utils: T8541: add write-config-file-value helper

Introduce src/helpers/write-config-file-value.py to patch a saved config.boot
from scripts by writing a quoted CLI path with an optional value (omit --value
to create a valueless node).

Extend vyos.utils.config.write_saved_value() to safely update saved configs:
create missing parent nodes before tagging (ConfigTree.set_tag requires nodes
to exist) and then write the requested value/valueless leaf back to disk.

Remove the legacy serial console activation script which is now replaced by the
helper-based approach. The helper is run once during ISO Image assembly and will
inject the approprate serial console defintiions into config.boot.default.

3 files changed, 77 insertions, 43 deletions

diff --git a/python/vyos/utils/config.py b/python/vyos/utils/config.py
index 1f067e91e..5bba69a61 100644
--- a/python/vyos/utils/config.py
+++ b/python/vyos/utils/config.py
@@ -44,6 +44,35 @@ def read_saved_value(path: list):
         return ' '.join(res)
     return res
 
+def write_saved_value(path: list, value=None, config_path: str=config_file):
+    """Write or replace a node in a saved configuration.
+
+    - If value is None, the node is treated as valueless.
+    - Tag nodes in the path are detected via XML reference tree and marked on
+      the ConfigTree prior to setting the node.
+    """
+    if not isinstance(path, list) or not path:
+        raise ValueError('path must be a non-empty list')
+
+    from vyos.configtree import ConfigTree
+    from vyos.utils.file import read_file
+    from vyos.utils.file import write_file
+
+    config_string = read_file(config_path)
+    ct = ConfigTree(config_string)
+
+    # ConfigTree.set_tag() requires the node to exist.
+    # Create missing nodes along the path so tag marking works even when
+    # writing a completely new subtree into config.boot.
+    for target in flag(path):
+        if not ct.exists(target):
+            ct.create_node(target)
+
+    set_tags(ct, path)
+
+    ct.set(path, value=value, replace=True)
+    write_file(config_path, ct.to_string())
+
 def flag(l: list) -> list:
     res = [l[0:i] for i,_ in enumerate(l, start=1)]
     return res
diff --git a/src/activation-scripts/05-serial_console.py b/src/activation-scripts/05-serial_console.py
deleted file mode 100755
index ccda2ac4e..000000000
--- a/src/activation-scripts/05-serial_console.py
+++ /dev/null
@@ -1,43 +0,0 @@
-# Copyright (C) VyOS Inc.
-#
-# This library is free software; you can redistribute it and/or
-# modify it under the terms of the GNU Lesser General Public
-# License as published by the Free Software Foundation; either
-# version 2.1 of the License, or (at your option) any later version.
-#
-# This library 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
-# Lesser General Public License for more details.
-#
-# You should have received a copy of the GNU Lesser General Public License
-# along with this library.  If not, see <http://www.gnu.org/licenses/>.
-
-from vyos.configtree import ConfigTree
-from vyos.system.image import is_live_boot
-from vyos.utils.kernel import get_kernel_serial_console
-from vyos.utils.serial import is_tty
-
-base = ['system', 'console', 'device']
-
-def activate(config: ConfigTree):
-    # Configure the kernel serial console only once during live boot. During
-    # installation, the user can define the console type (VT TTY or serial
-    # TTY). If this is not limited to live boot, the serial interface will
-    # always be re-added during system boot, even if it was removed from
-    # config.boot.
-    if not is_live_boot():
-        return
-
-    # Parse current kernel cmdline and continue only for valid serial console
-    # data. Prevent writing incomplete/invalid console settings to config.boot.
-    console_type, console_num, console_speed = get_kernel_serial_console()
-    device = f'{console_type}{console_num}'
-    if not is_tty(device) or not console_speed:
-        return
-
-    # The kernel was booted with a configured serial console, but no console
-    # is configured via CLI; align/fix the CLI configuration.
-    if not config.exists(base + [device]):
-        config.set(base + [device, 'speed'], value=console_speed)
-        config.set_tag(base)
diff --git a/src/helpers/write-config-file-value.py b/src/helpers/write-config-file-value.py
new file mode 100644
index 000000000..f3993223c
--- /dev/null
+++ b/src/helpers/write-config-file-value.py
@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+#
+# Copyright (C) VyOS Inc.
+#
+# 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/>.
+
+from argparse import ArgumentParser
+from shlex import split as shlex_split
+
+from vyos.utils.config import write_saved_value
+
+def _split_quoted(s: str) -> list[str]:
+    parts = shlex_split(s)
+    if not parts:
+        raise ValueError('empty string')
+    return parts
+
+if __name__ == '__main__':
+    parser = ArgumentParser()
+    parser.add_argument(
+        '--path',
+        required=True,
+        help='Quoted CLI path, e.g. "system console device ttyS1 speed"',
+    )
+    parser.add_argument(
+        '--value',
+        required=False,
+        help='Value for the node, e.g. "9600". If omitted, creates a valueless node.',
+    )
+    parser.add_argument(
+        '--config-file',
+        required=True,
+        help=f'Path to saved config.boot',
+    )
+    args = parser.parse_args()
+
+    path = _split_quoted(args.path)
+    write_saved_value(path, value=args.value, config_path=args.config_file)