feat(revpiconfig): Add CmdLineTxt class for managing cmdline.txt

Introduced the `CmdLineTxt` class to handle parsing, modifying, and
writing operations on the `cmdline.txt` file with thread safety.
Implemented methods for setting, removing keys, and ensuring safe file
operations.

Signed-off-by: Sven Sager <s.sager@kunbus.com>

src/revpi_middleware/dbus_middleware1/system_config/revpi_config.py

@@ -25,6 +25,7 @@ ConfigVariable = namedtuple("ConfigVariable", ["name", "value", "line_index"])
 LINUX_BT_CLASS_PATH = "/sys/class/bluetooth"
 LINUX_WLAN_CLASS_PATH = "/sys/class/ieee80211"
 CONFIG_TXT_LOCATIONS = ("/boot/firmware/config.txt", "/boot/config.txt")
+CMDLINE_TXT_LOCK = Lock()
 CONFIG_TXT_LOCK = Lock()
 
 
@@ -221,6 +222,87 @@ class RevPiConfig:
         return bool(self._wlan_class_path)
 
 
+class CmdLineTxt:
+    """
+    Represents operations on a `cmdline.txt` configuration file.
+
+    This class provides functionality to read, modify, and save the
+    `cmdline.txt` file commonly used for system configurations. It allows
+    setting key-value pairs, removing keys, and manages file locking to ensure
+    thread-safe modifications.
+    """
+
+    # Value is optional, "?:=" non-capturing the "="
+    re_name_value = re.compile(r"(?P<key>[^\s=]+)(?:=(?P<value>\S+))?")
+
+    def __init__(self):
+        self._cmdline_txt_path = ""
+        for path in CONFIG_TXT_LOCATIONS:
+            if exists(path):
+                self._cmdline_txt_path = path
+                break
+
+        if not self._cmdline_txt_path:
+            raise FileNotFoundError("no config.txt found")
+
+    def _get_cmdline_dict(self) -> dict:
+        with CMDLINE_TXT_LOCK:
+            with open(self._cmdline_txt_path, "r") as file:
+                cmdline = file.read()
+
+        return {
+            match.group("key"): match.group("value")
+            for match in self.re_name_value.finditer(cmdline)
+        }
+
+    def _write_cmdline_dict(self, cmdline_dict: dict) -> None:
+        with CMDLINE_TXT_LOCK:
+
+            tmp_path = f"{self._cmdline_txt_path}.tmp"
+            with open(tmp_path, "w") as file:
+                str_cmdline = ""
+                for key, value in cmdline_dict.items():
+                    if value is None:
+                        str_cmdline += f"{key} "
+                    else:
+                        str_cmdline += f"{key}={value} "
+
+                str_cmdline = str_cmdline.strip()
+                file.write(str_cmdline + "\n")
+
+            shutil.move(tmp_path, self._cmdline_txt_path)
+
+    def remove_key(self, key: str) -> None:
+        """
+        Removes a specified key from the config.txt file.
+
+        Parameters:
+        key: str
+            The key to be removed from the config.txt file.
+        """
+        dc_cmdline = self._get_cmdline_dict()
+        if key in dc_cmdline:
+            del dc_cmdline[key]
+            self._write_cmdline_dict(dc_cmdline)
+
+    def set_key_value(self, key: str, value: Optional[str] = None) -> None:
+        """
+        Sets a given key-value pair in the config.txt file. If the key does not
+        exist or the value differs from the current one, the pair is updated.
+        If the value is None, just the key is set without a value.
+
+        Parameters:
+        key: str
+            The key to set in the config.txt file.
+        value: Optional[str], default = None
+            The value to associate with the key, defaulting to None.
+        """
+        dc_cmdline = self._get_cmdline_dict()
+        if key not in dc_cmdline or dc_cmdline.get(key, value) != value:
+            dc_cmdline[key] = value
+            self._write_cmdline_dict(dc_cmdline)
+
+
 class ConfigTxt:
     """
     Configuration file handler for managing 'config.txt'.