31 ms·

Surely you need an alternative to Box<dyn Error> for reporting memory allocation failures?!

• dwattttt 4mo ago
  A &(dyn Error + 'static) should be fine for that; you don't need any allocated/variable sized data in a memory allocation failure.
  • dnautics 4mo ago
    stacktraces? might also be useful to know whether or not the latest allocand was a jumbo sized allocand that caused the failure?
    • dwattttt 4mo ago
      Do you really want that data passed back down to the caller of the allocation? From the description of the failure state you'd want to log that data instead: what's the caller of the allocation going to do if you tell it it failed with a crazy size? It already knows the size, it's the one who asked for it.
      • dnautics 4mo ago
        So, suppose it's a rust library -- you're locking me into whatever logging system the library author chooses? Maybe I'd like to consume the relevant data at the entry point and send it to a logging system of my choice.
        • cobbzilla 4mo ago
          usually “stdout” is good enough, wrapper/runner routes output to logserver for collation and search. who cares about formats as long as it’s reasonably structured and searchable?
        • sheept 4mo ago
          A Rust library likely wouldn't be returning an opaque Box<dyn Error> to begin with. Errors are part of a library's API—it's what allows consumers to handle them—so you'd define an enum of possible errors your library could produce and return that, which would be stored on the stack.
          • dnautics 4mo ago
            What about the data in the error payload?
            • yuriks 4mo ago
              That's part of the error enum. enum AllocError { SizeTooLarge { size: usize }, // etc. } This enum has a known size and doesn't require any dynamic allocations.
            • dwattttt 4mo ago
              I think this is a clash of terminology: a Rust enum isn't an integer with pretensions of an identity. You'd describe it as a tagged union in some languages. So when you say you'd return an error with extra information, what that information is is associated with the specific variant of the enum. Using yuriks AllocError as an example, if the error is SizeTooLarge, it has the size field. Other errors may have no additional data, others may have different data. When you return an error from your allocating function, it's a known size, the size of the largest enum variant + the discriminant (tag).
              • dnautics 4mo ago
                I'm aware that a rust enum isn't just an integer? How would it have a payload if it were just an integer? right. If allocerror only has size field, where do you stash the unwind information (which could be of arbitrary size)
                • dwattttt 4mo ago
                  There's a few confusing things here. For one, just because the allocator gave you an AllocError, that doesn't mean your function has to return an AllocError: you can return whatever error type you choose. If you want to collect a stack trace at that point, put one in there. What value would a stack trace that includes internal allocator functions be to you? What do you lose by having to collect the stack trace at the point where your function receives an AllocError?
        • pas 4mo ago
          it depends, if the functionality represented by the library is known to require a lot of memory (or simply allocation failures are an expected part of its operation), then it should be pretty much part of the API, probably with some tracing/diagnostics interface to get the required visibility into how much memory goes and where. but for most libraries I on allocation failure I don't expect any fancy logging system. maybe even panic is fine.
    • 63stack 4mo ago
      Any time I mention "but I would like stacktraces with my errors" I get told I'm doing it wrong.
      • simonask 4mo ago
        That's because the types of errors where you want a stack trace are a relatively small subset of all possible errors. Stack traces are only useful for errors that indicate a bug in the program, i.e. something a programmers has to respond to. It's not useful for the vast class of bugs that are a result of wrong input, wrong external state, or infrastructure issues. Rust projects tend to favor panicking over error handling for programmer bugs (which does indeed give you a stack trace depending on environment variables), or even better encoding the invariants in the type system, but there are cases where an error coming from a library are truly, actually unexpected, so both `anyhow` and `thiserror` do provide support for attaching a stack trace in those situations.
        • 63stack 4mo ago
          See? You get people explaining to you that you actually don't want a stack trace because xyz.
          • wtetzner 4mo ago
            This sounds disingenuous. They explained why the language doesn't force stack traces on all errors, and then explained how to get them if you want them.
            • 63stack 4mo ago
              I see a very opinionated explanation, not a "this is why the language does not" explanation. >Stack traces are only useful for errors that indicate a bug in the program, i.e. something a programmers has to respond to. It's not useful for the vast class of bugs that are a result of wrong input, wrong external state, or infrastructure issues. This is a personal opinion, not something you can declare as the objective truth. There is a lot of value in seeing what path the program took before it encountered a eg. validation error. >but there are cases where an error coming from a library are truly, actually unexpected, so both `anyhow` and `thiserror` do provide support for attaching a stack trace in those situations. This is wrong because it's up to the library to attach the stacktrace, not the userland code using the library, so saying "you can get them if you want them" is not true. If the author of the library did not decide to attach the stacktrace, your only option is wrapping it yourself, which you can only do if you already know up front all the paths that can fail. Also, you are not supposed to expose errors from a library with anyhow, they are only for application/top level code.
    • SkiFire13 4mo ago
      And how do you store a stacktrace without allocating?
    • imtringued 4mo ago
      If you let the allocation error panic you will get your stack trace. You can't have a stack trace on an error in the error path that failed to allocate. If you have a "jumbo sized" error and the error fails to allocate, it won't get reported. The only reporting you will get is that the error failed to allocate and this new allocation error overrides the error that failed to allocate.
    • mswphd 4mo ago
      if you have an OOM and want to log why, if your logging allocates it will likely fail as well. You could in principle work around this with enough effort, but "properly" handling OOM is typically much more trouble than its worth.
• loeg 4mo ago
  Anything other than panic/abort on allocation failure is outside the scope of the vast majority of programs, including anything using the standard library in Rust. I wouldn't worry about Box<dyn Error>.
• wyager 4mo ago
  You're already writing Rust in a very different style if you're writing the type of code that gracefully handles allocation failure. It's to Rust's immense credit that this type of coding is actually fairly well-supported (unlike in Go), but you're already a bit off the beaten path for stuff like error handling.
• imtringued 4mo ago
  Not sure what your problem is? If you need to handle an allocation error in the error path, then the error reporting path must abort, which means that the allocation error must be bubbled up. There is no real solution to an allocation error inside the error path. Even if you preallocate an arena for errors, the error might be large enough that it won't fit inside the arena. Hence the best thing you can do from that point onwards is to have an error enum with an AllocError variant that doesn't allocate. Said error won't contain any information beyond line numbers of the allocation error since you just don't have the space for it. In the end you will basically end up with panic free code, but the error still bubbles up like regular unwinding. So yeah you can do it, and I will do it in the future, but I personally think that the people who think this is some huge deal breaker don't understand the problem in the first place.
• afdbcreid 4mo ago
  The comparison here is to Go, which doesn't have any way to handle memory allocation failure AFAIK. In Rust you can do that (mostly on nightly although not only), and yes there are alternatives for this case, but it's rare anyway.