4 ms·

When can data races across processes happen? Are you talking about databases, services or IO and such?

• knuthsat 5y ago
  I guess the simplest example is shared memory between processes. Even Python has it: https://docs.python.org/3/library/multiprocessing.shared_memory.html https://docs.python.org/3/library/multiprocessing.shared_mem...
  • SCHiM 5y ago
    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.
    • pjmlp 5y ago
      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.
  • KptMarchewa 5y ago
    >Even Python It's not "even". Python specifically has it because it has no real threading.
    • bjourne 5y ago
      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.
      • nesarkvechnep 5y ago
        The Erlang VM does preemptive scheduling.
        • bjourne 5y ago
          No it doesn't.
          • grogers 5y ago
            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.
            • bjourne 5y ago
              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.
    • dangerbird2 5y ago
      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.
    • knuthsat 5y ago
      SharedMemory is a new thing in Python. Not even supported by all 3.x versions.
  • berkes 5y ago
    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.
• pjmlp 5y ago
  That and kind of data structure that can be shared via IPC mechanisms, some of them even transparent for the processes.
• SAI_Peregrinus 5y ago
  Environment variables. Locales. Quite a few other POSIX bits, really.
  • Kranar 5y ago
    It is not possible to have a data race with environment variables across multiple processes. Every process has its own copy of environment variables (in fact they have their own copy of the entire environment). I'm not sure what data race is possible across processes with locales, that's too vague of a claim to make.
    • berkes 5y ago
      One type of locales I know are the LC_ env vars. So there the "ENV is a copy" applies too. Another would be to read and write into locale files, such as JSON. But then the ame applies as with any database or IO: this is inherently race-condition-prone and that is by design. Maybe grandparent is thinking about locales in many web frameworks, that is some global var which should not be shared across users. So that if you set `Locale.current = "EN_GB"` that applies for any (email)notifications, errors, files, responses or such, being sent out during that request/response and during any jobs that request/response may spawn. In e.g. Rails this "somewhat global var" is a Frankenstein, but works suprisingly stable, actually.