Discovering a JDK Race Condition, and Debugging It in 30 Minutes with Fray

Post by gf000 » Sun, Jun 08, 2025, 6:01 PM UTC

brabel wrote: post #29

gf000 wrote: post #26

Earlier quoted context omitted.

This is more of a toy example for how a set of atomic changes can still end up in an inconsistent state, e.g. setting January the 31st and February 3rd in quick succession from two or more different threads may result in Feb 31st being visible from a third thread. This is not solved by Rust and your struct will even get the Sync trait automatically, which may be not be applicable as in this case.

Given your example, I am convinced you've never written any Rust. Of course it does stop you doing shit like that. But in this example, even Java does it properly, since the constructor runs to completion before any Object is accessible to any Thread, not just the one creating it. You need to validate the state of the object in the constructor to prevent that, but TBH why are we talking about this, it's almost comple…

Post by tialaramex » Sun, Jun 08, 2025, 7:03 PM UTC

gf000 wrote: post #32

brabel wrote: post #29

Earlier quoted context omitted.

Given your example, I am convinced you've never written any Rust. Of course it does stop you doing shit like that. But in this example, even Java does it properly, since the constructor runs to completion before any Object is accessible to any Thread, not just the one creating it. You need to validate the state of the object in the constructor to prevent that, but TBH why are we talking about this, it's almost comple…

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.

Post by gf000 » Sun, Jun 08, 2025, 7:33 PM UTC

tialaramex wrote: post #34

gf000 wrote: post #32

Earlier quoted context omitted.

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.

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.

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

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

Post by kbolino » Sun, Jun 08, 2025, 8:15 PM UTC

tialaramex wrote: post #35

nlitened wrote: post #23

Earlier quoted context omitted.

> “Make invalid states unrepresentable” I think this phrase sounds good but is not applicable to systems that touch messy reality. For example, I think it’s not even possible to apply it to the `tm` structure, as leap seconds are not known in advance.

I agree that messy reality can intervene, in the medium term (for about a decade) we'll need to handle leap seconds But we can do a lot without challenging the messy reality. 61 second minutes are (regrettably) a thing in some time systems, but negative 1 million second minutes are not a thing, there's no need for this to be a signed integer!

Post by tialaramex » Sun, Jun 08, 2025, 9:29 PM UTC

gf000 wrote: post #36

tialaramex wrote: post #34

Earlier quoted context omitted.

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.

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…