Handling connection reset errors in Go - error-handling

Question

On a plain Go HTTP handler, if I disconnect a client while still writting to the response, http.ResponseWritter.Write will return an error with a message like write tcp 127.0.0.1:60702: connection reset by peer.
Now from the syscall package, I have sysca.ECONNRESET, which has the message connection reset by peer, so they're not exactly the same.
How can I match them, so I know not to panic if it occurs ? On other ocasions I have been doing
if err == syscall.EAGAIN {
/* handle error differently */
}
for instance, and that worked fine, but I can't do it with syscall.ECONNRESET.
Update:
Because I'm desperate for a solution, for the time being I'll be doing this very dirty hack:
if strings.Contains(err.Error(), syscall.ECONNRESET.Error()) {
println("it's a connection reset by peer!")
return
}

Answer 1

The error you get has the underlying type *net.OpError, built here, for example.
You should be able to type-assert the error to its concrete type like this:
operr, ok := err.(*net.OpError)
And then access its Err field, which should correspond to the syscall error you need:
operr.Err.Error() == syscall.ECONNRESET.Error()

Answer 2

The answer by #zian is more useful than the accepted answer, but now on Go 1.13+ it is preferable to avoid manually unwrapping the errors:
if errors.Is(opErr,syscall.ECONNRESET) {
fmt.Println("Found a ECONNRESET")
}
This has the benefit that you can also use it more generally, such as after:
resp, err := http.Get("http://127.0.0.1:4444")
Here this err would otherwise have an extra layer of wrapping (*url.Error) and would be missed by the condition #zian used without explicitly unwrapping it a third time.

Answer 3

I came across this issue and the accepted answer was sufficient to point me in the right direction. However, the code it provides to check if the Error embedded inside *net.OpError is ECONNRESET is not complete, at least not for Golang 1.9.
The error embedded at OpError.Err is actually of type *os.SyscallError (https://golang.org/pkg/os/#SyscallError). The Write() function implemented by struct *net.netFD (which is what's being written to when sending a response over the network) looks like this:
func (fd *netFD) Write(p []byte) (nn int, err error) {
nn, err = fd.pfd.Write(p)
runtime.KeepAlive(fd)
return nn, wrapSyscallError("write", err)
}
And wrapSyscallError:
func wrapSyscallError(name string, err error) error {
if _, ok := err.(syscall.Errno); ok {
err = os.NewSyscallError(name, err)
}
return err
}
The error inside the *os.SyscallError struct can be directly compared against syscall.ECONNRESET.
So, given an error returned from a network write (e.g. a call to http.ResponseWritter.Write), the full code block to determine if that error is ECONNRESET is:
if opErr, ok := err.(*net.OpError); ok {
if syscallErr, ok := opErr.Err.(*os.SyscallError); ok {
if syscallErr.Err == syscall.ECONNRESET {
fmt.Println("Found a ECONNRESET")
}
}
}

Answer 4

#zian - thanks for your good solution to João Pinto's (and my) question : How can I match them, so I know not to panic if it occurs ?
As at go version 1.13, an improvement is to use the errors.Is function which does error unwrapping and testing sequentially 'under the hood'. For example :
if errors.Is(opErr,syscall.ECONNRESET) {
fmt.Println("Found a ECONNRESET")
}
#SteveCoffman - adding to your good answer, cheers!
Working with Errors in Go 1.13 - The Go Blog - Golang