VP9 parser: undo r34393 and fix incorrect return statement.

Some code was deleted in
https://webrtc-review.googlesource.com/c/src/+/224266/2/modules/video_coding/utility/vp9_uncompressed_header_parser.cc
since it was detected as unreachable.
The root cause was an early return that should have been a
RETURN_IF_FALSE(x).

Bug: webrtc:12924
Change-Id: Ifadded9bbb4748d56cf65c30fd8f87e92fde10d8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/225040
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34422}

modules/video_coding/BUILD.gn

@@ -985,6 +985,7 @@ if (rtc_include_tests) {
       "utility/qp_parser_unittest.cc",
       "utility/quality_scaler_unittest.cc",
       "utility/simulcast_rate_allocator_unittest.cc",
+      "utility/vp9_uncompressed_header_parser_unittest.cc",
       "video_codec_initializer_unittest.cc",
       "video_receiver_unittest.cc",
     ]

modules/video_coding/utility/vp9_uncompressed_header_parser.cc

@@ -300,7 +300,12 @@ bool Vp9ReadQp(BitstreamReader* br, FrameInfo* frame_info) {
 }
 
 bool Vp9ReadSegmentationParams(BitstreamReader* br) {
-  return br->IfNextBoolean([&] {    // segmentation_enabled
+  constexpr int kVp9MaxSegments = 8;
+  constexpr int kVp9SegLvlMax = 4;
+  constexpr int kSegmentationFeatureBits[kVp9SegLvlMax] = {8, 6, 2, 0};
+  constexpr bool kSegmentationFeatureSigned[kVp9SegLvlMax] = {1, 1, 0, 0};
+
+  RETURN_IF_FALSE(br->IfNextBoolean([&] {  // segmentation_enabled
     return br->IfNextBoolean([&] {  // update_map
       // Consume probs.
       for (int i = 0; i < 7; ++i) {
@@ -316,6 +321,19 @@ bool Vp9ReadSegmentationParams(BitstreamReader* br) {
         return true;
       });
     });
+  }));
+
+  return br->IfNextBoolean([&] {
+    RETURN_IF_FALSE(br->ConsumeBits(1));  // abs_or_delta
+    for (int i = 0; i < kVp9MaxSegments; ++i) {
+      for (int j = 0; j < kVp9SegLvlMax; ++j) {
+        RETURN_IF_FALSE(br->IfNextBoolean([&] {  // feature_enabled
+          return br->ConsumeBits(kSegmentationFeatureBits[j] +
+                                 kSegmentationFeatureSigned[j]);
+        }));
+      }
+    }
+    return true;
   });
 }
 

modules/video_coding/utility/vp9_uncompressed_header_parser_unittest.cc

@@ -0,0 +1,47 @@
+/*
+ *  Copyright (c) 2021 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "modules/video_coding/utility/vp9_uncompressed_header_parser.h"
+
+#include "test/gmock.h"
+#include "test/gtest.h"
+
+namespace webrtc {
+namespace vp9 {
+
+TEST(Vp9UncompressedHeaderParserTest, FrameWithSegmentation) {
+  // Uncompressed header from a frame generated with libvpx.
+  // Encoded QVGA frame (SL0 of a VGA frame) that includes a segmentation.
+  const uint8_t kHeader[] = {
+      0x87, 0x01, 0x00, 0x00, 0x02, 0x7e, 0x01, 0xdf, 0x02, 0x7f, 0x01, 0xdf,
+      0xc6, 0x87, 0x04, 0x83, 0x83, 0x2e, 0x46, 0x60, 0x20, 0x38, 0x0c, 0x06,
+      0x03, 0xcd, 0x80, 0xc0, 0x60, 0x9f, 0xc5, 0x46, 0x00, 0x00, 0x00, 0x00,
+      0x2e, 0x73, 0xb7, 0xee, 0x22, 0x06, 0x81, 0x82, 0xd4, 0xef, 0xc3, 0x58,
+      0x1f, 0x12, 0xd2, 0x7b, 0x28, 0x1f, 0x80, 0xfc, 0x07, 0xe0, 0x00, 0x00};
+
+  absl::optional<FrameInfo> frame_info =
+      ParseIntraFrameInfo(kHeader, sizeof(kHeader));
+  // Segmentation info is not actually populated in FrameInfo struct, but it
+  // needs to be parsed otherwise we end up on the wrong offset. The check for
+  // segmentation is thus that we have a valid return value.
+  ASSERT_TRUE(frame_info.has_value());
+
+  EXPECT_EQ(frame_info->is_keyframe, false);
+  EXPECT_EQ(frame_info->error_resilient, true);
+  EXPECT_EQ(frame_info->show_frame, true);
+  EXPECT_EQ(frame_info->base_qp, 185);
+  EXPECT_EQ(frame_info->frame_width, 320);
+  EXPECT_EQ(frame_info->frame_height, 240);
+  EXPECT_EQ(frame_info->render_width, 640);
+  EXPECT_EQ(frame_info->render_height, 480);
+}
+
+}  // namespace vp9
+}  // namespace webrtc