Implemented client/server protocol

Cargo.toml

@@ -7,5 +7,9 @@ edition = "2021"
 
 [dependencies]
 argh = "0.1.12"
+bincode = "1.3.3"
+crc32fast = "1.3.2"
 crossterm = "0.27.0"
+interprocess = "1.2.1"
 ratatui = "0.23.0"
+serde = "1.0.196"

src/bin/client.rs

@@ -1,3 +0,0 @@
-fn main() {
-    rmp::client::run().unwrap();
-}

src/client.rs

@@ -1,24 +1,38 @@
-use argh::FromArgs;
+use std::{
-use std::{error::Error, time::Duration};
+    error::Error,
+    sync::{Arc, Mutex},
+    time::Duration,
+};
+
+use crate::CliArgs;
+
+use self::{app::App, request_queue::request_queue_cleaner};
 
 pub mod app;
 pub mod crossterm;
+pub mod request_queue;
 pub mod ui;
 
-/// Demo
+pub fn run(args: CliArgs) -> Result<(), Box<dyn Error>> {
-#[derive(Debug, FromArgs)]
+    let message_queue = Arc::new(Mutex::new(vec![]));
-struct Cli {
+    let server_state = Arc::new(Mutex::new(None));
-    /// time in ms between two ticks.
+    let app = App {
-    #[argh(option, default = "250")]
+        title: "rmp - Rust Music Player".into(),
-    tick_rate: u64,
+        enhanced_graphics: args.enhanced_graphics,
-    /// whether unicode symbols are used to improve the overall look of the app
+        should_quit: false,
-    #[argh(option, default = "true")]
+        state: server_state.clone(),
-    enhanced_graphics: bool,
+        message_queue: message_queue.clone(),
-}
+    };
-
+    let thread_builder = std::thread::Builder::new().name("request_queue".into());
-pub fn run() -> Result<(), Box<dyn Error>> {
+    thread_builder
-    let cli: Cli = argh::from_env();
+        .spawn(move || {
-    let tick_rate = Duration::from_millis(cli.tick_rate);
+            request_queue_cleaner(
-    crossterm::run(tick_rate, cli.enhanced_graphics)?;
+                Duration::from_millis(args.message_rate),
+                message_queue.clone(),
+                server_state.clone(),
+            )
+        })
+        .unwrap();
+    crossterm::run(app, Duration::from_millis(args.tick_rate))?;
     Ok(())
 }

src/client/app.rs

@@ -1,226 +1,62 @@
-use std::{ops::Deref, time::Duration};
+use std::{
+    sync::{Arc, Mutex},
+    time::Duration,
+};
 
-const FILE_PATHS: [&str; 0] = [];
+use rmp::{
-
+    protocol::{Message, MessageType},
-#[derive(Debug, Default, Clone, Eq, PartialEq, Hash)]
+    ServerState,
-pub struct ListState {
+};
-    offset: usize,
-    selected: Option<usize>,
-}
-
-impl ListState {
-    pub fn offset(&self) -> usize {
-        self.offset
-    }
-
-    pub fn offset_mut(&mut self) -> &mut usize {
-        &mut self.offset
-    }
-
-    pub fn with_selected(mut self, selected: Option<usize>) -> Self {
-        self.selected = selected;
-        self
-    }
-
-    pub fn with_offset(mut self, offset: usize) -> Self {
-        self.offset = offset;
-        self
-    }
-
-    pub fn selected(&self) -> Option<usize> {
-        self.selected
-    }
-
-    pub fn select(&mut self, index: Option<usize>) {
-        self.selected = index;
-        if index.is_none() {
-            self.offset = 0;
-        }
-    }
-}
-
-pub struct StatefulList<T> {
-    pub state: ListState,
-    pub items: Vec<T>,
-}
-
-impl<T> StatefulList<T>
-where
-    T: Deref + Eq,
-{
-    pub fn with_items(items: Vec<T>) -> StatefulList<T> {
-        StatefulList {
-            state: ListState::default(),
-            items,
-        }
-    }
-
-    pub fn next(&mut self) {
-        let i = match self.state.selected() {
-            Some(i) => {
-                if i >= self.items.len() - 1 {
-                    0
-                } else {
-                    i + 1
-                }
-            }
-            None => 0,
-        };
-        self.state.select(Some(i));
-    }
-
-    pub fn previous(&mut self) {
-        let i = match self.state.selected() {
-            Some(i) => {
-                if i == 0 {
-                    self.items.len() - 1
-                } else {
-                    i - 1
-                }
-            }
-            None => 0,
-        };
-        self.state.select(Some(i));
-    }
-
-    pub fn index_of(&self, item: T) -> Option<usize> {
-        self.items.iter().position(|element| *element == item)
-    }
-
-    pub fn current(&self) -> Option<&T> {
-        self.state
-            .selected
-            .map_or(None, |index| self.items.get(index))
-    }
-
-    pub fn current_mut(&mut self) -> Option<&mut T> {
-        self.state
-            .selected
-            .map_or(None, |index| self.items.get_mut(index))
-    }
-}
-
-// TODO: Implement this
-#[derive(Default, Clone)]
-pub struct TrackMetadata {
-    pub duration: Option<Duration>,
-}
-
-pub struct PlayerState {
-    pub currently_playing: Option<String>,
-    pub is_playing: bool,
-    pub time_ratio: f64,
-}
-
-impl Default for PlayerState {
-    fn default() -> Self {
-        Self {
-            currently_playing: None,
-            is_playing: false,
-            time_ratio: 0.0,
-        }
-    }
-}
-
-impl PlayerState {
-    pub fn play(&mut self, path: &str) {
-        self.currently_playing = Some(path.to_string());
-        self.is_playing = true;
-        self.time_ratio = 0.0;
-        // TODO: Handle audio
-    }
-
-    pub fn toggle_pause(&mut self) {
-        self.is_playing = !self.is_playing;
-    }
-}
 
 pub struct App {
     pub title: String,
-    pub playlist: StatefulList<String>,
-    pub player_state: PlayerState,
     pub should_quit: bool,
     pub enhanced_graphics: bool,
+    pub message_queue: Arc<Mutex<Vec<Message>>>,
+    pub state: Arc<Mutex<Option<ServerState>>>,
 }
 
 impl App {
     pub fn new(title: &str, enhanced_graphics: bool) -> Self {
-        let mut playlist = StatefulList::<String> {
-            state: ListState::default(),
-            items: FILE_PATHS.iter().map(|path| path.to_string()).collect(),
-        };
-        playlist.state.selected = if playlist.items.len() > 0 {
-            Some(0)
-        } else {
-            None
-        };
         Self {
             title: title.to_string(),
-            playlist,
-            player_state: PlayerState::default(),
             should_quit: false,
             enhanced_graphics,
+            message_queue: Arc::new(Mutex::new(vec![])),
+            state: Arc::new(Mutex::new(None)),
         }
     }
 
-    pub fn play_next(&mut self) {
+    fn push_message(&mut self, message: Message) {
-        let current = if let Some(currently_playing) = self.player_state.currently_playing.clone() {
+        self.message_queue.lock().unwrap().push(message);
-            if let Some(current_index) = self
-                .playlist
-                .items
-                .iter()
-                .position(|path| *path == currently_playing)
-            {
-                current_index
-            } else {
-                return;
-            }
-        } else {
-            return;
-        };
-        let track = if current >= self.playlist.items.len() - 1 {
-            self.playlist.items[0].as_str()
-        } else {
-            self.playlist.items[current + 1].as_str()
-        };
-        self.player_state.play(track);
     }
 
-    pub fn play_previous(&mut self) {
+    pub fn connected(&self) -> bool {
-        let current = if let Some(currently_playing) = self.player_state.currently_playing.clone() {
+        self.state.lock().unwrap().is_some()
-            if let Some(current_index) = self
-                .playlist
-                .items
-                .iter()
-                .position(|path| *path == currently_playing)
-            {
-                current_index
-            } else {
-                return;
-            }
-        } else {
-            return;
-        };
-        let track = if current <= 0 {
-            self.playlist.items[self.playlist.items.len() - 1].as_str()
-        } else {
-            self.playlist.items[current - 1].as_str()
-        };
-        self.player_state.play(track);
     }
 
-    pub fn replay_current(&mut self) {
+    pub fn toggle_shuffle(&mut self) {
-        if let Some(current) = self.playlist.current() {
+        self.push_message(Message::new(MessageType::ToggleSuffle, None));
-            self.player_state.play(current)
+    }
-        }
+
+    pub fn toggle_next(&mut self) {
+        self.push_message(Message::new(MessageType::ToggleNext, None));
+    }
+
+    pub fn toggle_repeat(&mut self) {
+        self.push_message(Message::new(MessageType::ToggleRepeat, None));
+    }
+
+    pub fn fetch_state(&mut self) {
+        self.push_message(Message::new(MessageType::StateFetch, None));
     }
 
     pub fn on_key(&mut self, key: char) {
         match key {
-            'b' => self.play_previous(),
+            'S' => self.toggle_shuffle(),
-            'n' => self.play_next(),
+            'X' => self.toggle_next(),
+            'R' => self.toggle_repeat(),
             'q' => self.should_quit = true,
-            ' ' => self.player_state.toggle_pause(),
             _ => (),
         }
     }
@@ -229,33 +65,13 @@ impl App {
 
     pub fn on_right(&mut self) {}
 
-    pub fn on_up(&mut self) {
+    pub fn on_up(&mut self) {}
-        if self.playlist.items.len() > 0 {
-            self.playlist.previous()
-        }
-    }
 
-    pub fn on_down(&mut self) {
+    pub fn on_down(&mut self) {}
-        if self.playlist.items.len() > 0 {
-            self.playlist.next()
-        }
-    }
 
-    pub fn on_enter(&mut self) {
+    pub fn on_enter(&mut self) {}
-        if let Some(path) = self.playlist.current() {
-            self.player_state.play(path)
-        }
-    }
 
     pub fn on_tab(&mut self) {}
 
-    pub fn on_tick(&mut self, duration: Duration) {
+    pub fn on_tick(&mut self, duration: Duration) {}
-        let time_mod = if self.player_state.is_playing {
-            0.25
-        } else {
-            0.0
-        };
-        self.player_state.time_ratio =
-            (self.player_state.time_ratio + duration.as_secs_f64() * time_mod) % 1.0
-    }
 }

src/client/crossterm.rs

@@ -13,7 +13,7 @@ use ratatui::prelude::*;
 
 use super::{app::App, ui};
 
-pub fn run(tick_rate: Duration, enhanced_graphics: bool) -> Result<(), Box<dyn Error>> {
+pub fn run(app: App, tick_rate: Duration) -> Result<(), Box<dyn Error>> {
     // setup terminal
     enable_raw_mode()?;
     let mut stdout = io::stdout();
@@ -21,8 +21,6 @@ pub fn run(tick_rate: Duration, enhanced_graphics: bool) -> Result<(), Box<dyn E
     let backend = CrosstermBackend::new(stdout);
     let mut terminal = Terminal::new(backend)?;
 
-    // create app and run it
-    let app = App::new("rmp - Rust Music Player", enhanced_graphics);
     let res = run_app(&mut terminal, app, tick_rate);
 
     // restore terminal
@@ -56,19 +54,32 @@ fn run_app<B: Backend>(
         if crossterm::event::poll(timeout)? {
             if let Event::Key(key) = event::read()? {
                 if key.kind == KeyEventKind::Press {
-                    match key.code {
+                    if app.connected() {
-                        KeyCode::Char(c) => app.on_key(c),
+                        match key.code {
-                        KeyCode::Left => app.on_left(),
+                            KeyCode::Char(c) => app.on_key(c),
-                        KeyCode::Up => app.on_up(),
+                            KeyCode::Left => app.on_left(),
-                        KeyCode::Right => app.on_right(),
+                            KeyCode::Up => app.on_up(),
-                        KeyCode::Down => app.on_down(),
+                            KeyCode::Right => app.on_right(),
-                        KeyCode::Enter => app.on_enter(),
+                            KeyCode::Down => app.on_down(),
-                        KeyCode::Tab => app.on_tab(),
+                            KeyCode::Enter => app.on_enter(),
-                        _ => {}
+                            KeyCode::Tab => app.on_tab(),
+                            _ => {}
+                        }
+                    } else {
+                        // Allow quitting while in "Not connected" screen
+                        match key.code {
+                            KeyCode::Char(c) => {
+                                if c == 'q' {
+                                    app.should_quit = true;
+                                }
+                            }
+                            _ => (),
+                        }
                     }
                 }
             }
         }
+
         if last_tick.elapsed() >= tick_rate {
             app.on_tick(last_tick.elapsed());
             last_tick = Instant::now();

src/client/request_queue.rs

@@ -0,0 +1,94 @@
+use std::{
+    sync::{Arc, Mutex},
+    time::{Duration, Instant},
+};
+
+use interprocess::local_socket::LocalSocketStream;
+use rmp::{
+    protocol::{self, Message, MessageError, MessageType},
+    ServerState,
+};
+
+pub fn request_queue_cleaner(
+    message_rate: Duration,
+    queue: Arc<Mutex<Vec<Message>>>,
+    state: Arc<Mutex<Option<ServerState>>>,
+) {
+    let mut last_tick = Instant::now();
+    let mut should_connect = true;
+    let mut stream: Option<LocalSocketStream> = None;
+    loop {
+        if should_connect {
+            *state.lock().unwrap() = None;
+            stream = Some(connect());
+            should_connect = false;
+        }
+        match stream.as_mut() {
+            Some(mut stream) => {
+                for request in queue.lock().unwrap().drain(..) {
+                    if let Err(_) = protocol::send(&mut stream, &request) {
+                        should_connect = true;
+                        continue;
+                    }
+                    if let Ok(response) = protocol::receive(&mut stream) {
+                        match route_response(&response, &mut state.lock().unwrap()) {
+                            Err(error) => {
+                                eprintln!("{error:?}")
+                            }
+                            _ => {}
+                        }
+                    }
+                }
+
+                // HACK: keep updating state
+                queue
+                    .lock()
+                    .unwrap()
+                    .push(Message::new(MessageType::StateFetch, None));
+
+                let sleep_duration = message_rate
+                    .checked_sub(last_tick.elapsed())
+                    .unwrap_or_else(|| Duration::from_secs(0));
+                std::thread::sleep(sleep_duration);
+                last_tick = Instant::now();
+            }
+            None => should_connect = true,
+        }
+    }
+}
+
+/// Blocks thread until connected to socket
+fn connect() -> LocalSocketStream {
+    let path = rmp::os::get_socket_path().unwrap();
+    loop {
+        match LocalSocketStream::connect(path.clone()) {
+            Ok(stream) => return stream,
+            Err(_) => {}
+        }
+        std::thread::sleep(Duration::from_millis(100));
+    }
+}
+
+fn route_response(response: &Message, state: &mut Option<ServerState>) -> Result<(), String> {
+    match response.message_type {
+        MessageType::StateResponse => {
+            let body = response.body.as_ref().ok_or("Missing response body")?;
+            let response: ServerState =
+                bincode::deserialize(&body).map_err(|err| err.to_string())?;
+            *state = Some(response);
+        }
+        MessageType::NotImplementedAck => {
+            eprintln!("Server doesn't implement message")
+        }
+        MessageType::ProtocolError => {
+            let body = response.body.as_ref().ok_or("Missing response body")?;
+            let response: MessageError =
+                bincode::deserialize(&body).map_err(|err| err.to_string())?;
+            eprintln!("Server claims protocol error: {response:?}");
+        }
+        message_type => {
+            eprintln!("Message handling not implemented for client: {message_type:?}");
+        }
+    }
+    Ok(())
+}

src/client/ui.rs

@@ -1,54 +1,47 @@
 use ratatui::{prelude::*, widgets::*};
-use std::path::Path;
 
 use super::app::App;
 
 pub fn draw<B: Backend>(f: &mut Frame<B>, app: &mut App) {
-    let chunks = Layout::default()
+    if app.connected() {
-        .constraints([Constraint::Min(3), Constraint::Length(2)].as_ref())
+        let chunks = Layout::default()
-        .split(f.size());
+            .constraints([Constraint::Min(3), Constraint::Length(2)].as_ref())
-    draw_playlist(f, app, chunks[0]);
+            .split(f.size());
-    draw_player(f, app, chunks[1]);
+        draw_playlist(f, app, chunks[0]);
+        draw_player(f, app, chunks[1]);
+    } else {
+        draw_no_connection(f);
+    }
 }
 
 static PRIMARY_COLOR: Color = Color::Rgb(200, 150, 70);
 static SECONDARY_COLOR: Color = Color::Rgb(200, 200, 200);
 
+fn draw_no_connection<B: Backend>(f: &mut Frame<B>) {
+    let message = "Not connected";
+    let width = message.len() as u16 + 4;
+    let height = 3;
+
+    let x = (f.size().width as i16 - width as i16).max(0) as u16 / 2;
+    let y = (f.size().height as i16 - height as i16).max(0) as u16 / 2;
+
+    let area = Rect {
+        x,
+        y,
+        width: u16::min(width, f.size().width - x),
+        height: u16::min(height, f.size().height - y),
+    };
+    f.render_widget(
+        Paragraph::new(message)
+            .fg(SECONDARY_COLOR)
+            .block(Block::default().borders(Borders::ALL).fg(PRIMARY_COLOR))
+            .alignment(Alignment::Center),
+        area,
+    );
+}
+
 fn draw_playlist<B: Backend>(f: &mut Frame<B>, app: &App, area: Rect) {
-    let tracks: Vec<_> = app
+    let playlist = List::new(vec![])
-        .playlist
-        .items
-        .iter()
-        .enumerate()
-        .map(|(index, path)| {
-            let selected = app
-                .playlist
-                .state
-                .selected()
-                .map_or(false, |selected| index == selected);
-            let playing = app
-                .player_state
-                .currently_playing
-                .clone()
-                .map_or(false, |currently_playing| currently_playing == *path);
-            let mut style = Style::default();
-            match (selected, playing) {
-                (true, false) => {
-                    style.fg = Some(Color::Black);
-                    style.bg = Some(PRIMARY_COLOR);
-                }
-                (false, true) => style.fg = Some(PRIMARY_COLOR),
-                (true, true) => {
-                    style.fg = None;
-                    style.bg = Some(PRIMARY_COLOR);
-                }
-                (_, _) => (),
-            }
-            let content = Span::from(format_track_name(path));
-            ListItem::new(content).set_style(style)
-        })
-        .collect();
-    let playlist = List::new(tracks)
         .block(
             Block::default()
                 .borders(Borders::ALL)
@@ -59,22 +52,11 @@ fn draw_playlist<B: Backend>(f: &mut Frame<B>, app: &App, area: Rect) {
 }
 
 fn draw_player<B: Backend>(f: &mut Frame<B>, app: &App, area: Rect) {
-    let title_content = match app.player_state.currently_playing.as_ref() {
+    let title_content = vec![
-        Some(playing) => {
+        Span::from("[ "),
-            let symbol = match app.player_state.is_playing {
+        Span::from("??").fg(PRIMARY_COLOR),
-                true => Span::from("||").fg(PRIMARY_COLOR),
+        Span::from(" ]"),
-                false => Span::from("|>").fg(Color::Black).bg(PRIMARY_COLOR),
+    ];
-            };
-            vec![
-                Span::from("[ "),
-                symbol,
-                Span::from(" "),
-                Span::from(format_track_name(playing.as_str())),
-                Span::from(" ]"),
-            ]
-        }
-        None => vec![Span::from(" Nothing selected ")],
-    };
 
     let player = Gauge::default()
         .block(
@@ -85,19 +67,8 @@ fn draw_player<B: Backend>(f: &mut Frame<B>, app: &App, area: Rect) {
                 .title_position(block::Position::Top),
         )
         .gauge_style(Style::default().fg(PRIMARY_COLOR))
-        .ratio(app.player_state.time_ratio)
+        .ratio(0.25)
-        .label(if app.player_state.is_playing {
+        .label("[ PAUSED ]");
-            ""
-        } else {
-            "[ PAUSED ]"
-        });
 
     f.render_widget(player, area)
 }
-
-fn format_track_name(path: &str) -> String {
-    match Path::new(path).file_stem() {
-        Some(file_name) => file_name.to_string_lossy().to_string(),
-        None => path.to_string(),
-    }
-}

src/lib.rs

@@ -1 +1,76 @@
-pub mod client;
+use std::path::PathBuf;
+
+use serde::{Deserialize, Serialize};
+
+#[cfg(target_family = "unix")]
+#[path = "os_unix.rs"]
+pub mod os;
+pub mod protocol;
+
+#[derive(Serialize, Deserialize, Debug, Default)]
+pub struct ServerState {
+    pub playlist_params: PlaylistParams,
+    pub directory_playlist: Option<DirectoryPlaylist>,
+    pub queue_playlist: QueuePlaylist,
+}
+
+pub type PlaylistElement = PathBuf;
+
+pub enum PlaylistType {
+    Directory,
+    Queue,
+}
+
+#[derive(Serialize, Deserialize, Debug, Default)]
+pub struct Playlist {
+    pub items: Vec<PlaylistElement>,
+    pub current: Option<usize>,
+}
+
+impl Playlist {}
+
+#[derive(Serialize, Deserialize, Debug)]
+pub struct DirectoryPlaylist {
+    pub directory: PathBuf,
+    pub playlist: Playlist,
+}
+
+#[derive(Serialize, Deserialize, Debug, Default)]
+pub struct QueuePlaylist {
+    pub playlist: Playlist,
+}
+
+#[derive(Serialize, Deserialize, Debug)]
+pub struct PlaylistParams {
+    pub shuffle: bool,
+    pub next: bool,
+    pub repeat: bool,
+}
+
+impl Default for PlaylistParams {
+    fn default() -> Self {
+        Self {
+            shuffle: false,
+            next: true,
+            repeat: true,
+        }
+    }
+}
+
+pub mod server {
+    use std::{fmt::Debug, path::PathBuf};
+
+    #[derive(Debug)]
+    pub enum ServerError {
+        Other(String),
+        Io(std::io::Error),
+        AlreadyStarted,
+        MissingRuntimeDir(PathBuf),
+    }
+
+    impl ServerError {
+        pub fn from_debuggable(err: impl Debug) -> Self {
+            Self::Other(format!("Unexpected error: {err:?}").to_string())
+        }
+    }
+}

src/main.rs

@@ -0,0 +1,63 @@
+use argh::FromArgs;
+
+pub mod client;
+pub mod server;
+
+/// rmp: Rust Music Player
+#[derive(Debug, FromArgs)]
+pub struct CliArgs {
+    /// run the server
+    #[argh(switch, short = 's')]
+    server: bool,
+
+    /// randomize next track?
+    #[argh(option, default = "false")]
+    shuffle: bool,
+
+    /// change track after the current one is finished?
+    #[argh(option, default = "true")]
+    next: bool,
+
+    /// repeat the playlist (or track if 'next' is disabled) after it's finished
+    #[argh(option, default = "true")]
+    repeat: bool,
+
+    /// kill server
+    #[argh(switch, short = 'x')]
+    exit: bool,
+
+    /// don't start server even if it's not running
+    #[argh(switch, short = 'c')]
+    client_only: bool,
+
+    /// time in ms between two ticks.
+    #[argh(option, default = "250")]
+    tick_rate: u64,
+
+    /// interval in ms for clearing the request queue.
+    #[argh(option, default = "50")]
+    message_rate: u64,
+
+    /// whether unicode symbols are used to improve the overall look of the app
+    #[argh(option, default = "true")]
+    enhanced_graphics: bool,
+}
+
+fn main() {
+    let args: CliArgs = argh::from_env();
+
+    if args.exit {
+        server::kill();
+        return;
+    }
+
+    if args.server {
+        server::run(args).unwrap();
+        return;
+    }
+
+    if !args.client_only && !server::is_running().unwrap() {
+        server::run_in_background();
+    }
+    client::run(args).unwrap();
+}

src/os_unix.rs

@@ -0,0 +1,92 @@
+use std::{
+    fs,
+    path::{Path, PathBuf},
+    process::{id, Command, Stdio},
+};
+
+use super::server::ServerError;
+
+pub fn reserve_pid() -> Result<(), ServerError> {
+    let pid_path = get_pid_path()?;
+    is_running()?;
+
+    fs::write(&pid_path, id().to_string()).map_err(|err| ServerError::Io(err))?;
+    Command::new("chmod")
+        .args(&["600", &pid_path.to_string_lossy()])
+        .output()
+        .map_err(|err| ServerError::Io(err))?;
+    Ok(())
+}
+
+pub fn is_running() -> Result<bool, ServerError> {
+    let pid_path = get_pid_path()?;
+
+    match fs::read(&pid_path) {
+        Ok(old_pid) => {
+            let old_pid =
+                String::from_utf8(old_pid).map_err(|err| ServerError::from_debuggable(err))?;
+            let old_pid = old_pid.trim();
+            Ok(Command::new("ps")
+                .args(&["-p", old_pid])
+                .stdout(Stdio::null())
+                .stderr(Stdio::null())
+                .status()
+                .map_err(|err| ServerError::Io(err))?
+                .success())
+        }
+        _ => Ok(false),
+    }
+}
+
+pub fn run_in_background() -> Result<(), ServerError> {
+    let this = std::env::args().next().unwrap();
+    Command::new(this)
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
+        .args(&["-s"])
+        .spawn()
+        .map_err(|err| ServerError::Io(err))?;
+    Ok(())
+}
+
+pub fn kill() -> Result<(), ServerError> {
+    let pid_path = get_pid_path()?;
+    let socket_path = get_socket_path()?;
+    let pid = String::from_utf8(fs::read(&pid_path).map_err(|err| ServerError::Io(err))?)
+        .map_err(|err| ServerError::from_debuggable(err))?;
+    let pid = pid.trim();
+    Command::new("kill")
+        .arg(pid)
+        .spawn()
+        .map_err(|err| ServerError::Io(err))?;
+    Command::new("rm")
+        .args(&[
+            "-f",
+            &pid_path.to_string_lossy(),
+            &socket_path.to_string_lossy(),
+        ])
+        .spawn()
+        .map_err(|err| ServerError::Io(err))?;
+    Ok(())
+}
+
+pub fn get_socket_path() -> Result<PathBuf, ServerError> {
+    Ok(get_runtime_dir()?.join("rmp.socket"))
+}
+
+fn get_runtime_dir() -> Result<PathBuf, ServerError> {
+    let uid = String::from_utf8(
+        Command::new("id")
+            .arg("-u")
+            .output()
+            .map_err(|err| ServerError::Io(err))?
+            .stdout,
+    )
+    .map_err(|err| ServerError::from_debuggable(err))?;
+    let dir = Path::new("/run/user").join(uid.trim().to_string());
+    Ok(dir)
+}
+
+fn get_pid_path() -> Result<PathBuf, ServerError> {
+    Ok(get_runtime_dir()?.join("rmp.pid"))
+}

src/protocol.rs

@@ -0,0 +1,130 @@
+use std::{
+    fmt::Display,
+    io::{Read, Write},
+};
+
+use interprocess::local_socket::LocalSocketStream;
+use serde::{Deserialize, Serialize};
+
+use crate::ServerState;
+
+/// Prefix messages with this header
+pub const HEADER_MAGIC: [u8; 4] = [0xCA, 0xFE, 0xBA, 0xBE];
+/// Maximum allowed body size
+pub const MAX_BODY_LENGTH: usize = 10 * 1024 * 1024;
+
+#[derive(Debug, Serialize, Deserialize)]
+pub enum MessageError {
+    HeaderMismatch,
+    BodySizeLimit,
+    ChecksumMismatch,
+    ReadError,
+    DeserializationError,
+}
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone, Copy, Deserialize, Serialize)]
+pub enum MessageType {
+    /// Generic acknowledge
+    Ack = 0,
+    /// client/server did not know how to handle the request
+    NotImplementedAck,
+    /// Request was invalid
+    ProtocolError,
+    StateFetch,
+    StateResponse,
+    ToggleSuffle,
+    ToggleNext,
+    ToggleRepeat,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Message {
+    pub message_type: MessageType,
+    pub body: Option<Vec<u8>>,
+}
+
+impl Message {
+    pub fn new(message_type: MessageType, body: Option<&[u8]>) -> Self {
+        Self {
+            message_type,
+            body: body.map(|b| Vec::from(b)),
+        }
+    }
+
+    /// Message format (values are in little-endian):
+    /// offset | size | explanation
+    /// -------+------+-----------
+    /// 0x00   |  u32 | HEADER_MAGIC
+    /// 0x04   |  u32 | Body checksum
+    /// 0x08   |  u32 | Body length
+    /// 0x12   |    ? | Body
+    fn as_bytes(&self) -> Vec<u8> {
+        let magic = &HEADER_MAGIC[..];
+        let body = &bincode::serialize(self).unwrap();
+        let checksum = &crc32fast::hash(&body).to_le_bytes();
+        let body_length = &(body.len() as u32).to_le_bytes();
+        [magic, checksum, body_length, body].concat()
+    }
+
+    pub fn state_response(server_state: &ServerState) -> Result<Self, String> {
+        Ok(Self {
+            message_type: MessageType::StateResponse,
+            body: Some(bincode::serialize(server_state).map_err(|err| err.to_string())?),
+        })
+    }
+}
+
+impl Display for Message {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "{:?}\t{}",
+            self.message_type,
+            self.body
+                .as_ref()
+                .map_or("(no body)".into(), |body| format!("{} B", body.len()))
+        )
+    }
+}
+
+pub fn send(stream: &mut LocalSocketStream, message: &Message) -> Result<(), std::io::Error> {
+    stream.write_all(&message.as_bytes())?;
+    Ok(())
+}
+
+pub fn receive(stream: &mut LocalSocketStream) -> Result<Message, MessageError> {
+    let mut magic_buffer = vec![0; HEADER_MAGIC.len()];
+    if let Err(_) = stream.read_exact(&mut magic_buffer) {
+        return Err(MessageError::ReadError);
+    }
+    if magic_buffer != HEADER_MAGIC {
+        return Err(MessageError::HeaderMismatch);
+    }
+
+    let mut checksum_buffer = [0; 4];
+    if let Err(_) = stream.read_exact(&mut checksum_buffer) {
+        return Err(MessageError::ReadError);
+    }
+    let expected_checksum = u32::from_le_bytes(checksum_buffer);
+
+    let mut body_length_buffer = [0; 4];
+    if let Err(_) = stream.read_exact(&mut body_length_buffer) {
+        return Err(MessageError::ReadError);
+    }
+    let expected_body_length = u32::from_le_bytes(body_length_buffer) as usize;
+
+    if expected_body_length > MAX_BODY_LENGTH {
+        return Err(MessageError::BodySizeLimit);
+    }
+
+    let mut body_buffer = vec![0; expected_body_length];
+    if let Err(_) = stream.read_exact(&mut body_buffer) {
+        return Err(MessageError::ReadError);
+    }
+
+    if crc32fast::hash(&body_buffer) != expected_checksum {
+        return Err(MessageError::ChecksumMismatch);
+    }
+
+    bincode::deserialize(&body_buffer).map_err(|_| MessageError::DeserializationError)
+}

src/server.rs

@@ -0,0 +1,174 @@
+use interprocess::local_socket::{LocalSocketListener, LocalSocketStream};
+
+use rmp::{
+    os,
+    protocol::{Message, MessageType},
+    server::ServerError,
+    PlaylistParams, ServerState,
+};
+use std::{
+    fs,
+    sync::{Arc, Mutex},
+};
+
+use crate::CliArgs;
+
+pub mod audio_backend;
+
+#[derive(Debug)]
+pub struct Server {
+    pub state: ServerState,
+}
+
+impl Server {
+    pub fn from_state(state: ServerState) -> Self {
+        Self { state }
+    }
+}
+
+pub fn run(args: CliArgs) -> Result<(), ServerError> {
+    if let Err(err) = os::reserve_pid() {
+        let exit_code = handle_error(err);
+        std::process::exit(exit_code);
+    }
+    serve(ServerState {
+        playlist_params: PlaylistParams {
+            shuffle: args.shuffle,
+            next: args.next,
+            repeat: args.repeat,
+        },
+        ..Default::default()
+    })
+}
+
+pub fn kill() {
+    if let Err(err) = os::kill() {
+        let exit_code = handle_error(err);
+        std::process::exit(exit_code);
+    }
+}
+
+pub fn is_running() -> Result<bool, ServerError> {
+    match os::is_running() {
+        Ok(is_running) => Ok(is_running),
+        Err(err) => {
+            let exit_code = handle_error(err);
+            std::process::exit(exit_code);
+        }
+    }
+}
+
+pub fn run_in_background() {
+    if let Err(err) = os::run_in_background() {
+        let exit_code = handle_error(err);
+        std::process::exit(exit_code);
+    }
+}
+
+fn serve(state: ServerState) -> Result<(), ServerError> {
+    let socket_path = os::get_socket_path()?;
+    if socket_path.exists() {
+        fs::remove_file(&socket_path).map_err(|err| ServerError::Io(err))?;
+    }
+    let socket = LocalSocketListener::bind(socket_path).map_err(|err| ServerError::Io(err))?;
+    let server = Arc::new(Mutex::new(Server::from_state(state)));
+    println!("Waiting for connections...");
+    let mut session_counter = 0;
+    for message in socket.incoming() {
+        match message {
+            Ok(stream) => {
+                session_counter += 1;
+                let thread_builder =
+                    std::thread::Builder::new().name(format!("session_{session_counter}"));
+                let server = server.clone();
+                thread_builder
+                    .spawn(move || session_handler(stream, server))
+                    .unwrap();
+            }
+            Err(err) => {
+                let exit_code = handle_error(ServerError::Io(err));
+                std::process::exit(exit_code);
+            }
+        };
+    }
+    println!("Reached the end of an infinite loop?");
+    Ok(())
+}
+
+fn session_handler(mut stream: LocalSocketStream, server: Arc<Mutex<Server>>) {
+    let thread = std::thread::current();
+    let session_id = thread.name().unwrap_or("<unnamed>");
+    println!("[{session_id}] session created");
+    loop {
+        match rmp::protocol::receive(&mut stream) {
+            Ok(message) => {
+                println!("[{session_id}] rx {message}");
+                match route_request(&message, &mut server.lock().unwrap()) {
+                    Err(err) => {
+                        eprintln!("[{session_id}] rx Error: {err}");
+                    }
+                    Ok(response) => {
+                        println!("[{session_id}] tx {response}");
+                        rmp::protocol::send(&mut stream, &response).unwrap();
+                    }
+                }
+            }
+            Err(error) => match error {
+                rmp::protocol::MessageError::ReadError => {
+                    println!("[{session_id}] session terminated");
+                    return;
+                }
+                error => {
+                    eprintln!("[{session_id}] rx {error:?}");
+                    let body = bincode::serialize(&error).unwrap();
+                    let message = Message::new(MessageType::ProtocolError, Some(&body));
+                    println!("[{session_id}] tx {message}");
+                    rmp::protocol::send(&mut stream, &message).unwrap();
+                }
+            },
+        }
+    }
+}
+
+fn handle_error(err: ServerError) -> i32 {
+    match &err {
+        ServerError::Other(msg) => {
+            eprintln!("Unknown error: {msg}");
+            1
+        }
+        ServerError::Io(err) => {
+            eprintln!("IO error: {err}");
+            2
+        }
+        ServerError::AlreadyStarted => {
+            eprintln!("Server already running");
+            100
+        }
+        ServerError::MissingRuntimeDir(path) => {
+            eprintln!("Missing runtime directory: {}", path.to_string_lossy());
+            101
+        }
+    }
+}
+
+fn route_request(request: &Message, server: &mut Server) -> Result<Message, String> {
+    match request.message_type {
+        MessageType::StateFetch => {
+            return Message::state_response(&server.state);
+        }
+        MessageType::ToggleNext => {
+            server.state.playlist_params.next = !server.state.playlist_params.next;
+            return Message::state_response(&server.state);
+        }
+        MessageType::ToggleSuffle => {
+            server.state.playlist_params.shuffle = !server.state.playlist_params.shuffle;
+            return Message::state_response(&server.state);
+        }
+        MessageType::ToggleRepeat => {
+            server.state.playlist_params.repeat = !server.state.playlist_params.repeat;
+            return Message::state_response(&server.state);
+        }
+        _ => {}
+    }
+    Ok(Message::new(MessageType::NotImplementedAck, None))
+}

src/server/audio_backend.rs

@@ -0,0 +1,7 @@
+use std::{path::Path, time::Duration};
+
+pub trait AudioBackend {
+    fn seek_and_play(&mut self, file_path: &Path, seek: Duration) -> ();
+    fn pause_playback(&mut self) -> ();
+    fn continue_playback(&mut self) -> ();
+}