Rust has its own definition of what "safe" means, which is arguably a subset of memory safety.
You can have perfectly safe Rust code with data races, deadlocks, memory exhaustion, etc. Not even talking about logical errors.
No, you can't. Data races are explicitly one type of race condition that Rust protects you from. Anything else would indicate an unsound library or compiler bug. For reference, you can take a look at the non-exhaustive list of undefined behaviors[0]. None of those behaviours should be possible to trigger from safe Rust.
[0] https://doc.rust-lang.org/reference/behavior-considered-unde...
> However Rust does not prevent general race conditions.
> This is mathematically impossible in situations where you do not control the scheduler, which is true for the normal OS environment.
> For this reason, it is considered "safe" for Rust to get deadlocked or do something nonsensical with incorrect synchronization: this is known as a general race condition or resource race. Obviously such a program isn't very good, but Rust of course cannot prevent all logic errors.
> Are "data races" and "race condition" actually the same thing in context of concurrent programming
> No, they are not the same thing. They are not a subset of one another. They are also neither the necessary, nor the sufficient condition for one another.
The really curious thing here is that the Nomicon page also describes this distinction in great detail.
Rust can only offer its safety for the lifetimes it tracks, and this tracking is limited to a single instance of your process.
In other words, if you perform concurrent accesses through multiple threads spawned from a same process, you're safe from data races, at risk of race conditions. If you perform concurrent accesses through multiples processes, you're at risk of both.
That implies that even in a world where everything is safe Rust, you cannot guarantee that these two Rust processes will not data races together.
How would you do that in safe code? If I understand correctly, the problem you're referring to is the exact reason that mmap APIs in Rust are unsafe: https://docs.rs/memmap2/latest/memmap2/struct.Mmap.html#safe...
In highly concurrent/long lived systems, you often end up using a multiprocess approach between your producers and consumers rather than multithreading. This is because it allows you to dynamically spawn new processes to consume from a shared memory without going through a bastion process managing threaded consumers. e. g. It allows you to spawn at will newly developed consumers without redeploying and restarting the producers. Note that this does not mean a different codebase.
As for the expectations, I think it's fair to highlight it. Because people tend to think that a world of applications fully developed in Rust would somehow imply no data races. That's not the case: On a fully fault-free operating system, with only 100% safe Rust applications running, you can, and will, still run into data races, because in the real world applications cross process boundaries.
While I'm aware of some differences in opinion in the details, all of the definitions I'm aware of exclusively refer to multiple threads of execution writing to the same memory location, which makes the concept of a multi-process data race impossible. For example, Regehr: https://blog.regehr.org/archives/490
A data race happens when there are two memory accesses in a program where both:
target the same location
are performed concurrently by two threads
are not reads
are not synchronization operations
> The means by which this memory location was shared don't really matter.
It does in the context of this conversation, because you are claiming that safe Rust can create data races. Is there a safe API that lets you map addresses between processes? The closest thing I'm aware of is mmap, which is unsafe, for exactly this reason.
"Safe" and "unsafe" are scoped properties. They apply to the block of code that they encompass, by very definition of the fact that they are scoped. As long as an unsafe block respects the constraints that would be enforced if Rust could understand them, then that unsafe block can be wrapped in a safe block and exposed.
In other words, "unsafe" is not transitive as long as what is done in the unsafe block respects the constraints. Mapping a block of memory is not unsafe. AFAIK Rust is able to allocate memory, which means Rust calls mmap, and that is safe.
Thus, mmap being marked unsafe is not relevant here, because the actual unsafe block using mmap can be safe by knowledge of the programmer, while still inducing a data race later on (or not).
If your argument is that unsafety is transitive by virtue of calling something that might have unsafe side effects later on, then that is seriously broken. Because with the same logic, every file access could use /proc to overwrite your process memory or change your memory maps, should we mark the all unsafe? Or worst, you prone a "maybe unsafe" approach, where things are maybe transitively unsafe, in which case why bother, let's rename "unsafe" to "shady", or unsafety just becomes some qualitative measure.
It is not. I am in full agreement with this:
> As long as an unsafe block respects the constraints that would be enforced if Rust could understand them, then that unsafe block can be wrapped in a safe block and exposed.
But I am not sure what you mean by
> AFAIK Rust is able to allocate memory, which means Rust calls mmap, and that is safe.
liballoc is not required to call mmap, and even if it did, that mmap isn't shared with other processes, as far as I'm aware.
> while still inducing a data race later on
If this is possible, then it is not safe, otherwise you lose the properties we're in agreement about above: no data races are allowed in safe Rust.
> Because with the same logic, every file access could use /proc to overwrite your process memory
It is true that Linux specifically exposes the ability to mess with process memory via the filesystem API, and you can use that to subvert Rust's guarantees. But this one specific hole on one specific platform is considered to be outside of the scope of the ability of any programming language to guarantee. An operating system corrupting your memory is a completely different thing.
I doubt we're going to come to an agreement here, but I appreciate you elaborating, as now I think I understand your position here. It's not the one in general use though, so you're going to encounter these sorts of arguments in the future.