LD_LIBRARY_PATH contains an empty entry during build

[glandium]

Originally filed as rust-lang/rust#52693 but as far as I can tell, this is actually cargo's doing:

I was looking at the output of x.py with -vv for $reasons, and noticed that commands run with LD_LIBRARY_PATH set to multiple paths, separated with a colon, which is normal, but the last character is a colon, which effectively adds the current working directory in the list of paths used. This is generally considered bad practice.

FWIW, when LD_LIBRARY_PATH is set before running x.py, its original value ends up at the end of the LD_LIBRARY_PATH that had a trailing colon.

Cc: @alexcrichton

[alexcrichton]

I think the bug here is actually these lines --

https://github.com/rust-lang/rust/blob/45b48b9b6d70065a16cc119fe934ed342c664c78/src/bootstrap/bootstrap.py#L613-L621

Mind poking at those and seeing if it fixes the issue? If so mind refiling at rust-lang/rust? (or I can reopen the previous bug)

[glandium]

I tried that already, and this didn't change anything. Which is why I moved it here.

[ExpHP]

First, an important premise: I believe the standard behavior of most applications (at least in the UNIX world) is to treat PATH="" as equivalent to an unset PATH...and likewise for any other PATH-like environment variable.

If this premise is taken to be false, then as far as I can tell there is no problem.

...but! If this premise is taken to be true, then:

---

The lines in bootstrap.py are clearly wrong in the case where env["LD_LIBRARY_PATH"] == ''. and that issue should be reopened. However, judging from their context, it likely only affects the build of the bootstrap crate.

---

...and cargo (or the standard library) is also doing things a bit odd, as is easily demonstrated with this:

use std::env;
fn main() {
    println!("{:?}",
        env::split_paths(&env::var("LD_LIBRARY_PATH").unwrap()).collect::<Vec<_>>()
    );
}

(unset LD_LIBRARY_PATH && cargo run) (a truly-unset variable)

[
    "/home/lampam/cpp/throwaway/env/target/debug",
    "/home/lampam/cpp/throwaway/env/target/debug/deps",
    "/home/lampam/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib",
    "/home/lampam/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib"
]

LD_LIBRARY_PATH= cargo run (merely an empty string)

[
    "/home/lampam/cpp/throwaway/env/target/debug",
    "/home/lampam/cpp/throwaway/env/target/debug/deps",
    "/home/lampam/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib",
    "/home/lampam/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib",
    ""
]

If I had to guess, this is because cargo itself uses env::split_paths, and env::split_paths("") produces [""].