Calling MultiplayerAPI in a thread results in crashes.

[Leshy-YA]

Godot version

4.1

System information

Godot v4.1.stable - Fedora Linux 38 (KDE Plasma) - Vulkan (Forward+) - integrated Intel(R) Xe Graphics (TGL GT2) () - 11th Gen Intel(R) Core(TM) i7-11370H @ 3.30GHz (8 Threads)

Issue description

Whenever a MultiplayerAPI call is made in a function running in a thread, that thread will crash.

Issues started occurring in 4.1 in all 4.0 releases this worked fine.

Steps to reproduce

Start a thread.
Call multiplayer.get_unique_id() in that function.
The function will exit immediately.

NOTE: No debug information will be provided due to: #18330

Minimal reproduction project

multiplayer_thread_test.zip

[AThousandShips]

Multiplayer is only supported from the main thread, crash probably caused by manipulating the invalid result from get_multiplayer

• Related: [4.1.dev.3] Multiplayer game crash: multiplayer can only be manipulated from the main thread #77707

[Leshy-YA]

Multiplayer is only supported from the main thread (...)

I can't find any information in the docs of that being the case. Is that by design or just a long-standing issue?

[AThousandShips]

By design, see the linked issue

APIs are non-thread safe unless stated otherwise, see here

Including this:

Interacting with the active scene tree is NOT thread-safe.

[AThousandShips]

Should probably be more clearly documented though

[Leshy-YA]

APIs are non-thread safe unless stated otherwise, see here

Oh ok! It would have been useful to have that also in the MultiplayerAPI doc.
Thanks for the info!

[AThousandShips]

To clarify, the crash comes not from the thread safety as such, but from accessing methods on multiplayer when that is null, fixing the errors on threads, and documenting this should be the plan IMO

Not a regression as the change to enforce thread safety was intentional, see #75901

Will take a look at documenting this fact

Edit: Note that this crash would also happen if the node was not in the tree

[Chaosus]

Multiplayer is only supported from the main thread

IMO, even if it's not possible it shouldn't crash, instead an error should be printed to console/output.

[AThousandShips]

The problem is that if you fetch the multiplayer and then call something on it how do you avoid a crash then? The returned value from multiplayer will be null

You'll get an error but still a crash

[AThousandShips]

Calling things like is_multiplayer_authority used to crash but was fixed, but since it's up to the user to add null checks we can't fix user code that doesn't do that with multiplayer

I attempted to document this in the class reference but it wasn't approved, and didn't end up figuring out a good place to document it in the manual, but if someone could do that it'd be useful to add to it

Had forgotten that the decision was to close this after the PR was closed but didn't end up happening, so will close this and if improving documentation in the manual is desired it can be tracked over in godot-docs

But there are plenty of other methods that return null when called on a thread (including such fundamental methods as get_child) so not sure this is necessary, there is an error message printed if on the thread so it would be clear still