"Borrow" `Dxc` (lifetime) inside `DxcCompiler` and `DxcLibrary`

[Elabajaba]

update: This is due to dropping the Dxc object while holding onto the DxcCompiler and DxcLibrary objects and trying to use them after Dxc was dropped.

I've been trying to get this to work with wgpu, but I'm stuck trying to get it to create the DxcBlobEncoding to pass to DxcCompiler.compile.

It fails with code 0xc0000005: Invalid access to memory location. (os error 998) when calling

let blob = dxc_library
    .create_blob_with_encoding_from_str(&source);

The exception is in https://docs.rs/crate/com-rs/0.2.1/source/src/macros.rs in the innermost part of

        impl $iface {
            $($(#[$fn_attr])*
            pub unsafe fn $func(&self, $($i: $t),*) -> $rt {
                ((*self.vtable).$func)(self $(,$i)*)
            })*
        }

Passing the same &source to hassle_rs::utils::compile_hlsl works.

edit:edit: Link to WIP where the crash occurs

[MarijnS95]

Do you have your hassle-rs version matched up to the DXC library version, as per the readme? Is this occuring on Windows, Linux, or both?

[Elabajaba]

Turns out I wasn't storing the Dxc object (from Dxc::new). I was just creating the DxcCompiler and DxcLibrary and storing those, leading to dxc getting dropped.

On another note, are DxcCompiler and DxcLibrary thread safe? Should I be creating one of each at program start and reusing them, or should I create a new one every time I compile a new shader?

[MarijnS95]

Turns out I wasn't storing the Dxc object (from Dxc::new). I was just creating the DxcCompiler and DxcLibrary and storing those, leading to dxc getting dropped.

Ah yes, perhaps these should have a borrow or Arc or other lifetime mechanism to prevent the underlying libloading::Library from being dropped and the pointers loaded from it to become invalid.

On another note, are DxcCompiler and DxcLibrary thread safe? Should I be creating one of each at program start and reusing them, or should I create a new one every time I compile a new shader?

For that I defer you to microsoft/DirectXShaderCompiler#4818.

[Jasper-Bekkers]

Turns out I wasn't storing the Dxc object (from Dxc::new). I was just creating the DxcCompiler and DxcLibrary and storing those, leading to dxc getting dropped.

@MarijnS95 maybe we should make those constructors private to prevent these kinds of mistakes

On another note, are DxcCompiler and DxcLibrary thread safe? Should I be creating one of each at program start and reusing them, or should I create a new one every time I compile a new shader?

Yes - we are using it in a multi-threaded fashion without problems other then the one @MarijnS95 pointed out (which is linux specific).

[MarijnS95]

@Jasper-Bekkers it's not a faulty constructor that's wrongly called by the user, it is that these create_* constructors (implementation, returned object) don't set up a lifetime reference properly on Dxc. There's no other way to use these objects (other than going through utils::compile_hlsl() which is rather limited in functionality).