[Chromecast] Add padding bytes to the end of handshake packet

Audio packet can be sent after handshake packet, and thus handshake
packet must also has a multiple of 4 bytes to ensure alignment of audio
data.

Merge-With: eureka-internal/509721
Bug: internal: 176167293
Test: Add alignment check on receiver side and audio works fine.
Change-Id: I57343bf0238df64de47ec69ed5b64ed4eecd0a43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2607812
Commit-Queue: Kehuang Li <kehuangli@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840446}

chromecast/media/audio/capture_service/capture_service_receiver_unittest.cc

@@ -28,24 +28,26 @@ namespace capture_service {
 namespace {
 
 constexpr StreamInfo kStreamInfo =
-    StreamInfo{StreamType::kSoftwareEchoCancelled,
-               AudioCodec::kPcm,
-               1,
-               SampleFormat::PLANAR_FLOAT,
-               16000,
-               160};
-constexpr HandshakePacket kHandshakePacket =
-    HandshakePacket{0,
-                    static_cast<uint8_t>(MessageType::kHandshake),
-                    static_cast<uint8_t>(kStreamInfo.stream_type),
-                    static_cast<uint8_t>(kStreamInfo.audio_codec),
-                    static_cast<uint8_t>(kStreamInfo.sample_format),
-                    kStreamInfo.num_channels,
-                    kStreamInfo.frames_per_buffer,
-                    kStreamInfo.sample_rate};
-constexpr PcmPacketHeader kPcmAudioPacketHeader =
-    PcmPacketHeader{0, static_cast<uint8_t>(MessageType::kPcmAudio),
-                    static_cast<uint8_t>(kStreamInfo.stream_type), 0};
+    StreamInfo{.stream_type = StreamType::kSoftwareEchoCancelled,
+               .audio_codec = AudioCodec::kPcm,
+               .num_channels = 1,
+               .sample_format = SampleFormat::PLANAR_FLOAT,
+               .sample_rate = 16000,
+               .frames_per_buffer = 160};
+constexpr HandshakePacket kHandshakePacket = HandshakePacket{
+    .size = 0,  // dummy
+    .message_type = static_cast<uint8_t>(MessageType::kHandshake),
+    .stream_type = static_cast<uint8_t>(kStreamInfo.stream_type),
+    .audio_codec = static_cast<uint8_t>(kStreamInfo.audio_codec),
+    .sample_format = static_cast<uint8_t>(kStreamInfo.sample_format),
+    .num_channels = kStreamInfo.num_channels,
+    .num_frames = kStreamInfo.frames_per_buffer,
+    .sample_rate = kStreamInfo.sample_rate};
+constexpr PcmPacketHeader kPcmAudioPacketHeader = PcmPacketHeader{
+    .size = 0,  // dummy
+    .message_type = static_cast<uint8_t>(MessageType::kPcmAudio),
+    .stream_type = static_cast<uint8_t>(kStreamInfo.stream_type),
+    .timestamp_us = 0};
 
 class MockStreamSocket : public chromecast::MockStreamSocket {
  public:

chromecast/media/audio/capture_service/message_parsing_utils.cc

@@ -27,6 +27,10 @@ namespace {
 constexpr size_t kHandshakeHeaderBytes =
     sizeof(HandshakePacket) - sizeof(uint16_t);
 
+static_assert(sizeof(PcmPacketHeader) % 4 == 0,
+              "Size of PCM audio packet header must be a multiple of 4 bytes.");
+static_assert(sizeof(HandshakePacket) % 4 == 0,
+              "Size of handshake packet must be a multiple of 4 bytes.");
 static_assert(kPcmAudioHeaderBytes ==
                   sizeof(PcmPacketHeader) - sizeof(uint16_t),
               "Invalid message header size.");

chromecast/media/audio/capture_service/packet_header.h

@@ -23,8 +23,9 @@ struct __attribute__((__packed__)) PcmPacketHeader {
   int64_t timestamp_us;
 };
 
-// Memory block of a handshake packet. There is no size restriction for this
-// structure.
+// Memory block of a handshake packet. Audio packet may be sent after handshake
+// packet, and thus handshake packet must also have a multiple of 4 bytes, since
+// audio data must be aligned.
 struct __attribute__((__packed__)) HandshakePacket {
   uint16_t size;
   uint8_t message_type;
@@ -32,7 +33,9 @@ struct __attribute__((__packed__)) HandshakePacket {
   uint8_t audio_codec;
   uint8_t sample_format;
   uint8_t num_channels;
+  uint8_t padding_uint8;
   uint16_t num_frames;
+  uint16_t padding_uint16;
   uint32_t sample_rate;
 };