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, 70 insertions, 0 deletions

diff --git a/src/system/vyos-system-update-check.py b/src/system/vyos-system-update-check.py
new file mode 100755
index 000000000..c9597721b
--- /dev/null
+++ b/src/system/vyos-system-update-check.py
@@ -0,0 +1,70 @@
+#!/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 argparse
+import json
+import jmespath
+
+from pathlib import Path
+from sys import exit
+from time import sleep
+
+from vyos.util import call
+
+import vyos.version
+
+motd_file = Path('/run/motd.d/10-vyos-update')
+
+
+if __name__ == '__main__':
+    # Parse command arguments and get config
+    parser = argparse.ArgumentParser()
+    parser.add_argument('-c',
+                        '--config',
+                        action='store',
+                        help='Path to system-update-check configuration',
+                        required=True,
+                        type=Path)
+
+    args = parser.parse_args()
+    try:
+        config_path = Path(args.config)
+        config = json.loads(config_path.read_text())
+    except Exception as err:
+        print(
+            f'Configuration file "{config_path}" does not exist or malformed: {err}'
+        )
+        exit(1)
+
+    url_json = config.get('url')
+    local_data = vyos.version.get_full_version_data()
+    local_version = local_data.get('version')
+
+    while True:
+        remote_data = vyos.version.get_remote_version(url_json)
+        if remote_data:
+            url = jmespath.search('[0].url', remote_data)
+            remote_version = jmespath.search('[0].version', remote_data)
+            if local_version != remote_version and remote_version:
+                call(f'wall -n "Update available: {remote_version} \nUpdate URL: {url}"')
+                # MOTD used in /run/motd.d/10-update
+                motd_file.parent.mkdir(exist_ok=True)
+                motd_file.write_text(f'---\n'
+                                     f'Current version: {local_version}\n'
+                                     f'Update available: \033[1;34m{remote_version}\033[0m\n'
+                                     f'---\n')
+        # Check every 12 hours
+        sleep(43200)