How to check a net.Conn is closed?

Or how to check it is available for Read or Write in loop? If the conn is closed or not available, we should stop the loop.

For example:

package main

import "net"

func main() {
    conn, err := net.Dial("tcp", "127.0.0.1:1111")
    defer conn.Close()
    for {
        buf := make([]byte, 1, 1)
        n, err := conn.Read(buf)
        if err != nil {
            // currently we can only stop the loop
            // when occur any errors
            log.Fatal(err)
        }
    }
}

You can get a number of errors, depending on how the connection was closed. The only error that you can count on receiving from a Read is an io.EOF. io.EOF is the value use to indicate that a connection was closed normally.

Other errors can be checked against the net.Error interface for its Timeout and Temporary methods. These are usually of the type net.OpError. Any non-temporary error returned from a Write is fatal, as it indicates the write couldn't succeed, but note that due to the underlying network API, writes returning no error still aren't guaranteed to have succeeded.

In general you can just follow the io.Reader api.

> When Read encounters an error or end-of-file condition after successfully reading n > 0 bytes, it returns the number of bytes read. It may return the (non-nil) error from the same call or return the error (and n == 0) from a subsequent call. An instance of this general case is that a Reader returning a non-zero number of bytes at the end of the input stream may return either err == EOF or err == nil. The next Read should return 0, EOF.

If there was data read, you handle that first. After you handle the data, you can break from the loop on any error. If it was io.EOF, the connection is closed normally, and any other errors you can handle as you see fit.

If you are using Go 1.16 or newer and only working with the standard libraries (not some arbitrary networking stack), you can use a function like this to check the closed errors and handle them differently than other errors:

func isNetConnClosedErr(err error) bool {
	switch {
	case
		errors.Is(err, net.ErrClosed),
		errors.Is(err, io.EOF),
		errors.Is(err, syscall.EPIPE):
		return true
	default:
		return false
	}
}

Note that there is an os.ErrClosed error (alias for fs.ErrClosed) that you could add if you were dealing with files, but you don't need it when only using the net package. While your code only shows a client, there is probably a server side doing listener.Accept() that gets closed by a different go process and you don't want to log nasty errors when that happens, so you want to have net.ErrClosed in the list above. As for syscall.EPIPE, this comes in handy when writing to a remote end that already closed the connection. Remember that with networking connections, you may not get the EPIPE on the 1st write, as the OS may need to send some data to discover that the remote end was closed.