From c965bb77bd3fa16e3f4bcd947bbb347dcf192733 Mon Sep 17 00:00:00 2001
From: sfan5 <sfan5@live.de>
Date: Wed, 15 Oct 2025 00:21:07 +0200
Subject: [PATCH] Fix possible `sendPlayerPos` desync situation (#16498)

---
 src/client/client.cpp | 22 ++++++++++++++++------
 src/client/client.h   |  1 +
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/src/client/client.cpp b/src/client/client.cpp
index 2e585ea2f..78509baee 100644
--- a/src/client/client.cpp
+++ b/src/client/client.cpp
@@ -566,9 +566,8 @@ void Client::step(float dtime)
 	{
 		float &counter = m_playerpos_send_timer;
 		counter += dtime;
-		if((m_state == LC_Ready) && (counter >= m_recommended_send_interval))
-		{
-			counter = 0.0;
+		if (m_state == LC_Ready && counter >= m_recommended_send_interval) {
+			counter = 0;
 			sendPlayerPos();
 		}
 	}
@@ -1403,7 +1402,7 @@ void Client::sendPlayerPos()
 	f32 movement_speed = player->control.movement_speed;
 	f32 movement_dir = player->control.movement_direction;
 
-	if (
+	bool identical = (
 			player->last_position        == player->getPosition() &&
 			player->last_speed           == player->getSpeed()    &&
 			player->last_pitch           == player->getPitch()    &&
@@ -1413,8 +1412,19 @@ void Client::sendPlayerPos()
 			player->last_camera_inverted == camera_inverted       &&
 			player->last_wanted_range    == wanted_range          &&
 			player->last_movement_speed  == movement_speed        &&
-			player->last_movement_dir    == movement_dir)
-		return;
+			player->last_movement_dir    == movement_dir);
+
+	if (identical) {
+		// Since the movement info is sent non-reliable an unfortunate desync might
+		// occur if we stop sending and the last packet gets lost or re-ordered.
+		// To make this situation less likely we stop sending duplicate packets
+		// only after a delay.
+		m_playerpos_repeat_count++;
+		if (m_playerpos_repeat_count >= 5)
+			return;
+	} else {
+		m_playerpos_repeat_count = 0;
+	}
 
 	player->last_position        = player->getPosition();
 	player->last_speed           = player->getSpeed();
diff --git a/src/client/client.h b/src/client/client.h
index 4b36472f9..d21618e7a 100644
--- a/src/client/client.h
+++ b/src/client/client.h
@@ -478,6 +478,7 @@ private:
 	float m_connection_reinit_timer = 0.1f;
 	float m_avg_rtt_timer = 0.0f;
 	float m_playerpos_send_timer = 0.0f;
+	int m_playerpos_repeat_count = 0;
 	IntervalLimiter m_map_timer_and_unload_interval;
 
 	IWritableTextureSource *m_tsrc;