test(build_n_from_snapshot): Delete snapshot n after iteration n+2

If build_n_from_snapshot is asked to build many snapshots incrementally,
it doesn't clean up after itself properly: Each iteration ends by
creating a new snapshot of the VM, and this snapshot is passed to
iteration n+1. Here, a copy is created inside the new VMs chroot, and
iteration n+1 ends by creating a new snapshot for iteration n+2, and
deleting the copy of the snapshot inside the chroot. However, we never
delete the snapshot created in iteration n, and so with each iteration
more snapshots accumulate.

Fix this by having the function delete the snapshot created in iteration
n after iteration n+2 finished successfully.  The idea here is that in
case of failure, we will have the snapshot created in iteration n+1 (the
one which caused a failure in n+2), and also the snapshot created in n
(which was the last known snapshot to successfully go through a test
iteration, namely iteration n+1).

Signed-off-by: Patrick Roy <[email protected]>

Author: roypat

tests/framework/microvm.py

@@ -1158,7 +1158,7 @@ def build_from_snapshot(self, snapshot: Snapshot):
 
     def build_n_from_snapshot(
         self,
-        snapshot,
+        current_snapshot,
         nr_vms,
         *,
         uffd_handler_name=None,
@@ -1168,6 +1168,7 @@ def build_n_from_snapshot(
         """A generator of `n` microvms restored, either all restored from the same given snapshot
         (incremental=False), or created by taking successive snapshots of restored VMs
         """
+        last_snapshot = None
         for _ in range(nr_vms):
             microvm = self.build()
             microvm.spawn()
@@ -1176,27 +1177,33 @@ def build_n_from_snapshot(
                 spawn_pf_handler(
                     microvm,
                     uffd_handler(uffd_handler_name, binary_dir=self.binary_path),
-                    snapshot,
+                    current_snapshot,
                 )
 
-            snapshot_copy = microvm.restore_from_snapshot(snapshot, resume=True)
+            snapshot_copy = microvm.restore_from_snapshot(current_snapshot, resume=True)
 
             yield microvm
 
             if incremental:
-                new_snapshot = microvm.make_snapshot(snapshot.snapshot_type)
+                if last_snapshot is not None:
+                    last_snapshot.delete()
 
-                if snapshot.is_diff:
-                    new_snapshot = new_snapshot.rebase_snapshot(
-                        snapshot, use_snapshot_editor
+                next_snapshot = microvm.make_snapshot(current_snapshot.snapshot_type)
+
+                if current_snapshot.is_diff:
+                    next_snapshot = next_snapshot.rebase_snapshot(
+                        current_snapshot, use_snapshot_editor
                     )
 
-                snapshot = new_snapshot
+                last_snapshot = current_snapshot
+                current_snapshot = next_snapshot
 
             microvm.kill()
             snapshot_copy.delete()
 
-        snapshot.delete()
+        if last_snapshot is not None:
+            last_snapshot.delete()
+        current_snapshot.delete()
 
     def kill(self):
         """Clean up all built VMs"""