add tab completion for multiplayer #44

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1. YtvwlD

   added

   enhancementNew feature or requestNew feature or request

   on May 29, 2025
2. YtvwlD commented on Aug 17, 2025

   

   YtvwlD

   on Aug 17, 2025

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   I took a look at this last Thursday (together with @vitasomething) and this is more difficult than it might look like:

   telnet clients usually send a whole line at a time

   (This also applies to nc, even though it's technically not a telnet client.)

   This is RFC 1116 ("Telnet Linemode Option").
   This is a good idea, because it drastically reduces latency: Users can type and they get to see their characters immediately.

   But this doesn't work for us: When we want to complete, we need to see the part of the line that has already been entered.

   According to StackOverflow, we could instruct the client to send each char right as it is entered:

   # telnet: IAC WONT LINEMODE IAC WILL ECHO
   # see: https://stackoverflow.com/a/279271/2192464
   self.wfile.write(b"\377\375\042\377\373\001")
   # the client will ACK this, ignore this message
   self.rfile.read(65)

   This is ugly because this doesn't actually parses the client answer, but it basically works.

   But, this comes with two new problems:

   we're now responsible for line editing

   The easiest part of this is receiving each char, storing it in a buffer and returning it on enter:

   text = b""
   while True:
       char = self.file.read(1)
       if char == b"\0":
           break
       text += char

   But there's more problems here: What if the users presses backspace? Or even uses the arrow keys? This gets difficult.
   What if we're at the end of the line? How do we even know how wide the client's terminal is? (There's a telnet command for this!)

   It would be great if there were libraries for this. We'd need a telnet server library and a readline-like library and we'd need to stitch them together.

   But there just isn't? Apparently?

   There's telnetlib3 which seems pretty promising, but it's async, so it completely breaks cmd.
   And readline needs stdin to be the single real terminal – we want multiple connections with their own state at the same time.

   the client might not support this

   nc is not a telnet client. It'll probably print the IACs as garbled text and won't answer anything.
   This could be solved by first printing something like "Configuring your telnet client... If this doesn't work, press Enter.\r" and then the IAC. We would then need to match if the next character that the client sends is an IAC (0xff) or a newline.