Is there a good way to cancel a blocking read? - go

Question

I've got a command I have to run via the OS, let's call it 'login', that is interactive and therefore requires me to read from the stdin and pass it to the command's stdin pipe in order for it to work correctly. The only problem is the goroutine blocks on a read from stdin and I haven't been able to find a way to cancel a Reader in Go in order to get it to not hang on the blocking call. For example, from the perspective of the user, after the command looks as if it completed, you still have the opportunity to write to stdin once more (then the goroutine will move past the blocking read and exit)
Ideally I would like to avoid having to parse output from the command's StdoutPipe as that makes my code frail and prone to error if the strings of the login command were to change.
loginCmd := exec.Command("login")
stdin , err := loginCmd.StdinPipe()
if err != nil {
return err
}
out, err := loginCmd.StdoutPipe()
if err != nil {
return err
}
if err := loginCmd.Start(); err != nil {
return err
}
ctx, cancel := context.WithCancel(context.TODO())
var done sync.WaitGroup
done.Add(1)
ready := make(chan bool, 1)
defer cancel()
go func(ctx context.Context) {
reader := bufio.NewReader(os.Stdin)
for {
select {
case <- ctx.Done():
done.Done()
return
default:
//blocks on this line, if a close can unblock the read, then it should exit normally via the ctx.Done() case
line, err :=reader.ReadString('\n')
if err != nil {
fmt.Println("Error: ", err.Error())
}
stdin.Write([]byte(line))
}
}
}(ctx)
var bytesRead = 4096
output := make([]byte, bytesRead)
reader := bufio.NewReader(out)
for err == nil {
bytesRead, err = reader.Read(output)
if err != nil && err != io.EOF {
return err
}
fmt.Printf("%s", output[:bytesRead])
}
if err := loginCmd.Wait(); err != nil {
return err
}
cancel()
done.Wait()