From d23d04163d77c0a1779722263601cbbb651de296 Mon Sep 17 00:00:00 2001
From: thebongy <rishitbansal0@gmail.com>
Date: Thu, 16 Jan 2025 19:41:42 +0530
Subject: [PATCH] Fix to allow small negative jumps due to out of order packets
 in packet buffer

This resolves an issue where when packets appear out of order at the
beginning of a stream, packet_buffer.cc might drop the entire packet
buffer because it detects a "large negative jump" even though the
difference in sequence numbers is very minor and is caused by network
congestion / packet re-ordering. Currently, when the issue occurs, this
can cause video corruption/artifacts. More details and reproduction is
available on the attached webrtc bug report 390329776.

Bug: webrtc:390329776
Change-Id: Idb56eb2e066d596d8afd7ec904359baf0cb3feef
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/374540
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#43753}
---
 AUTHORS                                        |  1 +
 modules/video_coding/packet_buffer.cc          |  3 ++-
 modules/video_coding/packet_buffer_unittest.cc | 13 +++++++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/AUTHORS b/AUTHORS
index 46587ac6f3..b4e0e330fc 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -114,6 +114,7 @@ Ralph Giles <giles@ghostscript.com>
 Raman Budny <budnyjj@gmail.com>
 Ramprakash Jelari <ennajelari@gmail.com>
 Riku Voipio <riku.voipio@linaro.org>
+Rishit Bansal <rishitbansal0@gmail.com>
 Robert Bares <robert@bares.me>
 Robert Mader <robert.mader@collabora.com>
 Robert Mader <robert.mader@posteo.de>
diff --git a/modules/video_coding/packet_buffer.cc b/modules/video_coding/packet_buffer.cc
index df53e81685..718c4a86b9 100644
--- a/modules/video_coding/packet_buffer.cc
+++ b/modules/video_coding/packet_buffer.cc
@@ -82,7 +82,8 @@ PacketBuffer::InsertResult PacketBuffer::InsertPacket(
       return result;
     }
 
-    if (ForwardDiff<uint16_t>(first_seq_num_, seq_num) >= max_size_) {
+    if (ForwardDiff<uint16_t>(first_seq_num_, seq_num) >= max_size_ &&
+        ForwardDiff<uint16_t>(seq_num, first_seq_num_) >= max_size_ / 2) {
       // Large negative jump in rtp sequence number: clear the buffer and treat
       // latest packet as the new first packet.
       Clear();
diff --git a/modules/video_coding/packet_buffer_unittest.cc b/modules/video_coding/packet_buffer_unittest.cc
index 6c05d09277..2f4d808d1b 100644
--- a/modules/video_coding/packet_buffer_unittest.cc
+++ b/modules/video_coding/packet_buffer_unittest.cc
@@ -356,6 +356,19 @@ TEST_F(PacketBufferTest, FramesReordered) {
               StartSeqNumsAre(seq_num + 2));
 }
 
+TEST_F(PacketBufferTest, FramesReorderedReconstruction) {
+  Insert(100, kKeyFrame, kFirst, kNotLast, {}, 2);
+
+  Insert(98, kKeyFrame, kFirst, kNotLast, {}, 1);
+  EXPECT_THAT(Insert(99, kDeltaFrame, kNotFirst, kLast, {}, 1),
+              StartSeqNumsAre(98));
+
+  // Ideally frame with timestamp 2, seq No 100 should be
+  // reconstructed here from the first Insert() call in the test
+  EXPECT_THAT(Insert(101, kDeltaFrame, kNotFirst, kLast, {}, 2),
+              StartSeqNumsAre(100));
+}
+
 TEST_F(PacketBufferTest, InsertPacketAfterSequenceNumberWrapAround) {
   int64_t kFirstSeqNum = 0;
   uint32_t kTimestampDelta = 100;