomes true.
+	 *
+	 * Before querying metadata from the #InputStream,
+	 * InputStream::Update() must be called.
+	 *
+	 * Caller locks InputStream::mutex.
+	 */
+	virtual void OnInputStreamReady() noexcept = 0;
+
+	/**
+	 * Called when InputStream::IsAvailable() becomes true.
+	 *
+	 * Caller locks InputStream::mutex.
+	 */
+	virtual void OnInputStreamAvailable() noexcept = 0;
+};
+
+#endif