migration: Check in savevm_state_handler_insert for dups

Before finally register one SaveStateEntry, we detect for duplicated
entries.  This could be helpful to notify us asap instead of get
silent migration failures which could be hard to diagnose.

For example, this patch will generate a message like this (if without
previous fixes on x2apic) as long as we wants to boot a VM instance
with "-smp 200,maxcpus=288,sockets=2,cores=72,threads=2" and QEMU will
bail out even before VM starts:

savevm_state_handler_insert: Detected duplicate SaveStateEntry: id=apic, instance_id=0x0

Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231020090731.28701-10-quintela@redhat.com>

migration/savevm.c

@@ -237,6 +237,8 @@ static SaveState savevm_state = {
     .global_section_id = 0,
 };
 
+static SaveStateEntry *find_se(const char *idstr, uint32_t instance_id);
+
 static bool should_validate_capability(int capability)
 {
     assert(capability >= 0 && capability < MIGRATION_CAPABILITY__MAX);
@@ -716,6 +718,18 @@ static void savevm_state_handler_insert(SaveStateEntry *nse)
 
     assert(priority <= MIG_PRI_MAX);
 
+    /*
+     * This should never happen otherwise migration will probably fail
+     * silently somewhere because we can be wrongly applying one
+     * object properties upon another one.  Bail out ASAP.
+     */
+    if (find_se(nse->idstr, nse->instance_id)) {
+        error_report("%s: Detected duplicate SaveStateEntry: "
+                     "id=%s, instance_id=0x%"PRIx32, __func__,
+                     nse->idstr, nse->instance_id);
+        exit(EXIT_FAILURE);
+    }
+
     for (i = priority - 1; i >= 0; i--) {
         se = savevm_state.handler_pri_head[i];
         if (se != NULL) {