migration: Reject qmp_migrate_cancel after postcopy

After postcopy has started, it's not possible to recover the source
machine in case a migration error occurs because the destination has
already been changing the state of the machine. For that same reason,
it doesn't make sense to try to cancel the migration after postcopy
has started. Reject the cancel command during postcopy.

Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20250213175927.19642-6-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

1 files changed, 12 insertions, 1 deletions

diff --git a/migration/migration.c b/migration/migration.c
index c39cedef3b..48c9ad3c96 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2251,7 +2251,18 @@ static void qmp_migrate_finish(MigrationAddress *addr, bool resume_requested,
 
 void qmp_migrate_cancel(Error **errp)
 {
-    migration_cancel(NULL);
+    /*
+     * After postcopy migration has started, the source machine is not
+     * recoverable in case of a migration error. This also means the
+     * cancel command cannot be used as cancel should allow the
+     * machine to continue operation.
+     */
+    if (migration_in_postcopy()) {
+        error_setg(errp, "Postcopy migration in progress, cannot cancel.");
+        return;
+    }
+
+    migration_cancel();
 }
 
 void qmp_migrate_continue(MigrationStatus state, Error **errp)