net/context: ctx.Err() != error returned by ctxhttp.Do

[abhinav]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?

   go version go1.7rc1 darwin/amd64
   

2. What operating system and processor architecture are you using (go env)?

   GOARCH="amd64"
   GOBIN=""
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="darwin"
   GOOS="darwin"
   GOPATH="/Users/abg/dev/go"
   GORACE=""
   GOROOT="/Users/abg/.gimme/versions/go1.7rc1.darwin.amd64"
   GOTOOLDIR="/Users/abg/.gimme/versions/go1.7rc1.darwin.amd64/pkg/tool/darwin_amd64"
   CC="clang"
   GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/8z/qdzjsr3n5l72vdg67zr6xkjc0000gn/T/go-build692583142=/tmp/go-build -gno-record-gcc-switches -fno-common"
   CXX="clang++"
   CGO_ENABLED="1"
   

3. What did you do?

   The contract for ctxhttp.Do states that if a request timed out or got
   canceled, the error returned will be equal to ctx.Err(). This is true
   for Go 1.6 but is broken in 1.7rc1.

   I wrote a quick little program where the request always times out. It
   prints the returned error, the ctx.Err(), and whether they are equal.

   Full code:

   package main
   
   import (
       "fmt"
       "log"
       "net/http"
       "time"
   
       "golang.org/x/net/context"
       "golang.org/x/net/context/ctxhttp"
   )
   
   func main() {
       http.HandleFunc("/", func(http.ResponseWriter, *http.Request) {
           time.Sleep(1 * time.Second)
       })
       go http.ListenAndServe(":8080", nil)
   
       request, err := http.NewRequest("GET", "http://localhost:8080", nil)
       if err != nil {
           log.Fatal(err)
       }
   
       ctx, cancel := context.WithTimeout(context.Background(), 100*time.Millisecond)
       defer cancel()
   
       _, err = ctxhttp.Do(ctx, nil, request)
       if err == nil {
           log.Fatal("expected error")
       }
   
       fmt.Println("err =", err)
       fmt.Println("ctx.Err() =", ctx.Err())
       fmt.Println("err == ctx.Err() =", err == ctx.Err())
   }
   

4. What did you expect to see?

   Expected output:

   err = context deadline exceeded
   ctx.Err() = context deadline exceeded
   err == ctx.Err() = true
   

   This works as expected in 1.6.

5. What did you see instead?

   Output in 1.7rc1:

   err = Get http://localhost:8080: net/http: request canceled
   ctx.Err() = context deadline exceeded
   err == ctx.Err() = false
   

   This fails even if net/context is switched to Go 1.7's context package.

[gopherbot]

CL https://golang.org/cl/24977 mentions this issue.

[gopherbot]

CL https://golang.org/cl/24978 mentions this issue.