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diff --git a/src/server/PreProcessor.hpp b/src/server/PreProcessor.hpp
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+++ b/src/server/PreProcessor.hpp
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+/*
+  This file is part of Ingen.
+  Copyright 2007-2016 David Robillard <http://drobilla.net/>
+
+  Ingen is free software: you can redistribute it and/or modify it under the
+  terms of the GNU Affero General Public License as published by the Free
+  Software Foundation, either version 3 of the License, or any later version.
+
+  Ingen 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 Affero General Public License for details.
+
+  You should have received a copy of the GNU Affero General Public License
+  along with Ingen.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef INGEN_ENGINE_PREPROCESSOR_HPP
+#define INGEN_ENGINE_PREPROCESSOR_HPP
+
+#include <atomic>
+#include <thread>
+#include <mutex>
+
+#include "raul/Semaphore.hpp"
+
+namespace ingen {
+namespace server {
+
+class Engine;
+class Event;
+class PostProcessor;
+class RunContext;
+
+class PreProcessor
+{
+public:
+	explicit PreProcessor(Engine& engine);
+
+	~PreProcessor();
+
+	/** Return true iff no events are enqueued. */
+	inline bool empty() const { return !_head.load(); }
+
+	/** Enqueue an event.
+	 * This is safe to call from any non-realtime thread (it locks).
+	 */
+	void event(Event* ev, Event::Mode mode);
+
+	/** Process events for a cycle.
+	 * @return The number of events processed.
+	 */
+	unsigned process(RunContext&    context,
+	                 PostProcessor& dest,
+	                 size_t         limit = 0);
+
+protected:
+	void run();
+
+private:
+	enum class BlockState {
+		UNBLOCKED,      ///< Normal, unblocked execution
+		PRE_BLOCKED,    ///< Preprocess thread has enqueued blocking event
+		BLOCKED,        ///< Process thread has reached blocking event
+		PRE_UNBLOCKED,  ///< Preprocess thread has enqueued unblocking event
+		PROCESSING      ///< Process thread is executing all events in-between
+	};
+
+	void wait_for_block_state(const BlockState state) {
+		while (_block_state != state) {
+			std::this_thread::sleep_for(std::chrono::milliseconds(10));
+		}
+	}
+
+	Engine&                 _engine;
+	std::mutex              _mutex;
+	Raul::Semaphore         _sem;
+	std::atomic<Event*>     _head;
+	std::atomic<Event*>     _tail;
+	std::atomic<BlockState> _block_state;
+	bool                    _exit_flag;
+	std::thread             _thread;
+};
+
+} // namespace server
+} // namespace ingen
+
+#endif // INGEN_ENGINE_PREPROCESSOR_HPP