update-check: T3476: Allow update-check for VyOS images

Ability to autocheck available new images
Parse remote URL JSON image-version.json file and compare version
VyOS with a local current version, if find diff sent wall
message that the new image is available
Also, add op-mode command to check images "show system image"
With option "auto-check" check will be once per 12 hours

  set system update-check auto-check
  set system update-check url 'http://example.com/image-version.json'

If new version is available shows it per login (MOTD)

1 files changed, 93 insertions, 0 deletions

diff --git a/src/conf_mode/system_update_check.py b/src/conf_mode/system_update_check.py
new file mode 100755
index 000000000..08ecfcb81
--- /dev/null
+++ b/src/conf_mode/system_update_check.py
@@ -0,0 +1,93 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) 2022 VyOS maintainers and contributors
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 or later as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import os
+import json
+import jmespath
+
+from pathlib import Path
+from sys import exit
+
+from vyos.config import Config
+from vyos.util import call
+from vyos import ConfigError
+from vyos import airbag
+airbag.enable()
+
+
+base = ['system', 'update-check']
+service_name = 'vyos-system-update'
+service_conf = Path(f'/run/{service_name}.conf')
+motd_file = Path('/run/motd.d/10-vyos-update')
+
+
+def get_config(config=None):
+    if config:
+        conf = config
+    else:
+        conf = Config()
+
+    if not conf.exists(base):
+        return None
+
+    config = conf.get_config_dict(base, key_mangling=('-', '_'),
+                                  get_first_key=True, no_tag_node_value_mangle=True)
+
+    return config
+
+
+def verify(config):
+    # bail out early - looks like removal from running config
+    if config is None:
+        return
+
+    if 'url' not in config:
+        raise ConfigError('URL is required!')
+
+
+def generate(config):
+    # bail out early - looks like removal from running config
+    if config is None:
+        # Remove old config and return
+        service_conf.unlink(missing_ok=True)
+        # MOTD used in /run/motd.d/10-update
+        motd_file.unlink(missing_ok=True)
+        return None
+
+    # Write configuration file
+    conf_json = json.dumps(config, indent=4)
+    service_conf.write_text(conf_json)
+
+    return None
+
+
+def apply(config):
+    if config:
+        if 'auto_check' in config:
+            call(f'systemctl restart {service_name}.service')
+    else:
+        call(f'systemctl stop {service_name}.service')
+
+
+if __name__ == '__main__':
+    try:
+        c = get_config()
+        verify(c)
+        generate(c)
+        apply(c)
+    except ConfigError as e:
+        print(e)
+        exit(1)