serial: T7484: treat unavailable serial console devices as non-fatal

Instead of failing or starting agetty on an invalid device, we now check
whether the specified serial console device is a valid TTY. If it's not, we
display a warning to the user and skip starting the agetty process.

2 files changed, 20 insertions, 0 deletions

diff --git a/python/vyos/utils/serial.py b/python/vyos/utils/serial.py
index 68aad676e..36b483e91 100644
--- a/python/vyos/utils/serial.py
+++ b/python/vyos/utils/serial.py
@@ -116,3 +116,16 @@ def restart_login_consoles(prompt_user=False, quiet=True, devices: List[str]=[])
             cmd(f'systemctl stop {unit_name}')
 
     return True
+
+def is_tty(name: str) -> bool:
+    """ Check if a given device file (e.g. /dev/ttyS0) is a TTY (teletypewriter)
+    device in Linux
+    """
+    import os
+    path_tty = f'/dev/{name}'
+    if os.path.exists(path_tty):
+        with open(path_tty, 'rb') as f:
+            fd = f.fileno()
+            # True if filename is a TTY
+            return os.isatty(fd)
+    return False
diff --git a/src/conf_mode/system_console.py b/src/conf_mode/system_console.py
index 2f742ee07..fe910c1b0 100755
--- a/src/conf_mode/system_console.py
+++ b/src/conf_mode/system_console.py
@@ -17,9 +17,11 @@
 import os
 from pathlib import Path
 
+from vyos.base import Warning
 from vyos.config import Config
 from vyos.utils.process import call
 from vyos.utils.serial import restart_login_consoles
+from vyos.utils.serial import is_tty
 from vyos.system import grub_util
 from vyos.template import render
 from vyos import ConfigError
@@ -66,6 +68,8 @@ def verify(console):
             # and it can not be used as a serial interface
             if not os.path.isdir(by_bus_dir) or not os.path.exists(by_bus_device):
                 raise ConfigError(f'Device {device} does not support beeing used as tty')
+        if not is_tty(device):
+            Warning(f'Device "{device}" used for console is not a TTY!')
 
     return None
 
@@ -98,6 +102,9 @@ def generate(console):
                 raise ConfigError(f'Device {device} does not support beeing used as tty')
 
     for device, device_config in console['device'].items():
+        # Do not render getty configuration if specified device is not a TTY.
+        if not is_tty(device):
+            continue
         config_file = base_dir + f'/serial-getty@{device}.service'
         Path(f'{base_dir}/getty.target.wants').mkdir(exist_ok=True)
         getty_wants_symlink = base_dir + f'/getty.target.wants/serial-getty@{device}.service'