Add Pipe-to-command export backend

A first step towards allowing external encoders without intermediate
temporary file.

libs/audiographer/audiographer/general/cmdpipe_writer.h

@@ -0,0 +1,87 @@
+#ifndef AUDIOGRAPHER_CMDPIPE_WRITER_H
+#define AUDIOGRAPHER_CMDPIPE_WRITER_H
+
+#include <string>
+
+#include <boost/format.hpp>
+
+#include "audiographer/flag_debuggable.h"
+#include "audiographer/sink.h"
+#include "audiographer/types.h"
+
+#include "pbd/signals.h"
+#include "pbd/system_exec.h"
+
+namespace AudioGrapher
+{
+
+/** Writer for audio files using libsndfile.
+  * Only short, int and float are valid template parameters
+  */
+template <typename T = DefaultSampleType>
+class CmdPipeWriter
+  : public Sink<T>
+  , public Throwing<>
+  , public FlagDebuggable<>
+{
+public:
+	CmdPipeWriter (PBD::SystemExec* proc, std::string const& path)
+		: samples_written (0)
+		, _proc (proc)
+		, _path (path)
+	{
+		add_supported_flag (ProcessContext<T>::EndOfInput);
+	}
+
+	virtual ~CmdPipeWriter () {
+		delete _proc;
+	}
+
+	samplecnt_t get_samples_written() const { return samples_written; }
+	void       reset_samples_written_count() { samples_written = 0; }
+
+	void close (void)
+	{
+		_proc->terminate ();
+	}
+
+	virtual void process (ProcessContext<T> const & c)
+	{
+		check_flags (*this, c);
+
+		if (!_proc || !_proc->is_running()) {
+			throw Exception (*this, boost::str (boost::format
+						("Target encoder process is not running")));
+		}
+
+		const size_t bytes_per_sample = sizeof (T);
+		samplecnt_t const written = _proc->write_to_stdin ((const void*) c.data(), c.samples() * bytes_per_sample) / bytes_per_sample;
+		samples_written += written;
+
+		if (throw_level (ThrowProcess) && written != c.samples()) {
+			throw Exception (*this, boost::str (boost::format
+						("Could not write data to output file")));
+		}
+
+		if (c.has_flag(ProcessContext<T>::EndOfInput)) {
+			_proc->close_stdin ();
+			FileWritten (_path);
+		}
+	}
+
+	using Sink<T>::process;
+
+	PBD::Signal1<void, std::string> FileWritten;
+
+private:
+	CmdPipeWriter (CmdPipeWriter const & other) {}
+
+	samplecnt_t samples_written;
+	PBD::SystemExec* _proc;
+	std::string _path;
+};
+
+} // namespace
+
+#endif
+