flac encoder: enable Ogg FLAC and Ogg chaining

refactors GenerateOggSerial into a generic GenerateSerial
utility, under the util lib.

libFLAC may be encoded without Ogg support. If Ogg support is disabled,
libFLAC will still export Ogg-related methods (like setting a serial
number), and throw a runtime error when initializing an Ogg stream.

GenerateOggSerial does not depend on libogg. Refactoring it into
a generic GenerateSerial prevents having to add build-time checks
for libogg within the FLAC encoder plugin.

1 files changed, 46 insertions, 0 deletions

diff --git a/src/util/Serial.cxx b/src/util/Serial.cxx
new file mode 100644
index 000000000..c91fe62da
--- /dev/null
+++ b/src/util/Serial.cxx
@@ -0,0 +1,46 @@
+/*
+ * Copyright 2003-2021 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 "Serial.hxx"
+#include "Compiler.h"
+
+#include <atomic>
+#include <chrono>
+
+static std::atomic_uint next_serial;
+
+int
+GenerateSerial() noexcept
+{
+	unsigned serial = ++next_serial;
+	if (gcc_unlikely(serial < 16)) {
+		/* first-time initialization: seed with a clock value,
+		   which is random enough for our use */
+
+		/* this code is not race-free, but good enough */
+		using namespace std::chrono;
+		const auto now = steady_clock::now().time_since_epoch();
+		const auto now_ms = duration_cast<milliseconds>(now);
+		const unsigned seed = now_ms.count();
+		next_serial = serial = seed;
+	}
+
+	return serial;
+}
+