Skip daemon auto-start for Windows-mounted WSL homes (#50555)

## Why

The managed daemon writes an executable and Unix-style state beneath `CODEX_HOME`. Windows-mounted WSL filesystems (DrvFS/9p) do not reliably support the required permission semantics, which can cause startup to fail before the TUI opens.

## What changed

Detect WSL homes on 9p filesystems, checking an existing ancestor when `CODEX_HOME` does not yet exist. Keep the TUI in embedded mode instead of automatically starting the shared background server, and show a startup warning explaining the exclusion.

## Testing

Extend the daemon exclusion warning snapshot to cover the Windows-mounted WSL message.

GitOrigin-RevId: 995e6236d79d8190b0ab1e6b4761eeef3c068467

codex-rs/tui/src/daemon_startup.rs

@@ -15,6 +15,37 @@ const SERVER_FEATURES: [Feature; 4] = [
 ];
 
 pub(super) const FAILURE_HINT: &str = "To work without the background server, rerun the same command with --no-daemon (including resume or fork and its arguments).";
+pub(super) const WSL_DRVFS_EXCLUSION: &str = "a Windows-mounted WSL CODEX_HOME (DrvFS/9p)";
+
+pub(super) fn uses_wsl_drvfs(codex_home: &std::path::Path) -> bool {
+    // The managed daemon writes an executable and Unix-style state beneath CODEX_HOME.
+    // DrvFS does not reliably support the required permission semantics, so starting it
+    // there can fail before the TUI opens.
+    #[cfg(target_os = "linux")]
+    {
+        use std::os::fd::AsRawFd;
+
+        if !codex_utils_path::is_wsl() && std::env::var_os("WSL_INTEROP").is_none() {
+            return false;
+        }
+        let Some(directory) = codex_home
+            .ancestors()
+            .find_map(|path| std::fs::File::open(path).ok())
+        else {
+            return false;
+        };
+        let mut stats = std::mem::MaybeUninit::<libc::statfs>::uninit();
+        if unsafe { libc::fstatfs(directory.as_raw_fd(), stats.as_mut_ptr()) } != 0 {
+            return false;
+        }
+        unsafe { stats.assume_init() }.f_type as u64 == 0x0102_1997
+    }
+    #[cfg(not(target_os = "linux"))]
+    {
+        let _ = codex_home;
+        false
+    }
+}
 
 #[cfg(any(windows, test))]
 pub(super) const ELEVATED_LAUNCH_WARNING: &str = "Running as administrator: shared background server disabled. To enable it, restart Codex in a terminal without administrator permissions.";

codex-rs/tui/src/daemon_startup_tests.rs

@@ -431,6 +431,10 @@ fn daemon_exclusion_warning_snapshot() {
     let cell = crate::history_cell::StartupWarningsCell::new(vec![
         "Running without the shared background server: --strict-config requires embedded mode."
             .into(),
+        format!(
+            "Running without the shared background server: {} requires embedded mode.",
+            daemon_startup::WSL_DRVFS_EXCLUSION
+        ),
     ]);
     let text = cell
         .transcript_lines(/*width*/ 80)

codex-rs/tui/src/snapshots/codex_tui__daemon_startup_tests__daemon_exclusion_warning.snap

@@ -4,3 +4,5 @@ expression: text
 ---
 ⚠ Running without the shared background server: --strict-config requires
   embedded mode.
+⚠ Running without the shared background server: a Windows-mounted WSL CODEX_HOME
+  (DrvFS/9p) requires embedded mode.

codex-rs/tui/src/startup_orchestration.rs

@@ -533,6 +533,13 @@ pub(super) async fn run_main_inner(
         daemon_exclusion = Some("Bedrock sign-in");
         app_server_target = AppServerTarget::Embedded;
     }
+    if auto_start_daemon
+        && daemon_exclusion.is_none()
+        && matches!(&app_server_target, AppServerTarget::Embedded)
+        && daemon_startup::uses_wsl_drvfs(&codex_home)
+    {
+        daemon_exclusion = Some(daemon_startup::WSL_DRVFS_EXCLUSION);
+    }
     let mut daemon_features = daemon_startup::server_features(&cli_kv_overrides);
     // Disabling shared services requires confirmation, even on a fresh auto-start.
     daemon_features.retain(|_, enabled| *enabled);