Add a simple, safe gain PostProcessor.

chromecast/media/cma/backend/alsa/post_processor_factory.cc

Index: chromecast/media/cma/backend/alsa/post_processor_factory.cc
diff --git a/chromecast/media/cma/backend/alsa/post_processor_factory.cc b/chromecast/media/cma/backend/alsa/post_processor_factory.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c2f5379cc189280859c26c17454e9ae8947834af
--- /dev/null
+++ b/chromecast/media/cma/backend/alsa/post_processor_factory.cc
@@ -0,0 +1,44 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chromecast/media/cma/backend/alsa/post_processor_factory.h"
+
+#include "base/files/file_path.h"
+#include "base/memory/ptr_util.h"
+#include "base/scoped_native_library.h"
+#include "chromecast/public/media/audio_post_processor_shlib.h"
+
+namespace chromecast {
+namespace media {
+
+namespace {
+
+const char kSoCreateFunction[] = "AudioPostProcessorShlib_Create";
+
+}  // namespace
+
+using CreatePostProcessorFunction = AudioPostProcessor* (*)(const std::string&,
+                                                            int);
+
+PostProcessorFactory::PostProcessorFactory() = default;
+PostProcessorFactory::~PostProcessorFactory() = default;
+
+std::unique_ptr<AudioPostProcessor> PostProcessorFactory::CreatePostProcessor(
+    const std::string& library_path,
+    const std::string& config,
+    int channels) {
+  libraries_.push_back(base::MakeUnique<base::ScopedNativeLibrary>(
+      base::FilePath(library_path)));
+  CHECK(libraries_.back()->is_valid())
+      << "Could not open post processing library " << library_path;
+  auto create = reinterpret_cast<CreatePostProcessorFunction>(
+      libraries_.back()->GetFunctionPointer(kSoCreateFunction));
+
+  CHECK(create) << "Could not find " << kSoCreateFunction << "() in "
+                << library_path;
+  return base::WrapUnique(create(config, channels));
+}
+
+}  // namespace media
+}  // namespace chromecast