webgl2: Mesh without triangles panics

[rparrett]

Bevy version

0.15.0-rc.3

Relevant system information

AdapterInfo { name: "ANGLE (Apple, ANGLE Metal Renderer: Apple M1 Max, Unspecified Version)", vendor: 4203, device: 0, device_type: IntegratedGpu, driver: "", driver_info: "WebGL 2.0 (OpenGL ES 3.0 Chromium)", backend: Gl }

What you did

I experienced this panic in the wild while migrating an app that uses a lot of Mesh2d by way of bevy_prototype_lyon. It seems that the root cause is creating a Line with zero length in that plugin.

Minimal repro for Bevy 0.15

use bevy::{asset::RenderAssetUsages, prelude::*, render::mesh::PrimitiveTopology};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    commands.spawn(Camera2d);

    let mesh = Mesh::new(
        PrimitiveTopology::TriangleList,
        RenderAssetUsages::default(),
    );

    commands.spawn((
        Mesh2d(meshes.add(mesh)),
        MeshMaterial2d(materials.add(Color::hsl(180.0, 0.95, 0.7))),
    ));
}

Bevy 0.14 code tested

use bevy::{
    prelude::*,
    render::{mesh::PrimitiveTopology, render_asset::RenderAssetUsages},
    sprite::{MaterialMesh2dBundle, Mesh2dHandle},
};

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Startup, setup)
        .run();
}

fn setup(
    mut commands: Commands,
    mut meshes: ResMut<Assets<Mesh>>,
    mut materials: ResMut<Assets<ColorMaterial>>,
) {
    commands.spawn(Camera2dBundle::default());

    let mesh = Mesh::new(
        PrimitiveTopology::TriangleList,
        RenderAssetUsages::default(),
    );

    let color = Color::hsl(180., 0.95, 0.7);

    commands.spawn(MaterialMesh2dBundle {
        mesh: Mesh2dHandle(meshes.add(mesh)),
        material: materials.add(color),
        ..default()
    });
}

The exact type of mesh generated by a zero-length line w/ bevy_prototype_lyon (same panic)

let mesh = Mesh::new(
    PrimitiveTopology::TriangleList,
    RenderAssetUsages::default(),
)
.with_inserted_attribute(Mesh::ATTRIBUTE_POSITION, Vec::<[f32; 3]>::new())
.with_inserted_attribute(Mesh::ATTRIBUTE_COLOR, Vec::<[f32; 4]>::new())
.with_inserted_indices(Indices::U32(vec![]));

cargo run -p build-wasm-example -- --api webgl2 empty_mesh
basic-http-server examples/wasm/

What went wrong

wasm_example.js:1951 panicked at /Users/me/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wgpu-23.0.1/src/api/buffer.rs:684:9:
slice offset 0 is out of range for buffer of size 0

Stack:

Error
    at imports.wbg.__wbg_new_abda76e883ba8a5f (http://127.0.0.1:4000/target/wasm_example.js:1935:21)
    at sprite-e09298d3df91ece6.wasm.__wbg_new_abda76e883ba8a5f externref shim (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[87386]:0x22b19ec)
    at sprite-e09298d3df91ece6.wasm.console_error_panic_hook::hook::ha8c6c1fd53fdbabe (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[16852]:0x1840f09)
    at sprite-e09298d3df91ece6.wasm.core::ops::function::Fn::call::h4a1d224ff1d85143 (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[135054]:0x23915f6)
    at sprite-e09298d3df91ece6.wasm.std::panicking::rust_panic_with_hook::he5c089ac7305193e (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[35603]:0x1ee83b4)
    at sprite-e09298d3df91ece6.wasm.std::panicking::begin_panic_handler::{{closure}}::h010c94f3a1c5c766 (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[41310]:0x2000557)
    at sprite-e09298d3df91ece6.wasm.std::sys::backtrace::__rust_end_short_backtrace::hbe714695da4edadc (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[131865]:0x238a269)
    at sprite-e09298d3df91ece6.wasm.rust_begin_unwind (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[52617]:0x21540ee)
    at sprite-e09298d3df91ece6.wasm.core::panicking::panic_fmt::hdc8d2d914c0710e4 (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[54160]:0x2173e57)
    at sprite-e09298d3df91ece6.wasm.wgpu::api::buffer::check_buffer_bounds::h3364e293f2fdb994 (http://127.0.0.1:4000/target/wasm_example_bg.wasm:wasm-function[30214]:0x1d84ec2)

Additional information

This is a silly thing to be trying to do, but it did not panic in Bevy 0.14.

Possibly related to https://github.com/gfx-rs/wgpu/issues/3170#issuecomment-2491549161

webgpu and native on m1 mac seem to work okay.

[yshngg]

I ran your code on my Windows computer and a panic also occurred. However, it was due to another reason, which is as follows.

thread 'Compute Task Pool (7)' panicked at C:\Users\decli\.cargo\registry\src\index.crates.io-6f17d22bba15001f\bevy_render-0.15.0-rc.3\src\mesh\allocator.rs:656:48:
attempt to divide by zero
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Encountered a panic in system `bevy_render::mesh::allocator::allocate_and_free_meshes`!

Besides, this panic only occurs in version 0.15.0-rc.3.

[rparrett]

This new mesh allocator panic seems to be from me minimizing the repro excessively. The initial panic I experienced happens when there are some vertex attributes/indices, but they are empty vecs. This new panic happens when there are no vertex attributes or indices at all.

I did some additional testing of both scenarios on mac and windows, and edited the PR description with both code snippets I'm using to create the meshes.

platform	mesh attributes	result
win/nvidia/webgl2/chrome	No	Panic (bevy mesh allocator)
win/nvidia/webgl2/chrome	Yes	OK
win/nvidia/native	No	Panic (bevy mesh allocator)
win/nvidia/native	Yes	OK
m1/webgl2/chrome	No	Panic (wgpu buffer)
m1/webgl2/chrome	Yes	Panic (wgpu buffer)
m1/native	No	Panic (bevy mesh allocator)
m1/native	Yes	OK

Both of these scenarios did not panic in 0.14.2.

[BenjaminBrienen]

Is this a regression? Should it block 0.15?

[rparrett]

This is a regression and I don't think we should panic in either case.

Some considerations:

• I could see real situations where generating a mesh with attributes that are empty vecs would could be more convenient than removing a mesh.
• The panics are likely to be pretty mysterious to users
• But this is borderline user-error, and can be worked around by not spawning / generating these invalid meshes.

IMO, this shouldn't block 0.15.