net/http: source of errors is unclear #14203
Comments
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@jingweno would it be easier to return a more descriptive error rather than invent a mechanism to instrument call stacks at runtime ? |
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Can you run your code under a debugger? Setting a breakpoint at the offending error generating locations should get you the stack in question. |
It may be a yes in this case. But that would require waiting for the next Go version (if it's fixed), or I patch Go and create a customer build for myself. IMO a mechanism to instrument call stacks would be necessary.
Unfortunately not. It's a critical app in production and the best we could do is to log it and fix it later. |
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http.RoundTripper passes stuff between goroutines. A single stack trace won't yield the answers you seek. |
I may not fully understand how goroutines work internally. But it's a matter of traceability, right? Can we not trace the calls from one goroutine to another? |
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@jingweno, the error is sent over a channel. It's unclear which stack(s) at which time you want. There is no automatic answer here short of recording a full trace of the program and inspecting it later, probably. |
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I think this is not as complicated as you make out. As I understand it the On Wed, 3 Feb 2016, 10:34 Brad Fitzpatrick notifications@github.com wrote:
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To be clear, I agree with @davecheney. If there's an unstructured or useless error that needs to disambiguated, we can do that. |
Hmm...I may be oversimplifying the problem. These kinds of things happen for a distributed system as well: we keep track of callstacks for different microservices by carrying over a request id: https://brandur.org/request-ids. Can we do something similar for Goroutines? A unique "Goroutine ID" is generated before a piece of code is executed (the ID could come from an argument, as long as it's unique); The "Goroutine ID" is carried over for different goroutines; Stacks are correlated and aggregated by the "Goroutine ID". Maybe a deeper thought is needed though.
Yup, it would solve the problem. But I couldn't get away of waiting for the next Go release or making a custom build for Go if I want to use it right away as mentioned in #14203 (comment). I literally want to know what caused the errors now. |
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@jingweno, yes, we do the same thing with request tracing inside Google. See https://godoc.org/golang.org/x/net/trace and https://godoc.org/golang.org/x/net/context . Contexts need to be passed explicitly. Automatic contexts leads to tons of problems of confusingly-annotated traces (as witnessed in C++ and Java code trying to use TLS), which is why goroutines don't have accessible ID numbers. This won't be added to Go as an automatic thing.
Modify the Go code and see. The error can only come from a few places. |
The first is the when the client hung up on you. (the ReverseProxy CloseNotifier fired, and canceled the outbound request). The second is when the peer server you're making the request hung up on you. |
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Are those failed request POST request? ReverseProxy was using CloseNotifier
wrong. See #13666.
Although I only observe the problem with some Go tip revisions before the
fix, I still suggest you to try Go 1.6rc to see if it fixes the problem.
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@minux, that bug was never shipped. That was only during the Go 1.6 dev cycle. |
ianlancetaylor
changed the title
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@bradfitz Alright, I'm coming back to this. Would it be possible to add different error messages for https://github.com/golang/go/blob/release-branch.go1.5/src/net/http/transport.go#L1093-L1094? I could help with that but I need to figure out how to contribute first... Hopefully this could be fixed in time for 1.6. |
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And we're starting to get a couple "EOF" errors. The error type is |
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Also could @bradfitz explain a bit of b) of https://github.com/golang/go/blob/release-branch.go1.5/src/net/http/transport.go#L1194-L1199? We tracked down the downstream server did respond with 2xx but somehow the Proxy is responding with 5xx. |
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As described in #18272, it would also be nice if you could easily tell if an error returned by http.Transport.RoundTrip came from communicating with the target server or from reading request.Body (or a third place I'm not considering). |
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CL https://golang.org/cl/37495 mentions this issue. |
If I put a 10 millisecond sleep at testHookWaitResLoop, before the big select in (*persistConn).roundTrip, two flakes immediately started happening, TestTransportBodyReadError (#19231) and TestTransportPersistConnReadLoopEOF. The problem was that there are many ways for a RoundTrip call to fail (errors reading from Request.Body while writing the response, errors writing the response, errors reading the response due to server closes, errors due to servers sending malformed responses, cancelations, timeouts, etc.), and many of those failures then tear down the TCP connection, causing more failures, since there are always at least three goroutines involved (reading, writing, RoundTripping). Because the errors were communicated over buffered channels to a giant select, the error returned to the caller was a function of which random select case was called, which was why a 10ms delay before the select brought out so many bugs. (several fixed in my previous CLs the past few days). Instead, track the error explicitly in the transportRequest, guarded by a mutex. In addition, this CL now: * differentiates between the two ways writing a request can fail: the io.Copy reading from the Request.Body or the io.Copy writing to the network. A new io.Reader type notes read errors from the Request.Body. The read-from-body vs write-to-network errors are now prioritized differently. * unifies the two mapRoundTripErrorFromXXX methods into one mapRoundTripError method since their logic is now the same. * adds a (*Request).WithT(*testing.T) method in export_test.go, usable by tests, to call t.Logf at points during RoundTrip. This is disabled behind a constant except when debugging. * documents and deflakes TestClientRedirectContext I've tested this CL with high -count values, with/without -race, with/without delays before the select, etc. So far it seems robust. Fixes #19231 (TestTransportBodyReadError flake) Updates #14203 (source of errors unclear; they're now tracked more) Updates #15935 (document Transport errors more; at least understood more now) Change-Id: I3cccc3607f369724b5344763e35ad2b7ea415738 Reviewed-on: https://go-review.googlesource.com/37495 Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
bradfitz
added
Documentation
NeedsInvestigation
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/cc @tombergan |
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Hello all, func AfterRequestSent(err error) bool {
urlErr, ok := err.(*url.Error)
if !ok {
return true
}
opErr, ok := urlErr.Err.(*net.OpError)
if !ok {
return true
}
return opErr.Op != "dial" && opErr.Op != "write"
}According to you, is it correct one? Thx in adv. |
What version of Go are you using (go version)?
1.5.3.
What operating system and processor architecture are you using?
Linux. AMD 64.
What did you do?
I'm using
httputil.ReverseProxyto proxy to a host behind ELB, and I'm getting the errors "http: proxy error: net/http: request canceled" & "http: proxy error: net/http: transport closed before response was received" occationally. The line that printed the error is here. And the error came fromhttp.Transport.RoundTrip.After more digging, the errors could come from a couple places in
http.Transport.RoundTrip. I was trying to find a way to print the callstacks of the error fromhttp.Transport.RoundTripto understand why it was returned but there's currently no way to do so.Details can be found in this golang-nuts thread.
What did you expect to see?
I would expect I could instrument a method of either application code or Go's standard library and find out the callstacks at runtime.
What did you see instead?
No way to find out the callstack of a method from Go's stdlib. This is a feature request.
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