Note that Java, and the JVM, doesn't allow changing environment variables. It was the right choice, even if painful at times.
I am fairly certain that somewhere inside the polyhedron that satisfies those constraints, is a large subset that could be statically analyzed and proven sound. But I'm less certain if Rust could express it cleanly.
When using C++ I wanted programs to have a function that was called before main() and set up things that got sealed afterwards, like parsing command-line-arguments, the environment variables, loading runtime libraries, and maybe look at the local directory, but I'm not sure if it'll be a useful and meaningful distinction unless you restructure way too many things.
I remember that on the Fuchsia kernel programs needed to drop capabilities at some point, but the shift needed might be a hard sell given things already "work fine".
In Java you’d have the static initializers run before the main method starts. And in some languages that spreads to the imports which is usually where you get into these chicken and egg problems.
One of the solutions here is make the entry point small, and make 100% of bootstrapping explicit.
Which is to say: move everything into the main method.
I’ve seen that work. On the last project it got a little big, and I went in to straighten out some bits and reduce it. But at the end anyone could read for themselves the initialization sequence, without needing any esoteric knowledge.
(the argument that the horses have long bolted with respect to "just do the right think ok?!" here holds some water. I'm of the generation though where people on the internet could still tell each other they were wrong, and I assert that here; you're wrong if you believe a non-threadsafe unix interface is a bug. No matter what kind of restrictions around its use that means. You're still wrong if you assume the existence of such restrictions is a bug)
You can add `premain` function that calls `main` and set it as an entrypoint, you can implement pre-start logic in main and call main loop later.
This is how any sane program is written anyway: set up environment -> continue with business logic
Maybe it's possible, but if I need to review every library (and hope they don't break my assumptions later) I think I lost on building this separation in practical way.
This (obviously?) isn't "110%" perfect as the order of the constructor calls for several such objects may not be well-defined, and were they to create threads (who am I to suggest being reasonable ...) you end up with chicken-egg situations again.
There was one place and only one place where we violated that, and it was in code I worked on. It was a low level module used everywhere else for bootstrapping, and so we collectively decided to do something sneaky in order to avoid making the entire code base async.
And while I find that most of the time people can handle making one special case for a rule, it was a complicated system and even “we” screwed it up occasionally for a good long while.
The problem was we needed to make a consul call at startup and the library didn’t have a synchronous way to make that call. So all bootstrapping code had to call a function and await it, before loading other things that used that module. At the end we had about a dozen entry points (services, dev and diagnostic tools). And I always got blamed because nobody seemed to remember we decided this together.
I hate singletons. And I ended up with one of only two in the whole project, and that hatred still wasn’t enough to prevent hitting the classical problems with singletons.
They might do it for testing purposes.
I ended up implying some extra support when all I meant was “one could”.
If you're only reading environment variables you have no problem, though. It's only if you try to change them that it causes issues.
For setting, "only set environment variables in the Bash script that starts your program" might be a good rule.
I'd go further and say env should always be read-only and libraries should never even read env vars.
I mean, based on this issue I would say the only safe time is "at the start of the program, before any new threads may have been created".
But again, as others have said, there's no good reason I'm aware of to set environment variables in your own process, and when you spawn a new process you can give it its own environment with any changes you want.
struct BeforeEnvFreeze(());
struct AfterEnvFreeze(());
impl BeforeEnvFreeze {
pub fn new() -> Self { /* singleton check using a static AtomicBool or something */ Self(()) }
pub fn freeze(self) -> AfterEnvFreeze { AfterEnvFreeze(()) }
pub fn set_env(&self, ...) { ... }
}
impl AfterEnvFreeze {
pub fn spawn_thread(&self, ...) { ... }
}
fn main() {
let a = BeforeEnvFreeze::new();
a.set_env(...);
a.set_env(...);
//b.spawn_thread(...); // not available
let b = a.freeze(); // consumes `a`
b.spawn_thread(...);
//a.set_env(...); // not available
}
• Banning access to the relevant bits of Rust's stdlib, libc, etc. as a means of escaping this "safe" abstraction
• Conning your lead developer into accepting your handwave
• Setting up the appropriate VCS alerts so you have a chance to NAK "helpful" "utility" pull requests that undermine your "protections"
And of course, this all remains a hackaround for POSIX design flaws - your engineering time might be better spent ensuring or enforcing your libc is "fixed" via intentional memory leaks per e.g. https://github.com/bminor/glibc/commit/7a61e7f557a97ab597d6f... , which may ≈fix more than your Rust programs.
Not sure why would it be considered painful. Imo, use of setenv to modify your own variable, the definition of setenv is thread unsafe. So unless running a single threaded application it'd never make sense to call it.
Java does support running child processes with a designated env space (ProcessBuilder.environment is a modifiable map, copied from the current process), so inability to modify its own doesn't matter.
Personally I have never needed to change env variables. I consider them the same as the command line parameters.
Another reason why Java isn't the greatest language to create CLI tools with.
Linux and macOS both support per-thread working directory, although sadly through incompatible APIs.
Also, AFAIK, the Linux API can't restore the link between the process CWD and thread CWD once broken – you can change your thread's CWD back to the process CWD, but that thread won't pick up any future changes to the process CWD. By contrast, macOS has an API call to restore that link.
But I think it was actually possible to hack around up until Java 17.
Those are trivial things in around 100 lines of code and have been available since System.getenv() got back (it used to be deprecated and non-functional prior Java 1.5 or 2004)