when all I get is EOF from a net.TCPConn, how do I know if the connection has been dropped?

The godocs say:

"EOF is the error returned by Read when no more input is available. Functions should return EOF only to signal a graceful end of input. If the EOF occurs unexpectedly in a structured data stream, the appropriate error is either ErrUnexpectedEOF or some other error giving more detail."

and in my program I would love love to get ErrUnexpectedEOF, but I don't. I always get just EOF even when the client terminates in the middle of sending a file.

so, i'm stuck. Successful files that transfer 100% of the way thru send EOF at end. And files that quit 50% thru with error also send EOF as the error. I do not know the total size of the file before the transfer.

Is there any more information I could get from the net.TCPConn to know was this a full file or not?

TCP is a streaming protocol; a stream of bytes. That's why file transfer protocols invented/implemented upon TCP, like FTP (File Transfer Protocol).

But (not recommended/thought experiment) if you can send your files in chunks that does not contain a certain character/byte (like base64 + \n as that certain character), then you can use that certain character as delimiter.

Now if we handle our connections using a function similar to this:

func (srv *Server) handler(conn net.Conn) {
	//...

	defer srv.closeConnection(conn, nil)

	reader := bufio.NewReader(conn)
	writer := bufio.NewWriter(conn)

	//...

	conn.SetDeadline(time.Now().Add(conf.ClientTimeout))
		
	for {
		select {
		case <-conf.QuitSignal:
			return
		default:
			line, err := reader.ReadBytes('\n')
			if err != nil {
				return
			}

			//...
		}

		conn.SetDeadline(time.Now().Add(conf.ClientTimeout))
	}
}

func (srv *Server) closeConnection(conn net.Conn, err error) {
	conn.Close()
	//...
}

From the documentation of Reader.ReadBytes we read ReadBytes returns err != nil if and only if the returned data does not end in delimiter. This way we can recognize a broken chunk.