test: full snapshot resets dirty pages

Test that taking full snapshot is as described in documentation:

> the diff consists of the memory pages which have been dirtied since
> the last snapshot creation or since the creation of the microVM,
> whichever of these events was the most recent.

Co-authored-by: Pablo Barbáchano <[email protected]>
Signed-off-by: Roman Kovtyukh <[email protected]>

tests/integration_tests/test_dirty_pages_after_full_snapshot.py

@@ -0,0 +1,25 @@
+def test_dirty_pages_after_full_snapshot(uvm_plain):
+    """
+    Test if dirty pages are erased after making a full snapshot of a VM
+    """
+
+    vm_mem_size = 128
+    uvm = uvm_plain
+    uvm.spawn()
+    uvm.basic_config(mem_size_mib=vm_mem_size, track_dirty_pages=True)
+    uvm.add_net_iface()
+    uvm.start()
+    uvm.ssh.run("true")
+
+    snap_full = uvm.snapshot_full(vmstate_path="vmstate_full", mem_path="mem_full")
+    snap_diff = uvm.snapshot_diff(vmstate_path="vmstate_diff", mem_path="mem_diff")
+    snap_diff2 = uvm.snapshot_diff(vmstate_path="vmstate_diff2", mem_path="mem_diff2")
+
+    # file size is the same, but the `diff` snapshot is actually a sparse file
+    assert snap_full.mem.stat().st_size == snap_diff.mem.stat().st_size
+
+    # diff -> diff there should be no differences
+    assert snap_diff2.mem.stat().st_blocks == 0
+
+    # full -> diff there should be no differences
+    assert snap_diff.mem.stat().st_blocks == 0