test-driver.py: fix VM state directory deletion

The previous version of the code would only kick in if the state
directory path pointed at a *file*, which never occurs. Making that
codepath actually work reveals an ordering bug, which this patch fixes
as well.

It also replaces the confusing, imperative case log message "delete VM
state directory" with "deleting VM state directory".

Finally, we hint the user about how to prevent this deletion. IE. by
passing the --keep-vm-state flag.

Bug report:
NixOS#91046 (comment)

Credit goes to Edef for the rebase on top of a recent nixpkgs commit
and for writing most of this commit message.

Co-authored-by: edef <edef@edef.eu>

nixos/lib/test-driver/test-driver.py

@@ -217,7 +217,7 @@ def __init__(self, args: Dict[str, Any]) -> None:
                 match = re.search("run-(.+)-vm$", cmd)
                 if match:
                     self.name = match.group(1)
-
+        self.logger = args["log"]
         self.script = args.get("startCommand", self.create_startcommand(args))
 
         tmp_dir = os.environ.get("TMPDIR", tempfile.gettempdir())
@@ -227,15 +227,17 @@ def create_dir(name: str) -> str:
             os.makedirs(path, mode=0o700, exist_ok=True)
             return path
 
-        self.state_dir = create_dir("vm-state-{}".format(self.name))
+        self.state_dir = os.path.join(tmp_dir, f"vm-state-{self.name}")
+        if not args["keepVmState"]:
+            self.cleanup_statedir()
+        os.makedirs(self.state_dir, mode=0o700, exist_ok=True)
         self.shared_dir = create_dir("shared-xchg")
 
         self.booted = False
         self.connected = False
         self.pid: Optional[int] = None
         self.socket = None
         self.monitor: Optional[socket.socket] = None
-        self.logger: Logger = args["log"]
         self.allow_reboot = args.get("allowReboot", False)
 
     @staticmethod
@@ -780,9 +782,10 @@ def process_serial_output() -> None:
         self.log("QEMU running (pid {})".format(self.pid))
 
     def cleanup_statedir(self) -> None:
-        self.log("delete the VM state directory")
-        if os.path.isfile(self.state_dir):
+        if os.path.isdir(self.state_dir):
             shutil.rmtree(self.state_dir)
+            self.logger.log(f"deleting VM state directory {self.state_dir}")
+            self.logger.log("if you want to keep the VM state, pass --keep-vm-state")
 
     def shutdown(self) -> None:
         if not self.booted:
@@ -940,10 +943,10 @@ def subtest(name: str) -> Iterator[None]:
     for nr, vde_socket, _, _ in vde_sockets:
         os.environ["QEMU_VDE_SOCKET_{}".format(nr)] = vde_socket
 
-    machines = [create_machine({"startCommand": s}) for s in vm_scripts]
-    for machine in machines:
-        if not cli_args.keep_vm_state:
-            machine.cleanup_statedir()
+    machines = [
+        create_machine({"startCommand": s, "keepVmState": cli_args.keep_vm_state})
+        for s in vm_scripts
+    ]
     machine_eval = [
         "{0} = machines[{1}]".format(m.name, idx) for idx, m in enumerate(machines)
     ]