migration: Read state once

The 'status' field for the migration is updated normally using
an atomic operation from the migration thread.
Most readers of it aren't that careful, and in most cases it doesn't
matter.

In query_migrate->fill_source_migration_info the 'state'
is read twice; the first time to decide which state fields to fill in,
and then secondly to copy the state to the status field; that can end up
with a status that's inconsistent; e.g. setting up the fields
for 'setup' and then having an 'active' status.  In that case
libvirt gets upset by the lack of ram info.
The symptom is:
   libvirt.libvirtError: internal error: migration was active, but no RAM info was set

Read the state exactly once in fill_source_migration_info.

This is a possible fix for:
https://bugzilla.redhat.com/show_bug.cgi?id=2074205

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20220413113329.103696-1-dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -1084,6 +1084,7 @@ static void populate_disk_info(MigrationInfo *info)
 static void fill_source_migration_info(MigrationInfo *info)
 {
     MigrationState *s = migrate_get_current();
+    int state = qatomic_read(&s->state);
     GSList *cur_blocker = migration_blockers;
 
     info->blocked_reasons = NULL;
@@ -1103,7 +1104,7 @@ static void fill_source_migration_info(MigrationInfo *info)
     }
     info->has_blocked_reasons = info->blocked_reasons != NULL;
 
-    switch (s->state) {
+    switch (state) {
     case MIGRATION_STATUS_NONE:
         /* no migration has happened ever */
         /* do not overwrite destination migration status */
@@ -1148,7 +1149,7 @@ static void fill_source_migration_info(MigrationInfo *info)
         info->has_status = true;
         break;
     }
-    info->status = s->state;
+    info->status = state;
 }
 
 typedef enum WriteTrackingSupport {