migration: Don't set FAILED state when cancelling

The expected outcome from qmp_migrate_cancel() is that the source
migration goes to the terminal state
MIGRATION_STATUS_CANCELLED. Anything different from this is a bug when
cancelling.

Make sure there is never a state transition from an unspecified state
into FAILED. Code that sets FAILED, should always either make sure
that the old state is not CANCELLING or specify the old state.

Note that the destination is allowed to go into FAILED, so there's no
issue there.

(I don't think this is relevant as a backport because cancelling does
work, it just doesn't show the right state at the end)

Fixes: 3dde8fdbad ("migration: Merge precopy/postcopy on switchover start")
Fixes: d0edb8a173 ("migration: Create the postcopy preempt channel asynchronously")
Fixes: 8518278a6a ("migration: implementation of background snapshot thread")
Fixes: bf78a046b9 ("migration: refactor migrate_fd_connect failures")
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20250213175927.19642-7-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/migration.c

@@ -2648,7 +2648,10 @@ static int postcopy_start(MigrationState *ms, Error **errp)
     if (migrate_postcopy_preempt()) {
         migration_wait_main_channel(ms);
         if (postcopy_preempt_establish_channel(ms)) {
-            migrate_set_state(&ms->state, ms->state, MIGRATION_STATUS_FAILED);
+            if (ms->state != MIGRATION_STATUS_CANCELLING) {
+                migrate_set_state(&ms->state, ms->state,
+                                  MIGRATION_STATUS_FAILED);
+            }
             error_setg(errp, "%s: Failed to establish preempt channel",
                        __func__);
             return -1;
@@ -2986,7 +2989,9 @@ fail:
         error_free(local_err);
     }
 
-    migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
+    if (s->state != MIGRATION_STATUS_CANCELLING) {
+        migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
+    }
 }
 
 /**
@@ -3009,7 +3014,7 @@ static void bg_migration_completion(MigrationState *s)
         qemu_put_buffer(s->to_dst_file, s->bioc->data, s->bioc->usage);
         qemu_fflush(s->to_dst_file);
     } else if (s->state == MIGRATION_STATUS_CANCELLING) {
-        goto fail;
+        return;
     }
 
     if (qemu_file_get_error(s->to_dst_file)) {
@@ -3953,7 +3958,9 @@ void migration_connect(MigrationState *s, Error *error_in)
 
 fail:
     migrate_set_error(s, local_err);
-    migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
+    if (s->state != MIGRATION_STATUS_CANCELLING) {
+        migrate_set_state(&s->state, s->state, MIGRATION_STATUS_FAILED);
+    }
     error_report_err(local_err);
     migration_cleanup(s);
 }