net/http: Client returns "request canceled" on timeout

[abhinav]

Please answer these questions before submitting your issue. Thanks!

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

$ go version
go version go1.7.3 darwin/amd64

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

$ go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/abg/dev/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.7.3/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.7.3/libexec/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-build965147697=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"time"
)

func main() {
	req, err := http.NewRequest("GET", "https://httpbin.org/delay/3", nil)
	if err != nil {
		log.Fatal(err)
	}

	ctx, cancel := context.WithTimeout(context.Background(), 500*time.Millisecond)
	defer cancel()

	_, err = http.DefaultClient.Do(req.WithContext(ctx))
	fmt.Printf("err = %v (%T)\n", err, err)
	fmt.Printf("ctx.Err() = %v (%T)\n", ctx.Err(), ctx.Err())
}

What did you expect to see?

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

err should be a "context deadline exceeded" error and it should be the same as ctx.Error().

What did you see instead?

err and ctx.Error() are different. err is a "request canceled" error even though the context's cancel() hasn't been called yet.

err = Get https://httpbin.org/delay/3: net/http: request canceled while waiting for connection (*url.Error)
ctx.Err() = context deadline exceeded (context.deadlineExceededError)

Related to #16381, which was addressed in ctxhttp using,

if err != nil {
	select {
	case <-ctx.Done():
		err = ctx.Err()
	default:
	}
}

[bradfitz]

@odeke-em, was there a CL for this?

[odeke-em]

Yap, there is https://go-review.googlesource.com/#/c/24230 but it's been stalled. Also seems like this issue is a duplicate of #16094.