Fix toasts on Windows under WSL 2 (#7137)

Before this: no notifications or toasts when using Codex CLI in WSL 2.

After this: I get toasts from Codex

codex-rs/tui/src/notifications/mod.rs

@@ -0,0 +1,139 @@
+mod osc9;
+mod windows_toast;
+
+use std::env;
+use std::io;
+
+use codex_core::env::is_wsl;
+use osc9::Osc9Backend;
+use windows_toast::WindowsToastBackend;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum NotificationBackendKind {
+    Osc9,
+    WindowsToast,
+}
+
+#[derive(Debug)]
+pub enum DesktopNotificationBackend {
+    Osc9(Osc9Backend),
+    WindowsToast(WindowsToastBackend),
+}
+
+impl DesktopNotificationBackend {
+    pub fn osc9() -> Self {
+        Self::Osc9(Osc9Backend)
+    }
+
+    pub fn windows_toast() -> Self {
+        Self::WindowsToast(WindowsToastBackend::default())
+    }
+
+    pub fn kind(&self) -> NotificationBackendKind {
+        match self {
+            DesktopNotificationBackend::Osc9(_) => NotificationBackendKind::Osc9,
+            DesktopNotificationBackend::WindowsToast(_) => NotificationBackendKind::WindowsToast,
+        }
+    }
+
+    pub fn notify(&mut self, message: &str) -> io::Result<()> {
+        match self {
+            DesktopNotificationBackend::Osc9(backend) => backend.notify(message),
+            DesktopNotificationBackend::WindowsToast(backend) => backend.notify(message),
+        }
+    }
+}
+
+pub fn detect_backend() -> DesktopNotificationBackend {
+    if should_use_windows_toasts() {
+        tracing::info!(
+            "Windows Terminal session detected under WSL; using Windows toast notifications"
+        );
+        DesktopNotificationBackend::windows_toast()
+    } else {
+        DesktopNotificationBackend::osc9()
+    }
+}
+
+fn should_use_windows_toasts() -> bool {
+    is_wsl() && env::var_os("WT_SESSION").is_some()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::NotificationBackendKind;
+    use super::detect_backend;
+    use serial_test::serial;
+    use std::ffi::OsString;
+
+    struct EnvVarGuard {
+        key: &'static str,
+        original: Option<OsString>,
+    }
+
+    impl EnvVarGuard {
+        fn set(key: &'static str, value: &str) -> Self {
+            let original = std::env::var_os(key);
+            unsafe {
+                std::env::set_var(key, value);
+            }
+            Self { key, original }
+        }
+
+        fn remove(key: &'static str) -> Self {
+            let original = std::env::var_os(key);
+            unsafe {
+                std::env::remove_var(key);
+            }
+            Self { key, original }
+        }
+    }
+
+    impl Drop for EnvVarGuard {
+        fn drop(&mut self) {
+            unsafe {
+                match &self.original {
+                    Some(value) => std::env::set_var(self.key, value),
+                    None => std::env::remove_var(self.key),
+                }
+            }
+        }
+    }
+
+    #[test]
+    #[serial]
+    fn defaults_to_osc9_outside_wsl() {
+        let _wsl_guard = EnvVarGuard::remove("WSL_DISTRO_NAME");
+        let _wt_guard = EnvVarGuard::remove("WT_SESSION");
+        assert_eq!(detect_backend().kind(), NotificationBackendKind::Osc9);
+    }
+
+    #[test]
+    #[serial]
+    fn waits_for_windows_terminal() {
+        let _wsl_guard = EnvVarGuard::set("WSL_DISTRO_NAME", "Ubuntu");
+        let _wt_guard = EnvVarGuard::remove("WT_SESSION");
+        assert_eq!(detect_backend().kind(), NotificationBackendKind::Osc9);
+    }
+
+    #[cfg(target_os = "linux")]
+    #[test]
+    #[serial]
+    fn selects_windows_toast_in_wsl_windows_terminal() {
+        let _wsl_guard = EnvVarGuard::set("WSL_DISTRO_NAME", "Ubuntu");
+        let _wt_guard = EnvVarGuard::set("WT_SESSION", "abc");
+        assert_eq!(
+            detect_backend().kind(),
+            NotificationBackendKind::WindowsToast
+        );
+    }
+
+    #[cfg(not(target_os = "linux"))]
+    #[test]
+    #[serial]
+    fn stays_on_osc9_outside_linux_even_with_wsl_env() {
+        let _wsl_guard = EnvVarGuard::set("WSL_DISTRO_NAME", "Ubuntu");
+        let _wt_guard = EnvVarGuard::set("WT_SESSION", "abc");
+        assert_eq!(detect_backend().kind(), NotificationBackendKind::Osc9);
+    }
+}