test: Overwriting snapshot from which microvm is loaded

This commit adds a test that has a microvm overwrite the snapshot file
from which it was loaded with a new snapshot (e.g. a sort of "refresh"
operation on the snapshot to write any modifications done since loading
back to disk). Prior to this patch series, this caused both the snapshot
and the microvm to become corrupted, as the zeroing done by the
truncate/set_len pair would be reflected in the mmap'd memory region.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

tests/integration_tests/functional/test_snapshot_basic.py

@@ -458,3 +458,43 @@ def test_diff_snapshot_overlay(guest_kernel, rootfs, microvm_factory):
     # Run some command to check that the restored VM works
     rc, _, stderr = new_vm.ssh.run("true")
     assert rc == 0, stderr
+
+
+def test_snapshot_overwrite_self(guest_kernel, rootfs, microvm_factory):
+    """Tests that if we try to take a snapshot that would overwrite the
+    very file from which the current VM is stored, nothing happens.
+
+    Note that even though we map the file as MAP_PRIVATE, the documentation
+    of mmap does not specify what should happen if the file is changed after being
+    mmap'd (https://man7.org/linux/man-pages/man2/mmap.2.html). It seems that
+    these changes can propagate to the mmap'd memory region."""
+    base_vm = microvm_factory.build(guest_kernel, rootfs)
+    base_vm.spawn()
+    base_vm.basic_config()
+    base_vm.add_net_iface()
+    base_vm.start()
+
+    # Wait for microvm to be booted
+    rc, _, stderr = base_vm.ssh.run("true")
+    assert rc == 0, stderr
+
+    snapshot = base_vm.snapshot_full()
+    base_vm.kill()
+
+    vm = microvm_factory.build()
+    vm.spawn()
+    vm.restore_from_snapshot(snapshot, resume=True)
+
+    # When restoring a snapshot, vm.restore_from_snapshot first copies
+    # the memory file (inside of the jailer) to /mem.src
+    currently_loaded = Path(vm.chroot()) / "mem.src"
+
+    assert currently_loaded.exists()
+
+    vm.snapshot_full(mem_path="mem.src")
+    vm.resume()
+
+    # Check the overwriting the snapshot file from which this microvm was originally
+    # restored, with a new snapshot of this vm, does not break the VM
+    rc, _, stderr = vm.ssh.run("true")
+    assert rc == 0, stderr