Fix startup command and holder fallback

src/persist.rs

@@ -28,6 +28,7 @@ use std::time::Duration;
 /// One-byte commands a server sends to a holder over its control socket after
 /// the holder has handed back the master fd.
 const HOLDER_CMD_SHUTDOWN: u8 = b'X';
+const HOLDER_STARTUP_TIMEOUT: Duration = Duration::from_millis(750);
 
 /// Magic written into a holder's `meta` file, bumped if the on-disk layout
 /// changes so a stale holder from an incompatible build is ignored.
@@ -236,11 +237,14 @@ pub fn spawn_holder(
     // Wait (briefly) for the holder to come up. The holder forks/setsids before
     // creating the socket; once the socket exists we can connect. We also reap
     // the short-lived launcher child so it never becomes a zombie.
-    let deadline = std::time::Instant::now() + Duration::from_secs(5);
+    let deadline = std::time::Instant::now() + HOLDER_STARTUP_TIMEOUT;
     loop {
         if sock.exists() {
             break;
         }
+        if let Some(status) = child.try_wait().context("wait holder launcher")? {
+            bail!("holder launcher for session {session} exited before socket was ready: {status}");
+        }
         if std::time::Instant::now() >= deadline {
             let _ = child.kill();
             let _ = child.wait();