Summary

I'm building an internal system (hardware simulator, using Rust) to help test some Python-based services that talk to hardware. (The services talk to hardware via TTYs.) To trick the Python services into "believing" they're talking to the hardware they expect, I create some PTYs, where the master side is used by my simulator and the slave side of the PTY is given over to the Python-based service.

Since the Python service looks for its PTY by using a specific name, I create a symbolic link that points back to the slave PTY mentioned above, but using the name the Python service expects --or so I thought.

The problem is that the symlink function reports it's being successful, but no link actually exists in the file system. I'm not yet sure what I may be missing.

My question is: Does anyone know why this is happening and how to fix it?

Details

I'm using the std::os::unix::fs::symlink function to create the sym-link. The function's documentation is clear about its usage, e.g. symlink("a.txt", "b.txt") would create a sym-link called b.txt that points back to a.txt. (Assuming a.txt already exists.)

When I use the function in the simulator, I observe the following:

1. The function call returns successfully, and
2. No actual sym-link can be found in the file system.

I wrote a simple test program (below) to verify my usage, and it works fine:

use std::os::unix::fs::symlink;

fn main() {
    symlink("/tmp/src.txt", "/tmp/dst.txt").expect("symlink failed");
}

The above shows that I'm using it correctly, with dst.txt being the link that points back to src.txt. Also, the above would've failed if I had the arguments inverted or didn't have the correct permissions, so it's not that, either.

I also ran the simulator with strace to check what the lower-level syscall was actually doing (e.g. maybe that was failing but the Rust std library was not handling the error?), but it shows a successful return code:

symlink("/dev/pts/5", "/home/<user>/Projects/vpanel/ttymxc4") = 0

This is despite the fact that the symlink /home/<user>/Projects/vpanel/ttymxc4 does not really exist. A simple visual inspection with the ls command in the above directory does not show the file, the Python service complains that it cannot find the symlink to its PTY, and the simulator itself reports a panic! when it tries to clean up after itself but fails to find the symlink to remove it:

Drop for ServiceProxy { pty: OpenptyResult { master: 9, slave: 10 }, fspath: "/home/<user>/Projects/vpanel/ttymxc4", timeout: 8s }
thread 'tokio-runtime-worker' panicked at 'remove_file failed: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/proxy.rs:262:35
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
task join failed: JoinError::Panic(...)

The backtrace is not very helpful.

Reference Code

This is what the relevant code looks like in the actual simulator:

// main.rs
#[tokio::main]
async fn main() -> Result<(), Error> {
    let m = App::from(load_yaml!("cli.yaml")).get_matches();
    let ptys: Vec<&str> = m
        .values_of("ptys")
        .expect("Missing PTY paths")
        .collect();

    let proxies = ptys
        .iter()
        .map(|pty| ServiceProxy::new(*pty))
        .collect();

    // ...
}

// proxy.rs
impl ServiceProxy {
    pub fn new(symlink_path: &str) -> Self {
        let pty = openpty(None, None).expect("openpty failed");
        let cstr = unsafe { CStr::from_ptr(ttyname(pty.slave)) };
        let slave_path = String::from(cstr.to_str().expect("CStr::to_str failed"));
        
        // This call claims success, but no link ever shows up
        symlink(&slave_path, &symlink_path).expect("PTY symlink failed");

        // ...
    }
    // ...
}

impl ops::Drop for ServiceProxy {
    fn drop(&mut self) {
        eprintln!("Drop for {:?}", self);
        close(self.pty.master).expect("close master failed");
        close(self.pty.slave).expect("close slave failed");
        
        // The panic shown earlier comes from here
        remove_file(&self.fspath).expect("remove_file failed");
    }
}

Remarks

I don't think any of these should make a difference, but just in case

• the simulator is using tokio (async/await futures, tasks, etc);
• the simulator is working with PTYs instead of "regular" files like the short Rust test/example;
• a simple Python test script using os.symlink(...) works fine.

Update

I added the following code to the simulator, as a test, right after the symlink call:

if Path::new(&symlink_path).exists() {
    eprintln!("What?!: {}", symlink_path);
}
for p in std::fs::read_dir("/home/<user>/Projects/vpanel").unwrap() {
    eprintln!("{:?}", p.unwrap().path().display());
}

Interestingly, it lists the symlink as being present (irrelevant stuff omitted):

What?!: /home/<user>/Projects/vpanel/ttymxc4
...
"/home/<user>/Projects/vpanel/ttymxc4"
...

However, it's never listed by commands such as ls -la or anything. To make sure that there weren't any unexpected remove_file calls, I checked as follows:

$ find src -name '*.rs' | xargs grep remove_file
src/proxy.rs:    fs::remove_file,
src/proxy.rs:        remove_file(&self.fspath).expect("remove_file failed");

The only hit for an actual call in the code base is from the std::ops::Drop implementation. (The top hit is from the use std::{..., fs::remove_file, ...}; block.)

In short, there're no hidden/unexpected/accidental calls to remove_file after the symlink call. There's only the one we already knew about.

posted over 3 years ago

CC BY-SA 4.0

2y ago

ghost-in-the-zsh‭

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