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Comment by gf000

14 hours ago

Of course if you are creating a new object and you have an atomic handle to it, it is trivial to solve. Like, having immutable objects solves a lot of these problems.

But what I'm quite obviously talking about is a Rust struct with 3 atomic fields. Just because I can safely race on any of its fields, doesn't mean that the whole struct can safely be shared, yet it will be inferred to be Sync.

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gf000

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tialaramex  13 hours ago

Object mutability isn't relevant here. A Date type which is mutable can ensure that all mutations are valid, it just can't do so while retaining this clumsy "LOL I'm just a D-M-Y tuple" API.

We can see immediately that your type is broken because it allows us to directly set the date to February 31st, there's no concurrency bug needed, the type was always defective.

  • gf000  12 hours ago 

      void setDate(int month, int day) {
    if (notValidDate(month, date)) { throw; }

    this.month = month; // atomic
    this.day = day // atomic
  }

    Yet the whole function is not "atomic"/transactional/consistent, and two threads running simultaneously may surface the above error.

    Of course it can ensure that it is consistent, C code can also just ensure that it is memory safe. This is just not an inherent property, and in general you will mess it up.

    The only difference is that we can reliably solve memory safety issues (GC, Rusty's ownership model), but we have absolutely no way to solve concurrency issues in any model. The only solution is.. having a single thread.

    • tialaramex  10 hours ago

      But you were critiquing Rust's model, yet you've written C++ here. I agree it's perfectly easy to write the bug in C++.

      In Rust this improved type doesn't have the defect, to call Rust's analogue of your setDate function you must have the exclusive mutable reference, which means there's no concurrency problem.

      You have to do a whole lot of extra work to write the bug and why would you, just write what you meant and it behaves correctly.

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