add gui-startup and mux-startup events

Using the newly exposed-to-lua mux apis, you may now run some lua code
at GUI startup and/or mux startup, just prior to any default windows
being created.

If you happen to spawn any panes as a result of this, wezterm will
skip creating the default program.

```lua
local wezterm = require 'wezterm'
local mux = wezterm.mux

-- This produces a window split horizontally into three equal parts
wezterm.on("gui-startup", function()
  wezterm.log_info("doing gui startup")
  local tab, pane, window = mux.spawn_window{}
  mux.split_pane(pane, {size=0.3})
  mux.split_pane(pane, {size=0.5})
end)

wezterm.on("mux-startup", function()
  wezterm.log_info("doing mux startup")
  local tab, pane, window = mux.spawn_window{}
  mux.split_pane(pane, {size=0.5, direction="Top"})
end)

return {
  unix_domains = {
    {name="unix"}
  },
}
```

refs: #674
refs: #1949

Cargo.lock

@@ -4833,6 +4833,7 @@ dependencies = [
  "env-bootstrap",
  "libc",
  "log",
+ "mlua",
  "mux",
  "openssl",
  "portable-pty",

wezterm-gui/src/main.rs

@@ -224,7 +224,8 @@ async fn async_run_with_domain_as_default(
     mux.add_domain(&domain);
     mux.set_default_domain(&domain);
 
-    spawn_tab_in_default_domain_if_mux_is_empty(cmd).await
+    let is_connecting = true;
+    spawn_tab_in_default_domain_if_mux_is_empty(cmd, is_connecting).await
 }
 
 async fn async_run_mux_client(opts: ConnectCommand) -> anyhow::Result<()> {
@@ -273,10 +274,23 @@ fn run_mux_client(opts: ConnectCommand) -> anyhow::Result<()> {
 
 async fn spawn_tab_in_default_domain_if_mux_is_empty(
     cmd: Option<CommandBuilder>,
+    is_connecting: bool,
 ) -> anyhow::Result<()> {
     let mux = Mux::get().unwrap();
 
     let domain = mux.default_domain();
+
+    if !is_connecting {
+        let have_panes_in_domain = mux
+            .iter_panes()
+            .iter()
+            .any(|p| p.domain_id() == domain.domain_id());
+
+        if have_panes_in_domain {
+            return Ok(());
+        }
+    }
+
     let window_id = mux.new_empty_window(None);
 
     domain.attach(Some(*window_id)).await?;
@@ -381,7 +395,27 @@ async fn async_run_terminal_gui(
     if !opts.no_auto_connect {
         connect_to_auto_connect_domains().await?;
     }
-    spawn_tab_in_default_domain_if_mux_is_empty(cmd).await
+
+    async fn trigger_gui_startup(lua: Option<Rc<mlua::Lua>>) -> anyhow::Result<()> {
+        if let Some(lua) = lua {
+            let args = lua.pack_multi(())?;
+            config::lua::emit_event(&lua, ("gui-startup".to_string(), args))
+                .await
+                .map_err(|e| {
+                    log::error!("while processing gui-startup event: {:#}", e);
+                    e
+                })?;
+        }
+        Ok(())
+    }
+
+    promise::spawn::spawn(config::with_lua_config_on_main_thread(move |lua| {
+        trigger_gui_startup(lua)
+    }))
+    .await?;
+
+    let is_connecting = false;
+    spawn_tab_in_default_domain_if_mux_is_empty(cmd, is_connecting).await
 }
 
 #[derive(Debug)]

wezterm-mux-server/Cargo.toml

@@ -16,6 +16,7 @@ env-bootstrap = { path = "../env-bootstrap" }
 libc = "0.2"
 log = "0.4"
 mux = { path = "../mux" }
+mlua = "0.8.0-beta.4"
 openssl = "0.10"
 portable-pty = { path = "../pty", features = ["serde_support"]}
 promise = { path = "../promise" }

wezterm-mux-server/src/main.rs

@@ -199,10 +199,34 @@ fn run() -> anyhow::Result<()> {
     }
 }
 
+async fn trigger_mux_startup(lua: Option<Rc<mlua::Lua>>) -> anyhow::Result<()> {
+    if let Some(lua) = lua {
+        let args = lua.pack_multi(())?;
+        config::lua::emit_event(&lua, ("mux-startup".to_string(), args))
+            .await
+            .map_err(|e| {
+                log::error!("while processing mux-startup event: {:#}", e);
+                e
+            })?;
+    }
+    Ok(())
+}
+
 async fn async_run(cmd: Option<CommandBuilder>) -> anyhow::Result<()> {
     let mux = Mux::get().unwrap();
 
     let domain = mux.default_domain();
+
+    {
+        config::with_lua_config_on_main_thread(move |lua| trigger_mux_startup(lua)).await?;
+    }
+
+    let have_panes_in_domain = mux
+        .iter_panes()
+        .iter()
+        .any(|p| p.domain_id() == domain.domain_id());
+
+    if !have_panes_in_domain {
         let window_id = mux.new_empty_window(None);
         domain.attach(Some(*window_id)).await?;
 
@@ -211,6 +235,7 @@ async fn async_run(cmd: Option<CommandBuilder>) -> anyhow::Result<()> {
             .default_domain()
             .spawn(config.initial_size(0), cmd, None, *window_id)
             .await?;
+    }
     Ok(())
 }