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.
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.
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.
You can do better than the errors in other languages. You can provide all the relevant information in the emum variant.
enum MyApiBindingCrateError {
// You didn't provide an
// API key. Maybe we should
// design our interface to
// make this impossible
ApiKeyMissing,
// Client was unauthorized
// to make this request
AuthorizationError,
// The entity you requested
// did not exist (404'd)
NotFoundError,
// You're sending too many
// requests to the server
TooManyRequests,
// That specific error with
// the API
// Maybe users can't delete
// folders until they're empty
// Whatever
SpecificApiIssue1,
// Some other specific error
// with the API
SpecificApiError2,
// Server didn't respond the
// way we expected.
// Here's what it told us
UnexpectedHttpResponse {
// HTTP status code
status_code: StatusCode,
// If it had a
// string-encoded body
body: Option<String>,
},
// Unhandled Issue with IO
IoError(io::Error),
// Unhandled Issue with
// request library
ReqwestError(reqwest::Error),
}
The beauty with Rust is that you can create really detailed concrete errors at the crate level. Your callers will know exactly what the actual error states are.
Your application can be a little less structured if you want. Though with LLMs, I'm using anyhow and thiserror a lot less.
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).
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?
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.
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.
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.
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.
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.
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.