In your code, the timeout will always be reached and not cancelled, that is why you receive DeadlineExceeeded. We detect and treat these differently (as warnings rather than errors, in a logging sense), since theyre client triggered and dont indicate something wrong with the running of the server. Or have to overload the error handling in f to be quite different. (Questions rather than bug reports normally go to a forum, not the issue tracker; see https://golang.org/wiki/Questions. I'm seeing the pretty frequently and starting to realize that it's probably not harmful -- albeit pretty annoying to see in the logs daily. Golang and context - an explanation - Paul Di Gian's Blog i test exactly this code thousand times just for god damn you say this code happens error ! Yes, matching on error strings is typically bad practice. For instance, if I wrote a function like this: Now if g or h fail because the context was canceled, the error returned from f won't be context.Canceled. Println ( "trying", i ) try () } } func try () { mux := http. How does this impact our code? Here is one image illustrate the connection and stream of HTTP2 from web dev If you cancel the context in one gRPC call before the response comes back. @jtolio There's prior discussion on this in #28728. In that way we could map the errors in a fully compatible way. I think it would solve your issue. to your account. If so, then indeed we'd need to make sure they are nil / not-nil at the same times. Already on GitHub? WithContext returns a new Group and an associated Context derived from ctx. The text was updated successfully, but these errors were encountered: As with all the times I've tried before, I can't reproduce this. @andreimatei I need to write some tests with chains of contexts and multiple causes. Is this a bug or I am missing somethiong ? I don't see a Go bug. (The latter happens when the context is canceled: thectx.Done()channel closes, making it readable.). See #1229 (comment) for an example. i expect to see fine message instead of this. (1) if err.Error() == "dial tcp: operation was canceled" ctx1 is not canceled). Its simple and straightforward (and probably fine for many use cases). If I check the context for error before stream.Send() is called every time will it guarantee no data loss ? The core of the context package is the Context type: // A Context carries a deadline, cancellation signal, and request-scoped values // across API boundaries. // A DoneReasoner describes the reason a Context is done. In our system, each user has a list of friend ids which refer to another user, and we can get a friend id iterator from the Friend service. If it still occurs, hopefully then you can share more of the captured errors/output reported by chromedp. and while it's true that the cancel call is often just a deferred Do these exercise the case you were describing? issue number 364. By clicking Sign up for GitHub, you agree to our terms of service and We detect and treat these differently (as warnings rather than errors, in a logging sense), since they're client triggered and . And that if it's set on this client we shouldnt even need to bother with client.CalendarList.List().Context(), correct? So if the child routine does not have dependency on parent routine context, it is always a good practice to create a new context for background go routines. Writing into a full channel blocks forever until some reader makes space, and reading from an empty channel blocks forever until the writer adds a new value or closes the channel. CGO_CXXFLAGS="-g -O2" I think with your current patch that's not possible, because the key that Cause() looks for is not exported? I'm stuck again on the same subject a year later. This is set when the context cancels from ctx.Err(). A complete runnable program is good. It's also not nice because it forces us to allocate both our own context struct, and a WithCancel() context. It'd have to be a new channel per caller; I haven't thought through the performance implications. In such a case it The context.Context interface requires a method .Done () that, quoting from the documentation, returns a channel that's closed when work done on behalf of the context should be canceled. Well occasionally send you account related emails. Have a question about this project? is there any race condition ? (You're not. Could this be related to a simple timeout? Does Recv() have a timeout of sort ? So I personally do not agree with the premise that any Cause() call should be preceded by an Err() call (or, rather, I don't think we should design Cause() with that mindset). CancelFunc ctx, = context. A given WithContext call site may have many possible cancel call sites, Edit: Could please explain you can't see a reasonable way to change it when surely all that would be needed would be to rename Done() to Cancelled() and then re-create Done() as wrapper to Cancelled() which is deprecated? Important Use Cases of Context Request-Scoped Data: Getting the DB errors to manifest in my local machine is pretty difficult, so I can't really test it. Have a question about this project? And similarly for causes stemming from cancellation. Perhaps provide a context.WithDebug method that adds a Context value for storing debugging info. If you are using go routines, if the parent go routine finishes but child routine still runs in the background, and the child go routine had a context which is common to the parent go routine this can end up in a context cancelled, if the parent go routine closes context before exiting. This however is not as clear to people who have never used contexts before thus increasing their barrier of entry to the package (and ultimately the language as well). ctx . I just get other errors with curl (see #353 (comment)). While it makes sense in the context of a context, as the context has been cancelled it's work has been done. That could work. distinguish them at all. FWIW, here's what we're trying within CockroachDB: a colleague's PR and my commit on top of it. "revisionTime": "2017-07-19T21:11:51Z" => "revisionTime": "2018-05-30T06:29:46Z", "checksumSHA1": "hyK05cmzm+vPH1OO+F1AkvES3sw=", => "checksumSHA1": "xaHFy3C2djwUXtiURpX3/ruY8IA=", I'm not 100% certain we can address the issue, but I'll see what can be reasonably be done. CGO_ENABLED="1" Sign up for a free GitHub account to open an issue and contact its maintainers and the community. The returned 233 // context's Done channel is closed when the returned cancel function is called 234 // or when the parent context's Done channel is closed, whichever happens first. Why is {ni} used instead of {wo} in ~{ni}[]{ataru}? 72 comments matthewceravolo commented on Jul 12, 2018 edited edited "path": "golang.org/x/net/context", => "path": "golang.org/x/net/context", // Ignore this error. Suppose we have a web app that produces a list of friends for the logged in user. I had missed the developments around #28728 and their implications. "connection unexpectedly closed by client" would be much much better. Here's a small program that demonstrates what I'm talking about. I just wonder if you want the param to become lasterr when there isn't already one.). just send a GET request to localhost:8080 and hndlr handler will handle your request. It is one of the most important tools while working with concurrent programming in Go. #2159 Or maybe any DB error could fall into checking if the context has been canceled (ctx.Err() == context.Canceled)? If I have to implement a timeout for Recv() how do I unblock the Recv() call in client side ? Sign in If you want to attach a Done reason to a Context, I'd use a Context value. Heres a simple implementation of the server logic. What is known about the homotopy type of the classifier of subobjects of simplicial sets? Why do code answers tend to be given in Python when no language is specified in the prompt? @fraenkel I'm fine with whichever, but doesn't putting it on the client prevent me from needing to duplicate this context code across all calls? context package - context - Go Packages PKG_CONFIG="pkg-config" this definitely helps. This is overhead, though, so I'd make it opt-in in somehow. Implicitly or explicitly, I'd try to differentiate between the cancel function being called after the respective operation finished (because the API says that one must always call the cancel function), or if the cancel is called affirmatively to cancel an ongoing operation. That loses the error type and value identity. The context was not created "withcancel" option at client side. We read every piece of feedback, and take your input very seriously. WithCancel ( ctx ) defer () := cn. (Just x/net, or also x/oauth2?). I think here's a reproduction for this error we've been seeing. // without this, the "http: proxy error: context canceled" message might not appear. Context is an interface, presumably, because you want people to be able to implement their own. Is there any way to silence the message? If the mappers error exit and stop pulling data from thefriendIdschannel, the producer loop can deadlock. As you may have surmised from the code comments, we have a problemno error propagation. You signed in with another tab or window. Timeouts produce context.DeadlineExceeded. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. The purpose of this design is to provide backwards-compatibility for existing code while enabling users to add cancelation "causes" where they need them. <. The entry point for the contexts in golang is the context package. We didnt have to add a bunch of extra code, more channels, or additional synchronization ourselves. After a customer prospect shared a report from Emerge Tools, we were able to track down a quick win to reduce our SDK startup time on Android by 75%. If we'd replace net.errCanceled with fmt.Errorf("operation was canceled: %w", ctx.Err()), then only (1) and (2) would break. But isn't that a natural thing to do, since the user has to pass an error to WithDeadlineCause(,err)? 0 @mitar No, at this stage I'm just prototyping to get a feel for various ideas. Formerly at Square. Then, you'll add a main function that creates a context and calls doSomething using that context. You signed in with another tab or window. Each mapper looks up a single profile at a time, so we can look up as many profiles at a time as we have mappers. You are receiving this because you were mentioned. A given WithContext call site may have many possible cancel call sites, and while it's true that the cancel call is often just a deferred function, in some of the most interesting cases it is decidedly nontrivial. I believe this leaves 4 ways to act upon the error now. Reply to this email directly, view it on GitHub But I don't see any reasonable way that we can change it now. // DoneReason returns a human-readable description of the reason that ctx.Done() is closed. . If you're looking to pass a reason there's nothing stopping you from creating a custom context implementation. which is closed so I can not comment there . I suggest at this point I write up the current state as a Proposal and document your suggestions as Alternatives to Consider. September 2022 | No Comments Takeaways Errors as uint64 for better performance By convention, 0 is "OK" or "no error" Use the exhaustive linter with switch Use enums to define error cases Implement Stringer on enums for human readable error messages Use generic T ~uint64 to type custom structs to errors That leads me to think it's very tricky to foresee the need to check the context, as we'd see ctx.Err() coming back most of the time in testing. to your account. What did you expect to see? https://golang.org/cl/45370 is in that range and loosely matches @meirf's diagnosis. Capturing the stack trace at the cancel() call is likely to be uninteresting: usually cancel is called either via timer.AfterFunc (in the case of a deadline) or via defer, which may make it harder to debug if there are multiple defers in the same function. The code in WithTimeout and WithCancel can check for this debug value The cancellation of context happens in 2 steps. Do you mean a func net.IsCanceled(error) bool? I see a lot of context cancelled error in our project which is kind of annoying me. type Context interface { // Done returns a channel that is closed when this Context is canceled // or times out. Why Do We Need Cancellation? It'd be nice if the error extends ctx.Err() like in your example. I would state the expectation in the reverse: Cause(ctx) is only valid (defined) if ctx.Err() != nil (which is the same as when ctx.Done() is closed). I'm explicitly propagating causes from parents to children in propagateCancel and cancelCtx.cancelthe same places where the context's error gets propagated. This case looks unavoidable, but, as above, wed still prefer to detect and downgrade it to a warning. ), You can't rely on context.Canceled being the error returned, since (a) that would prevent the things you call from ever adding context. On Tue, Jul 17, 2018 at 3:52 PM Bryan C. Mills ***@***. Golangcontext - - I need to write more tests, particularly for cases where there are multiple causes in a context chain, and with mixed cancelation orders. Something like go/grpc's statusError seems nice. @bcmills I wouldn't use type assertion to introspect a Context because they are wrapped so often. The text was updated successfully, but these errors were encountered: Note that a context can also be "Done" if it times out, if it was created using context.WithTimeout or context.WithDeadline. Based on the feedback from several of you, I'll prototype an alternative Our handler code is not doing much actual, real CPU workit's just waiting on network responses from other services. This is very expensive. If your application is microservice based(or have several components which call each other using contexts), if microservice 1 calling microservice 2, and microservice 2 eplicitly closes or cancels context, even in that case you can get this error. And theres one more great thing I havent mentioned: error Groups compose (nest) perfectly well. Those changes seem to let us avoid the extra goroutine, which is great! net/http/httputil: ReverseProxy gives "context canceled - GitHub Inside the contexts directory use nano, or your favorite editor, to open the main.go file: nano main.go. In Running mode : Unexpected end of JSON input. "revision": "314dd2c0bf3ebd592ec0d20847d27e79d0dbe8dd", => "revision": "1e0a3fa8ba9a5c9eb35c271780101fdaf1b205d7", When an operation is explicitly canceled, I want to know who canceled it and why. The callee of the function then receives this signal to abort the current running operation. database/sql: "rollback due to context cancellation" error consistency, https://go-review.googlesource.com/c/go/+/145204. So if the callee has explicitly closed the context as shown above, this can result in a context cancelled error at caller. We have such a context implementation, but it's not a nice one because ctx.Err() still returns the vanilla context.Canceled error (as it also does in @Sajmani 's proposed patch). Left unchecked, leaked goroutines pin memory and process resources, eventually slowing down and then crashing the process. Thanks a lot for the explanation and guidance. If the parentContext(say, an http request context) is canceled, the groupContext. The net.errCanceled mapping is not exported, and its type errors.errorString has no properties other than the textual content. net: expose context Canceled/DeadlineExceeded error. Correct? My preference is to minimize API changes and improve debuggability in-place, but I understand performance concerns might make this difficult. If you want to attach a Done reason to a Context, I'd use a Context value. go - Handling Context Cancelled errors - Stack Overflow I believe the issue summary has additional concerns. Would you agree? Sign in 6 More posts you may like r/askmath Join 2 mo. (http. To see all available qualifiers, see our documentation. A call likeGetUserProfile(ctx, id)should halt if the supplied context is canceled, because the underlying remote calls (such as those in Gos http client librarywhen correctly usingrequest.WithContext) should check for context cancelation and exit quickly with acontext.Canceledorcontext.DeadlineExceedederror. (But I'm sure your advice will be useful to others who come looking for this stuff. To see all available qualifiers, see our documentation. I am new to Go and still learning it every signle day. But this then makes it difficult to determine whether ctx.Done was closed due to cancelation or deadline exceeded (the user would need to encode this into their cause). By clicking Sign up for GitHub, you agree to our terms of service and When I use an empty context with no timeout, everything works as expected. I have chain of 3 proxy servers, and middle server was returning "context canceled". What versions are you running? Are there any good tips or things to know about when investigating context cancelation errors? Using errgroup As you may have surmised from the code comments, we have a problemno error propagation. I don't see any deadline context set. In the main.go file, you'll create a doSomething function that accepts a context.Context as a parameter. last version of chromedp and chrome 74.0.3729.169. any reason why do you use EvaluateAsDevTools to set the value? Golang grpc: how to recover the grpc server from panic? https://godoc.org/github.com/chromedp/chromedp#Evaluate, when i use this code with net/http and wrapping a http handler on it it makes unexpected end of JSON without IDE , and with IDE debug mode it returns context canceled. You can't rely on context.Canceled being the error returned, since (a) that would prevent the things you call from ever adding context, and (b) that's not the only error that can signal that a context is done (e.g. Writers have no way to know no one is listening anymore. In our code, if the reducer loop exits with an error and stops pulling data from thefriendschannel, the workers can deadlock trying to write into a full channel. You switched accounts on another tab or window. I assume you want to have a timeout per service call rather than across the Client use. This is true for both Cause(
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