[BUG DEMO] invocation of cargo in workspace within build.rs causes hang in release

[vitiral]

I migrated my application to use a workspace because I have several repositories which depend on eachother. Unfortunately, this has lead to me hitting a bug where cargo hangs when attempting to compile the wasm frontend.

I have created a demo here to demonstrate the issue. the below state happens indefinitely, and does not happen in debug mode.

Suggested fix: use different file locks for different targets. Compiling x86_64-unknown-linux-gnu shouldn't block wasm32-unknown-unknown

$ cd ./demo
$ cargo build -p project_main
   Compiling project_main v0.1.0 (/home/rett/open/cargo/demo/project_main)
    Finished dev [unoptimized + debuginfo] target(s) in 0.57s
                                                                                                   
$ cargo build -p project_main --release -vvv
   Compiling project_main v0.1.0 (/home/rett/open/cargo/demo/project_main)
     Running `/home/rett/open/cargo/demo/target/release/build/project_main-f4448f50b4263e12/build-script-build`
[project_main 0.1.0] building via cargo-web
[project_main 0.1.0]     Blocking waiting for file lock on build directory
    Building [==================>                                        ] 1/3: project_main(build)

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dwijnand (or someone else) soon.

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[vitiral]

It looks like I am hitting something like #6412. However, with wasm frontends (probably) going to become more common this is no longer a "feature request for a really awkward use case." People will want to bundle their frontend (but use libraries that are related) and include_bytes! it, and they will want to do it all from their build.rs file.

Obviously a solution is to NOT use build.rs to build the application, but I don't think we want that :D

[vitiral]

Also, I can't simply remove project_frontend from the workspace because then I get this error:

$ cargo build
error: current package believes it's in a workspace when it's not:
current:   /home/rett/open/cargo/demo/project_wasm/Cargo.toml
workspace: /home/rett/open/cargo/demo/Cargo.toml

this may be fixable by adding `project_wasm` to the `workspace.members` array of the manifest located at: /home/rett/open/cargo/demo/Cargo.toml

[dwijnand]

Interesting!

Thanks for also linking the demo here in #6412.