telechat

A telnet chat server using distributed-process.

Build and run:

stack build
stack exec telechat

Connect:

telnet localhost 4444
sf@mymachine>telnet localhost 4444
	Trying 127.0.0.1...
	Connected to localhost.
	Escape character is '^]'.
	hello!
	this is a message from someone else!
	say something > why haven't I added identity yet? laziness.

TODO:

• Command line options to select port
• Identity/Authentication
• Connection limits / spam filtering?
• Better line editing

Design

• Each socket connection spawns a pair of child processes
  • the "reader" process
    • Interprets (and discards) telnet commands
    • parse remaining data to UTF-8
    • Interprets and control codes / removes unreadables from stream of client data.
  • the "writer" process
    • Receives commands from child reader
    • Renders the child's terminal
    • Receives chat messages from other clients
    • Broadcasts chat messages to other writers

TODO

Implementation

• high level design
  • Reader/Writer
  • Client identity
• Child processes
  • Reader
    • Discard telnet commands.
    • Parse UTF-8
    • Interpret control codes (DEL, Enter, etc.)
    • Filter remaining non-readable chars from chat messages
    • Forward messages to Writer
    • Write IO to wrap readingMachine
  • Writer
    • Update state of client terminal
    • Broadcast 'Send' messages to other writers
    • Draw client terminal
    • Convert 'Text' messages to valid telnet data (simply encodeUtf8 - see Telnet/UTF-8 note below)
    • Write IO to wrap writingMachine
    • Test that messages displayed on local terminal as well as remote

Performance

• Place limits on client input buffers (don't want to allow clients to DoS via unbounded input size)
• Use a more appropriate datastructure for editing text
• Reconsider use of Attoparsec for parsing?

Better line editing

• No sending of blank lines
• ctrl+c clears line
• ctrl+w "readline"-ish rubout behaviour
• ctrl+l clears screen (may have to implement more telnet!)
• Purify editing code?

Refactoring

• Test that "Chat" message is broadcast - refactor so Chat isn't added in Telechat.hs IO code

TODO: regression tests - already fixed, but make sure they don't break again.

• Reader/Writer dying means other process also dies (seems broken?)
• Backspace on empty buffer
• Unreadable characters don't crash client
• Test line clearing (ctrl+c) works (it does, but tessttttssss)
• Property based tests!
  • Check all combinations of valid + invalid inputs
  • Service does not drop input
  • Service does not hang (i.e. stopping machines!)

Telnet/UTF note

0xFF is not a valid UTF-8 byte, so we'll never see it after encoding a value of type 'Text'. This means we don't have to escape it in Telnet, so no extra work needs to be done. See https://en.wikipedia.org/wiki/UTF-8

Bugs / miscellaneous TODOs

• Place limits on length of parsed telnet commands. Probably if we go over ~1000 bytes, we've encountered a telnet command that has broken the parser.