Cargo build seems to hang on Arch

[dcellucci]

I am building opencv on an Arch VM, and I am having trouble getting the build to complete.
I don't get an error, but the build seems to hang at this point:

[opencv 0.70.0] === Waiting until the binding-generator binary is built...
[opencv 0.70.0]    Compiling libc v0.2.134
[opencv 0.70.0]    Compiling glob v0.3.0
[opencv 0.70.0]    Compiling memchr v2.5.0
[opencv 0.70.0]    Compiling clang-sys v1.4.0
[opencv 0.70.0]    Compiling cfg-if v1.0.0
[opencv 0.70.0]    Compiling libloading v0.7.3
[opencv 0.70.0]    Compiling aho-corasick v0.7.19
[opencv 0.70.0]    Compiling regex-syntax v0.6.27
[opencv 0.70.0]    Compiling regex v1.6.0
[opencv 0.70.0]    Compiling clang v2.0.0
[opencv 0.70.0]    Compiling percent-encoding v2.2.0
[opencv 0.70.0]    Compiling dunce v1.0.2
[opencv 0.70.0]    Compiling once_cell v1.15.0
[opencv 0.70.0]    Compiling maplit v1.0.2
[opencv 0.70.0]    Compiling opencv-binding-generator v0.48.0
[opencv 0.70.0]     Finished release [optimized] target(s) in 1m 18s
Building [=======================>   ] 34/37: opencv(build)

I installed the base packages using

pacman -S clang qt5-base opencv

and I built using

RUST_BACKTRACE=full cargo build -vv 

The full output is located here:
https://gist.github.com/dcellucci/fecbdbdaabc8c68981eef6d3c5b2d436

[twistedfall]

That's quite weird, can you please try doing a cargo clean beforehand and if that doesn't help try using the version 0.69.0 of the crate?

[dcellucci]

I have been running cargo clean each time, no help. Also downgrading to 0.69.0 didn't work.

[dcellucci]

ok after taking a closer look at the build process it seems like it's hanging on this line

let token = job_server.acquire().expect("Can't acquire token from job server");        

the thread is locked until it can acquire a token from the job server, so it appears to freeze. I will look into a solution.

UPDATE:
After reviewing jobserver generation code, I had to manually create a job server instead of relying on from_env(). A manually created jobserver successfully produces tokens and the rest of the build process seems to proceed without issues.

[twistedfall]

Thank you for the investigation, that's really helpful! Can you please tell me how exactly you created the jobserver manually?

[dcellucci]

Yes - sorry for not specifying that!

In build.rs, lines 228-238

opencv-rust/build.rs

Lines 228 to 238 in 007de72

	fn build_job_server() -> Option<jobserver::Client> {
	unsafe { jobserver::Client::from_env() }
	.or_else(|| {
	let num_jobs = env::var("NUM_JOBS").ok()
	.and_then(|jobs| jobs.parse().ok())
	.unwrap_or(2)
	.max(1);
	eprintln!("=== Creating a new job server with num_jobs: {}", num_jobs);
	jobserver::Client::new(num_jobs).ok()
	})
	}

Calling ::from_env() returns a client, but that client always has 0 jobs. Instead creating a new client from scratch seems to work fine. I did this by modifying the function to explicitly call the or_else code block. Not sure if this is a jobserver problem or what.

[twistedfall]

This should be fixed in 0.74.1