acpid: exit cleanly when acpi scheme already registered (hwd double-spawn)

drivers/acpid/src/main.rs

@@ -111,8 +111,21 @@ fn daemon(daemon: daemon::Daemon) -> ! {
         .subscribe(socket.inner().raw(), 1, EventFlags::READ)
         .expect("acpid: failed to register scheme socket for event queue");
 
-    register_sync_scheme(&socket, "acpi", &mut scheme)
-        .expect("acpid: failed to register acpi scheme to namespace");
+    if let Err(err) = register_sync_scheme(&socket, "acpi", &mut scheme) {
+        if err.errno == syscall::EEXIST {
+            // acpid is reachable via BOTH 41_acpid.service and hwd's ACPI
+            // backend (drivers/hwd/src/backend/acpi.rs spawns "acpid"), so a
+            // second instance races to register the "acpi" scheme. That is
+            // redundant, not fatal: exit cleanly so the duplicate does not
+            // abort and wedge the boot — the first instance keeps serving.
+            log::info!("acpid: 'acpi' scheme already registered; exiting duplicate instance");
+            std::process::exit(0);
+        }
+        panic!(
+            "acpid: failed to register acpi scheme to namespace: {}",
+            err
+        );
+    }
 
     libredox::call::setrens(0, 0).expect("acpid: failed to enter null namespace");