Allow empty video layer allocation extension

This patch adds support for sending zero video layer allocations
header extensions. This can be used to signal that a stream is
turned off.

Bug: webrtc:12000
Change-Id: Id18fbbff2216ca23179c58ef7bbe2ebea5e242af
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212743
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33541}

modules/rtp_rtcp/source/rtp_video_layers_allocation_extension.cc

@@ -201,17 +201,21 @@ SpatialLayersBitmasks SpatialLayersBitmasksPerRtpStream(
 //     Encoded (width - 1), 16-bit, (height - 1), 16-bit,  max frame rate 8-bit
 //     per spatial layer per RTP stream.
 //     Values are stored in (RTP stream id, spatial id) ascending order.
+//
+// An empty layer allocation (i.e nothing sent on ssrc) is encoded as
+// special case with a single 0 byte.
 
 bool RtpVideoLayersAllocationExtension::Write(
     rtc::ArrayView<uint8_t> data,
     const VideoLayersAllocation& allocation) {
-  if (allocation.active_spatial_layers.empty()) {
-    return false;
-  }
-
   RTC_DCHECK(AllocationIsValid(allocation));
   RTC_DCHECK_GE(data.size(), ValueSize(allocation));
 
+  if (allocation.active_spatial_layers.empty()) {
+    data[0] = 0;
+    return true;
+  }
+
   SpatialLayersBitmasks slb = SpatialLayersBitmasksPerRtpStream(allocation);
   uint8_t* write_at = data.data();
   // First half of the header byte.
@@ -276,10 +280,18 @@ bool RtpVideoLayersAllocationExtension::Parse(
   if (data.empty() || allocation == nullptr) {
     return false;
   }
+
+  allocation->active_spatial_layers.clear();
+
   const uint8_t* read_at = data.data();
   const uint8_t* const end = data.data() + data.size();
 
-  allocation->active_spatial_layers.clear();
+  if (data.size() == 1 && *read_at == 0) {
+    allocation->rtp_stream_index = 0;
+    allocation->resolution_and_frame_rate_is_valid = true;
+    return true;
+  }
+
   // Header byte.
   allocation->rtp_stream_index = *read_at >> 6;
   int num_rtp_streams = 1 + ((*read_at >> 4) & 0b11);
@@ -374,7 +386,7 @@ bool RtpVideoLayersAllocationExtension::Parse(
 size_t RtpVideoLayersAllocationExtension::ValueSize(
     const VideoLayersAllocation& allocation) {
   if (allocation.active_spatial_layers.empty()) {
-    return 0;
+    return 1;
   }
   size_t result = 1;  // header
   SpatialLayersBitmasks slb = SpatialLayersBitmasksPerRtpStream(allocation);

modules/rtp_rtcp/source/rtp_video_layers_allocation_extension_unittest.cc

@@ -19,15 +19,33 @@
 namespace webrtc {
 namespace {
 
-TEST(RtpVideoLayersAllocationExtension,
-     WriteEmptyLayersAllocationReturnsFalse) {
+TEST(RtpVideoLayersAllocationExtension, WriteEmptyLayersAllocationReturnsTrue) {
   VideoLayersAllocation written_allocation;
   rtc::Buffer buffer(
       RtpVideoLayersAllocationExtension::ValueSize(written_allocation));
-  EXPECT_FALSE(
+  EXPECT_TRUE(
       RtpVideoLayersAllocationExtension::Write(buffer, written_allocation));
 }
 
+TEST(RtpVideoLayersAllocationExtension,
+     CanWriteAndParseLayersAllocationWithZeroSpatialLayers) {
+  // We require the resolution_and_frame_rate_is_valid to be set to true in
+  // order to send an "empty" allocation.
+  VideoLayersAllocation written_allocation;
+  written_allocation.resolution_and_frame_rate_is_valid = true;
+  written_allocation.rtp_stream_index = 0;
+
+  rtc::Buffer buffer(
+      RtpVideoLayersAllocationExtension::ValueSize(written_allocation));
+  EXPECT_TRUE(
+      RtpVideoLayersAllocationExtension::Write(buffer, written_allocation));
+
+  VideoLayersAllocation parsed_allocation;
+  EXPECT_TRUE(
+      RtpVideoLayersAllocationExtension::Parse(buffer, &parsed_allocation));
+  EXPECT_EQ(written_allocation, parsed_allocation);
+}
+
 TEST(RtpVideoLayersAllocationExtension,
      CanWriteAndParse2SpatialWith2TemporalLayers) {
   VideoLayersAllocation written_allocation;