Segfault when MultiplayerSynchronizer node has different authority on different peers

[celeste-sinead]

Godot version

v4.0.beta.custom_build.e6751549c

System information

Ubuntu 22.04, linux 5.15.0-52-generic

Issue description

If MultiplayerSynchronizer instances with the same path have different multiplayer authorities set on different peers, peers will crash with segmentation faults.

Backtrace:

Thread 1 "godot" received signal SIGSEGV, Segmentation fault.
decode_uint32 (p_arr=0x7fff92c8cc1c <error: Cannot access memory at address 0x7fff92c8cc1c>) at ./core/io/marshalls.h:166
166			uint32_t b = *p_arr;
(gdb) bt
#0  decode_uint32 (p_arr=0x7fff92c8cc1c <error: Cannot access memory at address 0x7fff92c8cc1c>) at ./core/io/marshalls.h:166
#1  SceneReplicationInterface::on_sync_receive (this=<optimized out>, p_from=<optimized out>, p_buffer=<optimized out>, p_buffer_len=<optimized out>) at modules/multiplayer/scene_replication_interface.cpp:655
#2  0x00005555567fe3f3 in SceneMultiplayer::poll (this=0x7fffa80106f0) at modules/multiplayer/scene_multiplayer.cpp:81
#3  0x000055555812bfa9 in SceneTree::process (this=0x555560288d60, p_time=0.018055555555555554) at scene/main/scene_tree.cpp:453
#4  0x0000555555ffbc73 in Main::iteration () at main/main.cpp:3170
#5  0x0000555555fb9d81 in OS_LinuxBSD::run (this=this@entry=0x7fffffffda20) at platform/linuxbsd/os_linuxbsd.cpp:776
#6  0x0000555555fab2de in main (argc=<optimized out>, argv=0x7fffffffdf68) at platform/linuxbsd/godot_linuxbsd.cpp:73

Steps to reproduce

1. Add a MutliplayerSynchronizer to a scene. Configure it to sync any variable.
2. Call set_multiplayer_authority(multiplayer.get_unique_id()) on the synchronizer node at some point (the repro project does it in _ready but I don't think it particularly matters).
3. Connect at least 2 peers.
4. Should segfault ~immediately. Which peer appears to be random.

Minimal reproduction project

sync_segfault.zip

[akien-mga]

Is this still reproducible in 4.0 RC 2 or later?

[celeste-sinead]

I can still reproduce this on RC2 (v4.0.rc.custom_build.d2699dc7a) using the attached minimal reproduction project.

[Faless]

I can still reproduce this on RC2 (v4.0.rc.custom_build.d2699dc7a) using the attached minimal reproduction project.

The issue was fixed via #73216 which didn't make it in time for RC2 but will be in RC3.