system/bootdevice: Don't unregister reset handler in restore_boot_order()

Currently the qemu_register_reset() API permits the reset handler functions
registered with it to remove themselves from within the callback function.
This is fine with our current implementation, but is a bit odd, because
generally reset is supposed to be idempotent, and doesn't fit well in a
three-phase-reset world where a resettable object will get multiple
callbacks as the system is reset.

We now have only one user of qemu_register_reset() which makes use of
the ability to unregister itself within the callback:
restore_boot_order().  We want to change our implementation of
qemu_register_reset() to something where it would be awkward to
maintain the "can self-unregister" feature.  Rather than making that
reimplementation complicated, change restore_boot_order() so that it
doesn't unregister itself but instead returns doing nothing for any
calls after it has done the "restore the boot order" work.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240220160622.114437-4-peter.maydell@linaro.org
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>

system/bootdevice.c

@@ -101,20 +101,23 @@ void validate_bootdevices(const char *devices, Error **errp)
 void restore_boot_order(void *opaque)
 {
     char *normal_boot_order = opaque;
-    static int first = 1;
+    static int bootcount;
 
-    /* Restore boot order and remove ourselves after the first boot */
-    if (first) {
-        first = 0;
+    switch (bootcount++) {
+    case 0:
+        /* First boot: use the one-time config */
+        return;
+    case 1:
+        /* Second boot: restore normal boot order */
+        if (boot_set_handler) {
+            qemu_boot_set(normal_boot_order, &error_abort);
+        }
+        g_free(normal_boot_order);
+        return;
+    default:
+        /* Subsequent boots: keep using normal boot order */
         return;
     }
-
-    if (boot_set_handler) {
-        qemu_boot_set(normal_boot_order, &error_abort);
-    }
-
-    qemu_unregister_reset(restore_boot_order, normal_boot_order);
-    g_free(normal_boot_order);
 }
 
 void check_boot_index(int32_t bootindex, Error **errp)