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Comment by knuthsat | Hacker News Reader

I guess the simplest example is shared memory between processes.

Even Python has it: https://docs.python.org/3/library/multiprocessing.shared_mem...

Access to raw memory is locked behind the unsafe keyword though. Rust officially already does not guarantee any safety in that scenario even within 1 process.

There is always unsafe at some level on the standard library.

The point is that it doesn't protect the user of a crate that only exposes a fully safe API, unless they do digging to validate overall architecture safety.

>Even Python

It's not "even". Python specifically has it because it has no real threading.

Python has cooperative threading. It's the same threading model used in the Erlang VM, Julia and many other dynamically typed languages. But preemptive threading vs. cooperative threading is orthogonal to whether data races can happen. Java threads are preemptive but data races can still happen.

The Erlang VM does preemptive scheduling.

No it doesn't.

While this is technically true it's quite misleading. The VM itself uses cooperative scheduling, but the Erlang compiler emits something akin to yields appropriately, such the net effect is preemptive scheduling. You can break it by calling a NIF that doesn't do the yields appropriately, but that's not the norm.

Preemptive threading means that the running thread can be paused by the system without ANY cooperation from the thread itself. Emphasis on "any". Python works roughly the same as Erlang. Every N bytecodes it checks if a thread is waiting to run and switches to that. Both are variants of cooperative threading.

Python does have real threading. The `threading` module provides os-level threads and synchronization primitives. The only difference between this and multithreading in C or Java is that CPython's GIL prevents more than one thread executing bytecode at a time. This prevents parallelism, but not concurrency.

Note this does not mean that python code is thread-safe by default. At most, you can theoretically rely on bytecode operations to be atomic, which means you'll need to synchronize multi-threaded code with mutexes, semaphores and higher-level synchronization constructs.

SharedMemory is a new thing in Python. Not even supported by all 3.x versions.

But this is specifically about Rust.

What data races between processes, other than Disk/IO, databases, or external services, can a Rust program have?

I explicitely exclude the whole category of external services, since that is "by design" really. And the whole reason for ACID, global mutexes, transactions and CRDTs.

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