Refactor link module for use in one thread

While `link` module is still spawning new threads to handle new
connections and reading data from connected clients - it now collects
all the received messages in one thread, simplifying their handling.

Cargo.lock

@@ -3,4 +3,45 @@
 [[package]]
 name = "avarice"
 version = "0.1.0"
+dependencies = [
+ "custom_error 1.9.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.126 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde_json 1.0.66 (registry+https://github.com/rust-lang/crates.io-index)",
+]
 
+[[package]]
+name = "custom_error"
+version = "1.9.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "itoa"
+version = "0.4.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "ryu"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "serde"
+version = "1.0.126"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "serde_json"
+version = "1.0.66"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "itoa 0.4.7 (registry+https://github.com/rust-lang/crates.io-index)",
+ "ryu 1.0.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "serde 1.0.126 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[metadata]
+"checksum custom_error 1.9.2 (registry+https://github.com/rust-lang/crates.io-index)" = "4f8a51dd197fa6ba5b4dc98a990a43cc13693c23eb0089ebb0fcc1f04152bca6"
+"checksum itoa 0.4.7 (registry+https://github.com/rust-lang/crates.io-index)" = "dd25036021b0de88a0aff6b850051563c6516d0bf53f8638938edbb9de732736"
+"checksum ryu 1.0.5 (registry+https://github.com/rust-lang/crates.io-index)" = "71d301d4193d031abdd79ff7e3dd721168a9572ef3fe51a1517aba235bd8f86e"
+"checksum serde 1.0.126 (registry+https://github.com/rust-lang/crates.io-index)" = "ec7505abeacaec74ae4778d9d9328fe5a5d04253220a85c4ee022239fc996d03"
+"checksum serde_json 1.0.66 (registry+https://github.com/rust-lang/crates.io-index)" = "336b10da19a12ad094b59d870ebde26a45402e5b470add4b5fd03c5048a32127"

Cargo.toml

@@ -7,3 +7,6 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+custom_error="1.*"
+serde ="1.*"
+serde_json="1.*"

src/link/message.rs

@@ -0,0 +1,38 @@
+//! Implements Avarice message: target service, message type, json parameters
+use serde_json;
+use serde_json::json;
+use std::fmt;
+
+pub struct AvariceMessage {
+    pub service: String,
+    pub message_type: String,
+    pub parameters: serde_json::Value,
+}
+
+impl AvariceMessage {
+    /// Parses JSON form of a message into `AvariceMessage` struct
+    pub fn from(message_str: &str) -> Option<AvariceMessage> {
+        let mut message_json: serde_json::Value = serde_json::from_str(message_str).unwrap();
+        let message_json = message_json.as_object_mut()?;
+        Some(AvariceMessage {
+            service: message_json.remove("s")?.as_str()?.to_owned(),
+            message_type: message_json.remove("t")?.as_str()?.to_owned(),
+            parameters: message_json.remove("p")?,
+        })
+    }
+}
+
+impl fmt::Display for AvariceMessage {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(
+            f,
+            "{}",
+            json!({
+                "s": self.service,
+                "t": self.message_type,
+                "p": self.parameters,
+            })
+            .to_string()
+        )
+    }
+}

src/link/mod.rs

@@ -0,0 +1,111 @@
+//! This module provides a simple interface to exchange messages (`message::AvariceMessage`) between
+//! Avarice and ue-server. The model is simple - we create an mpsc-channel of `Sender` and
+//! `Receiver`, then pass `Sender` to the `network` sub-module and wait for its messages about
+//! ue-servers' connections using `Receiver`.
+use std::collections::HashMap;
+use std::error::Error;
+use std::io::Write;
+use std::net::{SocketAddr, TcpStream};
+use std::sync::mpsc::{channel, Receiver};
+pub use writer::MessageWriter;
+
+mod message;
+mod network;
+mod reader;
+mod writer;
+pub use message::AvariceMessage;
+pub use network::{run_server, NetworkMessage};
+
+/// For collecting messages from all connected ue-servers and providing a way to reply back.
+pub struct AvariceServer {
+    connected_links: HashMap<SocketAddr, Link>,
+    receiver: Receiver<NetworkMessage>,
+}
+
+/// For representing a link to one of the connected ue-servers, can be used to send messages back.
+/// To receive messages use `AvariceServer`'s `next()` method instead.
+pub struct Link {
+    ue_server_address: SocketAddr,
+    writer: MessageWriter,
+    writing_stream: TcpStream,
+}
+
+impl AvariceServer {
+    /// Blocks until a new message arrives from one of the ue-servers. Returns a pair of `Link`,
+    /// corresponding to the ue-server that sent next message and `AvariceMessage`,
+    /// representing that message.
+    pub fn next(&mut self) -> Option<(&mut Link, AvariceMessage)> {
+        loop {
+            match self.receiver.recv() {
+                Ok(NetworkMessage::ConnectionEstablished(ue_server_address, writing_stream)) => {
+                    //  If `ue_server_address` was already present in `self.connected_links`
+                    //  hash map, then it means we have failed to clean it up after it
+                    //  has disconnected. We can just throw away the old value here.
+                    self.connected_links.insert(
+                        ue_server_address,
+                        Link {
+                            ue_server_address,
+                            writing_stream,
+                            writer: MessageWriter::new(),
+                        },
+                    );
+                    continue;
+                }
+                Ok(NetworkMessage::ConnectionLost(ue_server_address)) => {
+                    self.connected_links.remove(&ue_server_address);
+                    continue;
+                }
+                Ok(NetworkMessage::InvalidDataReceived(ue_server_address)) => {
+                    self.connected_links.remove(&ue_server_address);
+                    continue;
+                }
+                Ok(NetworkMessage::UEReceivedUpdate(ue_server_address, ue_received_bytes)) => {
+                    if let Some(link) = self.connected_links.get_mut(&ue_server_address) {
+                        link.update_ue_received_bytes(ue_received_bytes)
+                    }
+                    continue;
+                }
+                Ok(NetworkMessage::MessageReceived(ue_server_address, message)) => {
+                    //  Not having a link with key `ue_server_address` should be impossible here
+                    return self
+                        .connected_links
+                        .get_mut(&ue_server_address)
+                        .and_then(|x| Some((x, message)));
+                }
+                _ => return None,
+            }
+        }
+    }
+}
+
+impl Link {
+    pub fn send(&mut self, message: AvariceMessage) {
+        self.writer.push(&message.to_string());
+        self.flush();
+    }
+
+    pub fn socket_address(&self) -> SocketAddr {
+        self.ue_server_address
+    }
+
+    fn update_ue_received_bytes(&mut self, ue_received_bytes: u64) {
+        self.writer.update_ue_received_bytes(ue_received_bytes);
+        self.flush();
+    }
+
+    fn flush(&mut self) {
+        if let Some(bytes) = self.writer.try_pop() {
+            self.writing_stream.write_all(&bytes).unwrap();
+        }
+    }
+}
+
+/// Creates a new `AvariceServer` that will listen for ue-server connections on the specified port.
+pub fn start_avarice(port: u16) -> Result<AvariceServer, Box<dyn Error>> {
+    let (sender, receiver) = channel();
+    run_server(port, sender)?;
+    Ok(AvariceServer {
+        connected_links: HashMap::new(),
+        receiver,
+    })
+}

src/link/network.rs

@@ -0,0 +1,106 @@
+//!     Implements a network model where messages from all the ue-servers are collected in a single
+//! main thread. For that we spawn a new thread that listens for new connections, which in turn
+//! spawns a new thread for every connected ue-server to handle reading data from it.
+//!     Since all reading is handled in ue-servers' own threads, to collect messages they have
+//! received in the main thread we use `std::sync::mpsc::Sender`. Conversely, all the writing to
+//! ue-server is handled in the main thread itself. Writing `TcpStream` is sent to the main thread
+//! by the same `std::sync::mpsc::Sender` object.
+use super::message::AvariceMessage;
+pub use super::reader::MessageReader;
+use std::error::Error;
+use std::io::Read;
+use std::net::{SocketAddr, TcpListener, TcpStream};
+use std::sync::mpsc::Sender;
+use std::thread;
+
+pub struct UEConnection {
+    pub address: SocketAddr,
+    pub reader: MessageReader,
+    pub reading_stream: TcpStream,
+    pub message_sender: Sender<NetworkMessage>,
+}
+
+/// Possible messages to the main thread
+pub enum NetworkMessage {
+    ConnectionEstablished(SocketAddr, TcpStream),
+    InvalidDataReceived(SocketAddr),
+    ConnectionLost(SocketAddr),
+    MessageReceived(SocketAddr, AvariceMessage),
+    UEReceivedUpdate(SocketAddr, u64),
+}
+
+pub fn run_server(port: u16, message_sender: Sender<NetworkMessage>) -> Result<(), Box<dyn Error>> {
+    let address = SocketAddr::from(([0, 0, 0, 0], port));
+    let listener = TcpListener::bind(address)?;
+    thread::spawn(move || loop {
+        //  Listen to new (multiple) connection
+        let (reading_stream, address) = listener.accept().unwrap();
+        let writing_stream = reading_stream.try_clone().unwrap();
+        message_sender
+            .send(NetworkMessage::ConnectionEstablished(
+                address,
+                writing_stream,
+            ))
+            .unwrap();
+        //  On connection - spawn a new thread
+        let sender_clone = message_sender.clone();
+        thread::spawn(move || {
+            manage_connection(UEConnection {
+                reader: MessageReader::new(),
+                message_sender: sender_clone,
+                reading_stream,
+                address,
+            })
+        });
+    });
+    Ok(())
+}
+
+fn manage_connection(mut connection: UEConnection) {
+    let mut buffer = [0; 1024];
+    loop {
+        //  Reading cycle
+        match connection.reading_stream.read(&mut buffer) {
+            Ok(n) => connection.reader.push(&buffer[..n]).unwrap(),
+            _ => {
+                connection
+                    .message_sender
+                    .send(NetworkMessage::ConnectionLost(connection.address))
+                    .unwrap();
+                return;
+            }
+        };
+        if connection.reader.is_broken() {
+            connection
+                .message_sender
+                .send(NetworkMessage::InvalidDataReceived(connection.address))
+                .unwrap();
+            return;
+        }
+        //  Decoding cycle
+        while let Some(text_message) = connection.reader.pop() {
+            if let Some(avarice_message) = AvariceMessage::from(&text_message) {
+                connection
+                    .message_sender
+                    .send(NetworkMessage::MessageReceived(
+                        connection.address,
+                        avarice_message,
+                    ))
+                    .unwrap();
+            } else {
+                connection
+                    .message_sender
+                    .send(NetworkMessage::InvalidDataReceived(connection.address))
+                    .unwrap();
+                return;
+            }
+        }
+        connection
+            .message_sender
+            .send(NetworkMessage::UEReceivedUpdate(
+                connection.address,
+                connection.reader.ue_received_bytes(),
+            ))
+            .unwrap();
+    }
+}

src/link/reader.rs

@@ -0,0 +1,321 @@
+//! Implements reader that receives data from UE2 game server.
+
+use std::collections::VecDeque;
+use std::str;
+
+extern crate custom_error;
+use custom_error::custom_error;
+
+//  Defines how many bytes is used to encode "AMOUNT" field in the response from ue-server about
+//  amount of bytes it received since the last update
+const UE_RECEIVED_FIELD_SIZE: usize = 2;
+//  Defines how many bytes is used to encode "LENGTH" field, describing length of
+//  next JSON message from ue-server
+const UE_LENGTH_FIELD_SIZE: usize = 4;
+//  Arbitrary value indicating that next byte sequence from ue-server reports amount of bytes
+//  received by that server so far.
+const HEAD_UE_RECEIVED: u8 = 85;
+//  Arbitrary value indicating that next byte sequence from ue-server contains JSON message.
+const HEAD_UE_MESSAGE: u8 = 42;
+//  Maximum allowed size of JSON message sent from ue-server.
+const MAX_UE_MESSAGE_LENGTH: usize = 25 * 1024 * 1024;
+
+custom_error! { pub ReadingStreamError
+    InvalidHead{input: u8}          = "Invalid byte used as a HEAD: {input}",
+    MessageTooLong{length: usize}   = "Message to receive is too long: {length}",
+    InvalidUnicode                  = "Invalid utf-8 was received",
+    BrokenStream                    = "Used stream is broken"
+}
+
+enum ReadingState {
+    Head,
+    ReceivedBytes,
+    Length,
+    Payload,
+}
+
+/// For converting byte stream that is expected from the ue-server into actual messages.
+///
+/// Expected format is a sequence of either:
+///
+/// | Data | Length |
+/// |---------|---------|
+/// | [`HEAD_UE_RECEIVED`] (marker byte)     | 1 byte    |
+/// | Amount of bytes received by ue-server since last update | 2 bytes: u16 BE|
+///
+/// or
+///
+/// | Data | Length |
+/// |---------|---------|
+/// | [`HEAD_UE_MESSAGE`] (marker byte) | 1 byte|
+/// | `LENGTH` of the JSON message in utf8 encoding | 4 bytes: u32 BE|
+/// | UTF8-encoded string | `LENGTH` bytes|
+///
+/// On any invalid input enters into a failure state (can be checked by `is_broken()`) and never recovers from it.
+/// Use either `push_byte()` or `push()` to input byte stream from ue-server and `pop()` to
+/// retrieve resulting messages.
+pub struct MessageReader {
+    is_broken: bool,
+    reading_state: ReadingState,
+    read_bytes: usize,
+    length_buffer: [u8; 4],
+    current_message_length: usize,
+    current_message: Vec<u8>,
+    read_messages: VecDeque<String>,
+    ue_received_bytes: u64,
+}
+
+impl MessageReader {
+    pub fn new() -> MessageReader {
+        MessageReader {
+            is_broken: false,
+            reading_state: ReadingState::Head,
+            read_bytes: 0,
+            length_buffer: [0; 4],
+            current_message_length: 0,
+            //  Will be recreated with `with_capacity` in `push_byte()`
+            current_message: Vec::new(),
+            //  This value should be more than enough for typical use
+            read_messages: VecDeque::with_capacity(100),
+            ue_received_bytes: 0,
+        }
+    }
+
+    pub fn push_byte(&mut self, input: u8) -> Result<(), ReadingStreamError> {
+        if self.is_broken {
+            return Err(ReadingStreamError::BrokenStream);
+        }
+        match &self.reading_state {
+            ReadingState::Head => {
+                if input == HEAD_UE_RECEIVED {
+                    self.change_state(ReadingState::ReceivedBytes);
+                } else if input == HEAD_UE_MESSAGE {
+                    self.change_state(ReadingState::Length);
+                } else {
+                    self.is_broken = true;
+                    return Err(ReadingStreamError::InvalidHead { input });
+                }
+            }
+            ReadingState::ReceivedBytes => {
+                self.length_buffer[self.read_bytes] = input;
+                self.read_bytes += 1;
+                if self.read_bytes >= UE_RECEIVED_FIELD_SIZE {
+                    self.ue_received_bytes += u64::from(array_of_u8_to_u16(self.length_buffer));
+                    self.change_state(ReadingState::Head);
+                }
+            }
+            ReadingState::Length => {
+                self.length_buffer[self.read_bytes] = input;
+                self.read_bytes += 1;
+                if self.read_bytes >= UE_LENGTH_FIELD_SIZE {
+                    self.current_message_length = array_of_u8_to_u32(self.length_buffer) as usize;
+                    if self.current_message_length > MAX_UE_MESSAGE_LENGTH {
+                        self.is_broken = true;
+                        return Err(ReadingStreamError::MessageTooLong {
+                            length: self.current_message_length,
+                        });
+                    }
+                    self.current_message = Vec::with_capacity(self.current_message_length);
+                    self.change_state(ReadingState::Payload);
+                }
+            }
+            ReadingState::Payload => {
+                self.current_message.push(input);
+                self.read_bytes += 1;
+                if self.read_bytes >= self.current_message_length {
+                    match str::from_utf8(&self.current_message) {
+                        Ok(next_message) => self.read_messages.push_front(next_message.to_owned()),
+                        _ => {
+                            self.is_broken = true;
+                            return Err(ReadingStreamError::InvalidUnicode);
+                        }
+                    };
+                    self.current_message.clear();
+                    self.current_message_length = 0;
+                    self.change_state(ReadingState::Head);
+                }
+            }
+        }
+        Ok(())
+    }
+
+    pub fn push(&mut self, input: &[u8]) -> Result<(), ReadingStreamError> {
+        for &byte in input {
+            self.push_byte(byte)?;
+        }
+        Ok(())
+    }
+
+    pub fn pop(&mut self) -> Option<String> {
+        self.read_messages.pop_back()
+    }
+
+    pub fn ue_received_bytes(&self) -> u64 {
+        self.ue_received_bytes
+    }
+
+    pub fn is_broken(&self) -> bool {
+        self.is_broken
+    }
+
+    fn change_state(&mut self, next_state: ReadingState) {
+        self.read_bytes = 0;
+        self.reading_state = next_state;
+    }
+}
+
+fn array_of_u8_to_u16(bytes: [u8; 4]) -> u16 {
+    (u16::from(bytes[0]) << 8) + u16::from(bytes[1])
+}
+
+fn array_of_u8_to_u32(bytes: [u8; 4]) -> u32 {
+    (u32::from(bytes[0]) << 24)
+        + (u32::from(bytes[1]) << 16)
+        + (u32::from(bytes[2]) << 8)
+        + (u32::from(bytes[3]))
+}
+
+#[test]
+fn message_push_byte() {
+    let mut reader = MessageReader::new();
+    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(13).unwrap();
+    reader.push_byte(b'H').unwrap();
+    reader.push_byte(b'e').unwrap();
+    reader.push_byte(b'l').unwrap();
+    reader.push_byte(b'l').unwrap();
+    reader.push_byte(b'o').unwrap();
+    reader.push_byte(b',').unwrap();
+    reader.push_byte(b' ').unwrap();
+    reader.push_byte(b'w').unwrap();
+    reader.push_byte(b'o').unwrap();
+    reader.push_byte(b'r').unwrap();
+    reader.push_byte(b'l').unwrap();
+    reader.push_byte(b'd').unwrap();
+    reader.push_byte(b'!').unwrap();
+    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(3).unwrap();
+    reader.push_byte(b'Y').unwrap();
+    reader.push_byte(b'o').unwrap();
+    reader.push_byte(b'!').unwrap();
+    assert_eq!(reader.pop().unwrap(), "Hello, world!");
+    assert_eq!(reader.pop().unwrap(), "Yo!");
+    assert_eq!(reader.pop(), None);
+}
+
+#[test]
+fn received_push_byte() {
+    let mut reader = MessageReader::new();
+    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0xf3).unwrap();
+    assert_eq!(reader.ue_received_bytes(), 0xf3);
+    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
+    reader.push_byte(0xb2).unwrap();
+    reader.push_byte(0x04).unwrap();
+    assert_eq!(reader.ue_received_bytes(), 0xb2_f7); // 0xf7 = 0x04 + 0xf3
+    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
+    reader.push_byte(231).unwrap();
+    assert_eq!(reader.ue_received_bytes(), 0xb2_f7);
+}
+
+#[test]
+fn mixed_push_byte() {
+    let mut reader = MessageReader::new();
+    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0xf3).unwrap();
+    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(3).unwrap();
+    reader.push_byte(b'Y').unwrap();
+    reader.push_byte(b'o').unwrap();
+    reader.push_byte(b'!').unwrap();
+    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
+    reader.push_byte(0xb2).unwrap();
+    reader.push_byte(0x04).unwrap();
+    assert_eq!(reader.ue_received_bytes(), 0xb2_f7); // 0xf7 = 0x04 + 0xf3
+    assert_eq!(reader.pop().unwrap(), "Yo!");
+    assert_eq!(reader.pop(), None);
+}
+
+#[test]
+fn pushing_many_bytes_at_once() {
+    let mut reader = MessageReader::new();
+    reader
+        .push(&[
+            HEAD_UE_RECEIVED,
+            0,
+            0xf3,
+            HEAD_UE_MESSAGE,
+            0,
+            0,
+            0,
+            3,
+            b'Y',
+            b'o',
+            b'!',
+            HEAD_UE_RECEIVED,
+            0xb2,
+            0x04,
+        ])
+        .unwrap();
+    assert_eq!(reader.ue_received_bytes(), 0xb2_f7);
+    assert_eq!(reader.pop().unwrap(), "Yo!");
+    assert_eq!(reader.pop(), None);
+}
+
+#[test]
+fn generates_error_invalid_head() {
+    let mut reader = MessageReader::new();
+    reader.push_byte(HEAD_UE_RECEIVED).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0xf3).unwrap();
+    assert!(!reader.is_broken());
+    reader
+        .push_byte(25)
+        .expect_err("Testing failing on incorrect HEAD");
+    assert!(reader.is_broken());
+}
+
+#[test]
+fn generates_error_message_too_long() {
+    let mut reader = MessageReader::new();
+    let huge_length = MAX_UE_MESSAGE_LENGTH + 1;
+    let bytes = (huge_length as u32).to_be_bytes();
+
+    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
+    reader.push_byte(bytes[0]).unwrap();
+    reader.push_byte(bytes[1]).unwrap();
+    reader.push_byte(bytes[2]).unwrap();
+    assert!(!reader.is_broken());
+    reader
+        .push_byte(bytes[3])
+        .expect_err("Testing failing on exceeding allowed message length");
+    assert!(reader.is_broken());
+}
+
+#[test]
+fn generates_error_invalid_unicode() {
+    let mut reader = MessageReader::new();
+    reader.push_byte(HEAD_UE_MESSAGE).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(0).unwrap();
+    reader.push_byte(2).unwrap();
+    reader.push_byte(0b11010011).unwrap(); // start of 2-byte sequence
+    assert!(!reader.is_broken());
+    //  Bytes inside multi-byte code point have to have `1` for their high bit
+    reader
+        .push_byte(0b01010011)
+        .expect_err("Testing failing on incorrect unicode");
+    assert!(reader.is_broken());
+}

src/link/writer.rs

@@ -0,0 +1,169 @@
+//! Implements writer that sends data to UE2 game server.
+
+use std::cmp::{max, min};
+use std::collections::VecDeque;
+use std::convert::TryFrom;
+use std::iter::Extend;
+
+//  Maximum amount of bytes ue-server is able to receive at once
+const UE_INPUT_BUFFER: usize = 4095;
+
+/// For converting text messages into chunks of bytes that can be sent to the ue-server.
+///
+/// Every string message is converted into a length-prefixed array of utf8 bytes:
+///
+/// | Data | Length |
+/// |---------|---------|
+/// | Message `LENGTH` | 4 bytes: u32 BE |
+/// | UTF8-encoded string | `LENGTH` bytes|
+///
+/// Resulting byte sequences from all messages are then concatenated (in the same order as they were
+/// "written") into a single data stream. Bytes from the data stream are returned in chunks of
+/// size no more than `UE_INPUT_BUFFER`. New chunk is returned only when `MessageWriter` knows
+/// that ue-server's buffer has enough space to accept it.
+///
+/// Use `push()` to input string messages and `try_pop()` to retrieve next chunk.
+/// NOTE: `try_pop()` can return `None` even if not all message data has been returned,
+/// in case `MessageWriter` thinks that ue-server's buffer does not have enough space.
+///
+/// Call `update_ue_received_bytes()` to update `MessageWriter`'s information about
+/// how many bytes ue-server has received so far. This can signal that its buffer has enough
+/// free space. `MessageWriter` assumes that all data returned by its `try_pop()` method is sent
+/// to ue-server.
+///
+/// Use `is_empty()` method to check for whether `MessageWriter` still has some data to return
+/// (possibly after ``update_ue_received_bytes()`).
+pub struct MessageWriter {
+    sent_bytes: u64,
+    ue_received_bytes: u64,
+    pending_data: VecDeque<u8>,
+}
+
+impl MessageWriter {
+    pub fn new() -> MessageWriter {
+        MessageWriter {
+            sent_bytes: 0,
+            ue_received_bytes: 0,
+            //  This value should be more than enough for typical use:
+            //  it will take at least one second to send this much data to a 30 tick rate ue-server.
+            pending_data: VecDeque::with_capacity(30 * UE_INPUT_BUFFER),
+        }
+    }
+
+    pub fn push(&mut self, message: &str) {
+        let message_as_utf8 = message.as_bytes();
+        let message_as_utf8 = [
+            &(message_as_utf8.len() as u32).to_be_bytes(),
+            message_as_utf8,
+        ]
+        .concat();
+        self.pending_data.extend(message_as_utf8.into_iter());
+    }
+
+    pub fn try_pop(&mut self) -> Option<Vec<u8>> {
+        if self.is_empty() {
+            return None;
+        }
+        let chunk_size = min(self.available_ue_buffer_capacity(), self.pending_data.len());
+        if chunk_size == 0 {
+            return None;
+        }
+        let mut bytes_to_send = Vec::with_capacity(chunk_size);
+        for next_byte in self.pending_data.drain(..chunk_size) {
+            bytes_to_send.push(next_byte);
+        }
+        self.sent_bytes += bytes_to_send.len() as u64;
+        Some(bytes_to_send)
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.pending_data.is_empty()
+    }
+
+    /// Takes total amount of bytes received so far by the ue-server, not just bytes received after
+    /// the last `update_ue_received_bytes()` call.
+    pub fn update_ue_received_bytes(&mut self, ue_received_bytes: u64) {
+        self.ue_received_bytes = max(ue_received_bytes, self.ue_received_bytes);
+    }
+
+    fn available_ue_buffer_capacity(&self) -> usize {
+        match usize::try_from(self.sent_bytes - self.ue_received_bytes).ok() {
+            Some(ue_buffered_bytes) => max(0, UE_INPUT_BUFFER - ue_buffered_bytes),
+            _ => 0,
+        }
+    }
+}
+
+#[test]
+fn writer_contents_after_creation() {
+    let writer = MessageWriter::new();
+    assert_eq!(writer.is_empty(), true);
+    assert_eq!(writer.available_ue_buffer_capacity(), UE_INPUT_BUFFER);
+}
+
+#[test]
+fn writer_content_after_push() {
+    let mut writer = MessageWriter::new();
+    writer.push("Hello, world!");
+    assert_eq!(writer.is_empty(), false);
+    assert_eq!(writer.available_ue_buffer_capacity(), UE_INPUT_BUFFER);
+}
+
+#[test]
+fn writing_single_short_message() {
+    let mut writer = MessageWriter::new();
+    writer.push("Hello, world!");
+    let resulting_bytes = writer.try_pop().unwrap();
+    let expected_bytes = [
+        0, 0, 0, 13, // Bytes in the message
+        b'H', b'e', b'l', b'l', b'o', b',', b' ', b'w', b'o', b'r', b'l', b'd', b'!',
+    ];
+    assert_eq!(writer.is_empty(), true);
+    assert_eq!(
+        writer.available_ue_buffer_capacity(),
+        UE_INPUT_BUFFER - "Hello, world!".len() - 4
+    );
+    assert_eq!(resulting_bytes, expected_bytes);
+    assert_eq!(writer.sent_bytes, expected_bytes.len() as u64);
+}
+
+#[test]
+fn writing_first_chunk_of_single_long_message() {
+    let mut writer = MessageWriter::new();
+    //  Because we also have to pass message length, this will go over the sending limit
+    let long_message = "Q".repeat(UE_INPUT_BUFFER);
+    writer.push(&long_message);
+    let resulting_bytes = writer.try_pop().unwrap();
+    assert_eq!(writer.is_empty(), false);
+    assert_eq!(resulting_bytes.len(), UE_INPUT_BUFFER);
+    assert_eq!(writer.available_ue_buffer_capacity(), 0);
+    //  Bytes in message = 4095 = 0x0fff
+    assert_eq!(resulting_bytes[0], 0);
+    assert_eq!(resulting_bytes[1], 0);
+    assert_eq!(resulting_bytes[2], 0x0f);
+    assert_eq!(resulting_bytes[3], 0xff);
+    for &byte in resulting_bytes[4..].iter() {
+        assert_eq!(byte, b'Q');
+    }
+    assert_eq!(writer.try_pop(), None);
+    assert_eq!(writer.is_empty(), false);
+}
+
+#[test]
+fn writing_second_chunk_of_single_long_message() {
+    let mut writer = MessageWriter::new();
+    //  Because we also have to pass lengths, this will go over the sending limit
+    let long_message = "Q".repeat(UE_INPUT_BUFFER);
+    writer.push(&long_message);
+    //  This pops all but 4 bytes of `long_message`, that were required to encode message length
+    let first_bytes = writer.try_pop().unwrap();
+    writer.update_ue_received_bytes(first_bytes.len() as u64);
+    let resulting_bytes = writer.try_pop().unwrap();
+    assert_eq!(
+        writer.available_ue_buffer_capacity(),
+        UE_INPUT_BUFFER - resulting_bytes.len()
+    );
+    assert_eq!(writer.is_empty(), true);
+    //  Bytes left for the next chunk = 4
+    assert_eq!(resulting_bytes, [b'Q', b'Q', b'Q', b'Q'])
+}

src/main.rs

@@ -1,13 +1,14 @@
-use std::env;
-use std::path::Path;
-mod unreal_config;
+// ! This utility provides a way to communicate with Unreal Engine 2 servers running Acedia mutator.
+mod link;
+use link::start_avarice;
 
 fn main() {
-    let args: Vec<String> = env::args().collect();
-    let filename = &args[1];
-    let config = unreal_config::load_file(Path::new(filename));
-    match config {
-        Ok(config) => print!("{}", config),
-        _ => (),
+    let mut server = start_avarice(1234).unwrap();
+    while let Some((link, message)) = server.next() {
+        println!("{}: {}", link.socket_address(), message.to_string());
+        if message.message_type != "end" {
+            link.send(message);
+        }
     }
+    println!("Avarice has shut down!");
 }