Add headless mode

This patch adds a daemon mode to Alacritty which allows starting the
Alacritty process without spawning an initial window.

While this does not provide any significant advantage over the existing
behavior of always spawning a window, it does integrate nicer with some
setups and is a pretty trivial addition.

9 files changed, 58 insertions, 37 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7dff2d54..8d9d463a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,7 @@ Notable changes to the `alacritty_terminal` crate are documented in its
 - Support relative path imports from config files
 - `alacritty migrate` support for TOML configuration changes
 - Support for Unicode 16 characters
+- Headless mode using `alacritty --daemon`
 
 ### Changed
 
diff --git a/alacritty/src/cli.rs b/alacritty/src/cli.rs
index 2b4afa02..bb0a24f4 100644
--- a/alacritty/src/cli.rs
+++ b/alacritty/src/cli.rs
@@ -26,7 +26,7 @@ pub struct Options {
     pub print_events: bool,
 
     /// Generates ref test.
-    #[clap(long)]
+    #[clap(long, conflicts_with("daemon"))]
     pub ref_test: bool,
 
     /// X11 window ID to embed Alacritty within (decimal or hexadecimal with "0x" prefix).
@@ -62,6 +62,10 @@ pub struct Options {
     #[clap(short, conflicts_with("quiet"), action = ArgAction::Count)]
     verbose: u8,
 
+    /// Do not spawn an initial window.
+    #[clap(long)]
+    pub daemon: bool,
+
     /// CLI options for config overrides.
     #[clap(skip)]
     pub config_options: ParsedOptions,
diff --git a/alacritty/src/event.rs b/alacritty/src/event.rs
index 23575e21..f0159060 100644
--- a/alacritty/src/event.rs
+++ b/alacritty/src/event.rs
@@ -1,6 +1,7 @@
 //! Process window events.
 
 use crate::ConfigMonitor;
+use glutin::config::GetGlConfig;
 use std::borrow::Cow;
 use std::cmp::min;
 use std::collections::{HashMap, HashSet, VecDeque};
@@ -16,7 +17,8 @@ use std::{env, f32, mem};
 
 use ahash::RandomState;
 use crossfont::Size as FontSize;
-use glutin::display::{Display as GlutinDisplay, GetGlDisplay};
+use glutin::config::Config as GlutinConfig;
+use glutin::display::GetGlDisplay;
 use log::{debug, error, info, warn};
 use winit::application::ApplicationHandler;
 use winit::event::{
@@ -80,7 +82,7 @@ pub struct Processor {
     initial_window_error: Option<Box<dyn Error>>,
     windows: HashMap<WindowId, WindowContext, RandomState>,
     proxy: EventLoopProxy<Event>,
-    gl_display: Option<GlutinDisplay>,
+    gl_config: Option<GlutinConfig>,
     #[cfg(unix)]
     global_ipc_options: ParsedOptions,
     cli_options: CliOptions,
@@ -121,7 +123,7 @@ impl Processor {
             cli_options,
             proxy,
             scheduler,
-            gl_display: None,
+            gl_config: None,
             config: Rc::new(config),
             clipboard,
             windows: Default::default(),
@@ -138,12 +140,16 @@ impl Processor {
     pub fn create_initial_window(
         &mut self,
         event_loop: &ActiveEventLoop,
-        options: WindowOptions,
     ) -> Result<(), Box<dyn Error>> {
+        let options = match self.initial_window_options.take() {
+            Some(options) => options,
+            None => return Ok(()),
+        };
+
         let window_context =
             WindowContext::initial(event_loop, self.proxy.clone(), self.config.clone(), options)?;
 
-        self.gl_display = Some(window_context.display.gl_context().display());
+        self.gl_config = Some(window_context.display.gl_context().config());
         self.windows.insert(window_context.id(), window_context);
 
         Ok(())
@@ -155,7 +161,7 @@ impl Processor {
         event_loop: &ActiveEventLoop,
         options: WindowOptions,
     ) -> Result<(), Box<dyn Error>> {
-        let window = self.windows.iter().next().as_ref().unwrap().1;
+        let gl_config = self.gl_config.as_ref().unwrap();
 
         // Override config with CLI/IPC options.
         let mut config_overrides = options.config_overrides();
@@ -164,9 +170,14 @@ impl Processor {
         let mut config = self.config.clone();
         config = config_overrides.override_config_rc(config);
 
-        #[allow(unused_mut)]
-        let mut window_context =
-            window.additional(event_loop, self.proxy.clone(), config, options, config_overrides)?;
+        let window_context = WindowContext::additional(
+            gl_config,
+            event_loop,
+            self.proxy.clone(),
+            config,
+            options,
+            config_overrides,
+        )?;
 
         self.windows.insert(window_context.id(), window_context);
         Ok(())
@@ -210,16 +221,11 @@ impl ApplicationHandler<Event> for Processor {
     fn resumed(&mut self, _event_loop: &ActiveEventLoop) {}
 
     fn new_events(&mut self, event_loop: &ActiveEventLoop, cause: StartCause) {
-        if cause != StartCause::Init {
+        if cause != StartCause::Init || self.cli_options.daemon {
             return;
         }
 
-        let initial_window_options = match self.initial_window_options.take() {
-            Some(initial_window_options) => initial_window_options,
-            None => return,
-        };
-
-        if let Err(err) = self.create_initial_window(event_loop, initial_window_options) {
+        if let Err(err) = self.create_initial_window(event_loop) {
             self.initial_window_error = Some(err);
             event_loop.exit();
             return;
@@ -338,7 +344,13 @@ impl ApplicationHandler<Event> for Processor {
                     window_context.display.make_not_current();
                 }
 
-                if let Err(err) = self.create_window(event_loop, options.clone()) {
+                if self.gl_config.is_none() {
+                    // Handle initial window creation in daemon mode.
+                    if let Err(err) = self.create_initial_window(event_loop) {
+                        self.initial_window_error = Some(err);
+                        event_loop.exit();
+                    }
+                } else if let Err(err) = self.create_window(event_loop, options.clone()) {
                     error!("Could not open window: {:?}", err);
                 }
             },
@@ -375,7 +387,7 @@ impl ApplicationHandler<Event> for Processor {
                 self.scheduler.unschedule_window(window_context.id());
 
                 // Shutdown if no more terminals are open.
-                if self.windows.is_empty() {
+                if self.windows.is_empty() && !self.cli_options.daemon {
                     // Write ref tests of last window to disk.
                     if self.config.debug.ref_test {
                         window_context.write_ref_test_results();
@@ -439,7 +451,7 @@ impl ApplicationHandler<Event> for Processor {
             info!("Exiting the event loop");
         }
 
-        match self.gl_display.take() {
+        match self.gl_config.take().map(|config| config.display()) {
             #[cfg(not(target_os = "macos"))]
             Some(glutin::display::Display::Egl(display)) => {
                 // Ensure that all the windows are dropped, so the destructors for
diff --git a/alacritty/src/ipc.rs b/alacritty/src/ipc.rs
index d06d395e..3d14c4ce 100644
--- a/alacritty/src/ipc.rs
+++ b/alacritty/src/ipc.rs
@@ -20,13 +20,13 @@ const ALACRITTY_SOCKET_ENV: &str = "ALACRITTY_SOCKET";
 
 /// Create an IPC socket.
 pub fn spawn_ipc_socket(options: &Options, event_proxy: EventLoopProxy<Event>) -> Option<PathBuf> {
-    // Create the IPC socket and export its path as env variable if necessary.
+    // Create the IPC socket and export its path as env.
+
     let socket_path = options.socket.clone().unwrap_or_else(|| {
         let mut path = socket_dir();
         path.push(format!("{}-{}.sock", socket_prefix(), process::id()));
         path
     });
-    env::set_var(ALACRITTY_SOCKET_ENV, socket_path.as_os_str());
 
     let listener = match UnixListener::bind(&socket_path) {
         Ok(listener) => listener,
@@ -36,6 +36,11 @@ pub fn spawn_ipc_socket(options: &Options, event_proxy: EventLoopProxy<Event>) -
         },
     };
 
+    env::set_var(ALACRITTY_SOCKET_ENV, socket_path.as_os_str());
+    if options.daemon {
+        println!("ALACRITTY_SOCKET={}; export ALACRITTY_SOCKET", socket_path.display());
+    }
+
     // Spawn a thread to listen on the IPC socket.
     thread::spawn_named("socket listener", move || {
         let mut data = String::new();
diff --git a/alacritty/src/window_context.rs b/alacritty/src/window_context.rs
index 062f9ef0..cfc3cd96 100644
--- a/alacritty/src/window_context.rs
+++ b/alacritty/src/window_context.rs
@@ -9,7 +9,7 @@ use std::os::unix::io::{AsRawFd, RawFd};
 use std::rc::Rc;
 use std::sync::Arc;
 
-use glutin::config::GetGlConfig;
+use glutin::config::Config as GlutinConfig;
 use glutin::display::GetGlDisplay;
 #[cfg(all(feature = "x11", not(any(target_os = "macos", windows))))]
 use glutin::platform::x11::X11GlConfigExt;
@@ -119,18 +119,14 @@ impl WindowContext {
 
     /// Create additional context with the graphics platform other windows are using.
     pub fn additional(
-        &self,
+        gl_config: &GlutinConfig,
         event_loop: &ActiveEventLoop,
         proxy: EventLoopProxy<Event>,
         config: Rc<UiConfig>,
         options: WindowOptions,
         config_overrides: ParsedOptions,
     ) -> Result<Self, Box<dyn Error>> {
-        // Get any window and take its GL config and display to build a new context.
-        let (gl_display, gl_config) = {
-            let gl_context = self.display.gl_context();
-            (gl_context.display(), gl_context.config())
-        };
+        let gl_display = gl_config.display();
 
         let mut identity = config.window.identity.clone();
         options.window_identity.override_identity_config(&mut identity);
@@ -147,11 +143,8 @@ impl WindowContext {
 
         // Create context.
         let raw_window_handle = window.raw_window_handle();
-        let gl_context = renderer::platform::create_gl_context(
-            &gl_display,
-            &gl_config,
-            Some(raw_window_handle),
-        )?;
+        let gl_context =
+            renderer::platform::create_gl_context(&gl_display, gl_config, Some(raw_window_handle))?;
 
         // Check if new window will be opened as a tab.
         #[cfg(target_os = "macos")]
diff --git a/extra/completions/_alacritty b/extra/completions/_alacritty
index a9260d5c..0ccce66f 100644
--- a/extra/completions/_alacritty
+++ b/extra/completions/_alacritty
@@ -27,9 +27,10 @@ _alacritty() {
 '*-o+[Override configuration file options \[example\: '\''cursor.style="Beam"'\''\]]:OPTION: ' \
 '*--option=[Override configuration file options \[example\: '\''cursor.style="Beam"'\''\]]:OPTION: ' \
 '--print-events[Print all events to STDOUT]' \
-'--ref-test[Generates ref test]' \
+'(--daemon)--ref-test[Generates ref test]' \
 '(-v)*-q[Reduces the level of verbosity (the min level is -qq)]' \
 '(-q)*-v[Increases the level of verbosity (the max level is -vvv)]' \
+'--daemon[Do not spawn an initial window]' \
 '--hold[Remain open after child process exit]' \
 '-h[Print help]' \
 '--help[Print help]' \
diff --git a/extra/completions/alacritty.bash b/extra/completions/alacritty.bash
index 2c10d47c..e2213b75 100644
--- a/extra/completions/alacritty.bash
+++ b/extra/completions/alacritty.bash
@@ -61,7 +61,7 @@ _alacritty() {
 
     case "${cmd}" in
         alacritty)
-            opts="-q -v -e -T -o -h -V --print-events --ref-test --embed --config-file --socket --working-directory --hold --command --title --class --option --help --version msg migrate help"
+            opts="-q -v -e -T -o -h -V --print-events --ref-test --embed --config-file --socket --daemon --working-directory --hold --command --title --class --option --help --version msg migrate help"
             if [[ ${cur} == -* || ${COMP_CWORD} -eq 1 ]] ; then
                 COMPREPLY=( $(compgen -W "${opts}" -- "${cur}") )
                 return 0
diff --git a/extra/completions/alacritty.fish b/extra/completions/alacritty.fish
index b7fa8ce6..7cfc3337 100644
--- a/extra/completions/alacritty.fish
+++ b/extra/completions/alacritty.fish
@@ -1,6 +1,6 @@
 # Print an optspec for argparse to handle cmd's options that are independent of any subcommand.
 function __fish_alacritty_global_optspecs
-	string join \n print-events ref-test embed= config-file= socket= q v working-directory= hold e/command= T/title= class= o/option= h/help V/version
+	string join \n print-events ref-test embed= config-file= socket= q v daemon working-directory= hold e/command= T/title= class= o/option= h/help V/version
 end
 
 function __fish_alacritty_needs_command
@@ -36,6 +36,7 @@ complete -c alacritty -n "__fish_alacritty_needs_command" -l print-events -d 'Pr
 complete -c alacritty -n "__fish_alacritty_needs_command" -l ref-test -d 'Generates ref test'
 complete -c alacritty -n "__fish_alacritty_needs_command" -s q -d 'Reduces the level of verbosity (the min level is -qq)'
 complete -c alacritty -n "__fish_alacritty_needs_command" -s v -d 'Increases the level of verbosity (the max level is -vvv)'
+complete -c alacritty -n "__fish_alacritty_needs_command" -l daemon -d 'Do not spawn an initial window'
 complete -c alacritty -n "__fish_alacritty_needs_command" -l hold -d 'Remain open after child process exit'
 complete -c alacritty -n "__fish_alacritty_needs_command" -s h -l help -d 'Print help'
 complete -c alacritty -n "__fish_alacritty_needs_command" -s V -l version -d 'Print version'
diff --git a/extra/man/alacritty.1.scd b/extra/man/alacritty.1.scd
index c8a67bb8..86630a65 100644
--- a/extra/man/alacritty.1.scd
+++ b/extra/man/alacritty.1.scd
@@ -21,6 +21,10 @@ set of features with high performance.
 
 	Remain open after child process exits.
 
+*--daemon*
+
+	Do not spawn an initial window.
+
 *--print-events*
 
 	Print all events to STDOUT.