migration: Add migrate_add_blocker_internal()

An internal version that removes -only-migratable implications.  It can be used
for temporary migration blockers like dump-guest-memory.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

include/migration/blocker.h

@@ -25,6 +25,22 @@
  */
 int migrate_add_blocker(Error *reason, Error **errp);
 
+/**
+ * @migrate_add_blocker_internal - prevent migration from proceeding without
+ *                                 only-migrate implications
+ *
+ * @reason - an error to be returned whenever migration is attempted
+ *
+ * @errp - [out] The reason (if any) we cannot block migration right now.
+ *
+ * @returns - 0 on success, -EBUSY on failure, with errp set.
+ *
+ * Some of the migration blockers can be temporary (e.g., for a few seconds),
+ * so it shouldn't need to conflict with "-only-migratable".  For those cases,
+ * we can call this function rather than @migrate_add_blocker().
+ */
+int migrate_add_blocker_internal(Error *reason, Error **errp);
+
 /**
  * @migrate_del_blocker - remove a blocking error from migration
  *

migration/migration.c

@@ -2049,15 +2049,8 @@ void migrate_init(MigrationState *s)
     s->threshold_size = 0;
 }
 
-int migrate_add_blocker(Error *reason, Error **errp)
+int migrate_add_blocker_internal(Error *reason, Error **errp)
 {
-    if (only_migratable) {
-        error_propagate_prepend(errp, error_copy(reason),
-                                "disallowing migration blocker "
-                                "(--only-migratable) for: ");
-        return -EACCES;
-    }
-
     /* Snapshots are similar to migrations, so check RUN_STATE_SAVE_VM too. */
     if (runstate_check(RUN_STATE_SAVE_VM) || !migration_is_idle()) {
         error_propagate_prepend(errp, error_copy(reason),
@@ -2070,6 +2063,18 @@ int migrate_add_blocker(Error *reason, Error **errp)
     return 0;
 }
 
+int migrate_add_blocker(Error *reason, Error **errp)
+{
+    if (only_migratable) {
+        error_propagate_prepend(errp, error_copy(reason),
+                                "disallowing migration blocker "
+                                "(--only-migratable) for: ");
+        return -EACCES;
+    }
+
+    return migrate_add_blocker_internal(reason, errp);
+}
+
 void migrate_del_blocker(Error *reason)
 {
     migration_blockers = g_slist_remove(migration_blockers, reason);