net/http: Export request errors

[stevenosborne-wf]

tl;dr

From the net/http package, export errTimeout, errRequestCanceled, and errRequestCanceledConn.

Request timeout

When making an http request with a timeout, the http client.Do throws an unexported errTimeout. This makes parsing the error problematic. For example:

package main

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

func main() {
        client := http.DefaultClient
        request, _ := http.NewRequest("GET", "https://google.com", nil)
        ctx, _ := context.WithTimeout(context.Background(), 1*time.Millisecond)
        request = request.WithContext(ctx)
        if response, err := client.Do(request); err != nil {
                // Untyped error. Have to scrape error message to find out if it's a
                // timeout.
                fmt.Println(err)
        } else {
                fmt.Println(response.StatusCode)
        }
}

It would be safer to export this error. For example:

package main

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

func main() {
        client := http.DefaultClient
        request, _ := http.NewRequest("GET", "https://google.com", nil)
        ctx, _ := context.WithTimeout(context.Background(), 1*time.Millisecond)
        request = request.WithContext(ctx)
        if response, err := client.Do(request); err != nil {
                // Untyped error. Have to scrape error message to find out if it's a
                // timeout.
                if err == http.ErrTimeout {
                        // Retry request with a back off.
                }
        } else {
                fmt.Println(response.StatusCode)
        }
}

[davecheney]

Assert if the error implements Timeout() and use that.

if err, ok := err.(net.Error); ok {
      if err.Timeout() { 
            // whatever
      }
}

[bradfitz]

Dup of #15935, #14203, #13667

[stevenosborne-wf]

@bradfitz I did a quick search and couldn't find a similar issue. Thanks for pointing those out.

[stevenosborne-wf]

@davecheney Thanks