Add XWayland support (#16)

* save dev state

* fix xwayland-satellite supervisor lifecycle, readiness and cleanup

* remove wayland scanner protocol generated files

* default xwayland enabled with path

* remove jargons

* address x11 review: park unconfirmed cleanup, never signal a reaped group, fix storm test and notify dir mode

* fix resize tests: never build Size values from negative dimensions

* add min-width/min-height window rules that replace client minimum sizes

* rename effective_size_constraints to effective_minimum_size and minor cleanups

compositor/src/config/xwayland.rs

@@ -0,0 +1,101 @@
+use std::path::PathBuf;
+
+use serde::Deserialize;
+
+#[derive(Clone, Copy, Debug, Default, Deserialize, PartialEq, Eq)]
+#[serde(rename_all = "kebab-case")]
+pub(crate) enum XwaylandStartup {
+    #[default]
+    OnDemand,
+    Eager,
+}
+
+#[derive(Clone, Debug, Deserialize, PartialEq, Eq)]
+#[serde(default, deny_unknown_fields)]
+pub(crate) struct XwaylandConfig {
+    pub enabled: bool,
+    pub startup: XwaylandStartup,
+    pub path: PathBuf,
+}
+
+impl Default for XwaylandConfig {
+    fn default() -> Self {
+        Self {
+            enabled: true,
+            startup: XwaylandStartup::OnDemand,
+            path: PathBuf::from("xwayland-satellite"),
+        }
+    }
+}
+
+impl XwaylandConfig {
+    pub fn validate(&self) -> Result<(), String> {
+        if self.enabled && self.path.as_os_str().is_empty() {
+            return Err("xwayland.path must name an executable".into());
+        }
+
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn defaults_enable_on_demand_with_satellite_on_path() {
+        let config = XwaylandConfig::default();
+
+        assert!(config.enabled);
+        assert_eq!(config.startup, XwaylandStartup::OnDemand);
+        assert_eq!(config.path, PathBuf::from("xwayland-satellite"));
+        assert_eq!(config.validate(), Ok(()));
+    }
+
+    #[test]
+    fn disabled_config_does_not_require_a_path() {
+        let config = XwaylandConfig {
+            enabled: false,
+            path: PathBuf::new(),
+            ..XwaylandConfig::default()
+        };
+
+        assert_eq!(config.validate(), Ok(()));
+    }
+
+    #[test]
+    fn enabled_config_rejects_an_empty_path() {
+        let config = XwaylandConfig {
+            enabled: true,
+            path: PathBuf::new(),
+            ..XwaylandConfig::default()
+        };
+
+        assert_eq!(
+            config.validate(),
+            Err("xwayland.path must name an executable".to_owned())
+        );
+    }
+
+    #[test]
+    fn startup_mode_accepts_kebab_case() {
+        #[derive(Deserialize)]
+        struct Wrapper {
+            startup: XwaylandStartup,
+        }
+
+        assert_eq!(
+            serde_json::from_str::<Wrapper>(r#"{"startup":"eager"}"#)
+                .unwrap()
+                .startup,
+            XwaylandStartup::Eager
+        );
+        assert_eq!(
+            serde_json::from_str::<Wrapper>(r#"{"startup":"on-demand"}"#)
+                .unwrap()
+                .startup,
+            XwaylandStartup::OnDemand
+        );
+        assert!(serde_json::from_str::<Wrapper>(r#"{"startup":"eagerly"}"#).is_err());
+    }
+}