Concurrent SPIR-V codegen building causes issues

[nazar-pc]

When building multiple crates that depend on SPIR-V codegen, compilation can, unfortunately, fail.

The error is not very descriptive, but I think it is caused by some kind of race:

  Error: `rustc_codegen_spirv` build failed

https://github.com/nazar-pc/abundance/actions/runs/28211711059/job/83574463880

It'd be great if necessary locking was used such that this doesn't cause issues.

The reason I'm now hitting it is because I'm using parallel steps in CI with different target directories concurrently, so SPIR-V codegen apparently ends up being compiled multiple times unnecessarily (it is normally cached, hence only needs to be compiled once) and that process breaks somewhere along the way.

[nazar-pc]

In CI I had to build shader crate separately from everything else, which essentially defeated the parallelism and I think made things a bit worse even.

[Firestar99]

Here's what cargo-gpu is doing:

• figure out the spirv-std version in your project
• [race starts]
• if a rustc_codegen_spirv.so is not found of that version:
  • create a dummy crate depending on rustc_codegen_spirv
  • cargo metadata creates Cargo.lock and makes cargo download rustc_codegen_spirv
  • read rustc nightly from downloaded rustc_codegen_spirv
  • delete Cargo.lock
  • cargo +nightly-XX build
  • move rustc_codegen_spirv.so into correct dir
  • [race ends]
  • delete target
• build shader crates

The dirty fix would be running cargo gpu install --shader-crate <path> before you initiate the full build, so the if condition is always skipped and no build fails. But I'll go ahead and add a lock mechanism for cargo-gpu right now.

[Firestar99]

#622 should fix it
(This has been on my personal todo list for quite some time, since most of my personal projects have two shader crates and the first build always fails due to this)