Add a new stage for setting kernel parameters via sysctl.d

Add a new stage `org.osbuild.sysctld` for setting kernel parameters at
boot by creating a configuration file in /usr/lib/sysctl.d. At least
one parameter must be specified for the stage.

Add unit test for the new stage.

Fix #790

Signed-off-by: Tomas Hozza <thozza@redhat.com>

stages/org.osbuild.sysctld

@@ -0,0 +1,106 @@
+#!/usr/bin/python3
+"""
+Configure kernel parameters at boot via sysctl.d.
+
+This stage creates a sysctl.d configuration file with the given name in
+/usr/lib/sysctl.d. Provided list of parameters is written as separate lines
+into the configuration file. At least one parameter must be specified.
+
+The syntax of the sysctl.d file is simply:
+<token> = <value>
+
+If a line begins with a single "-", any attempts to set the value that fail
+will be ignored. (from sysct.conf(5) man page)
+
+The <token> can be a glob pattern.
+
+A key may be explicitly excluded from being set by any matching glob patterns
+by specifying the key name prefixed with a "-" character and not followed
+by "=". (from sysctl.d(5) man page)
+"""
+
+
+import sys
+
+import osbuild.api
+
+
+SCHEMA = r"""
+"definitions": {
+  "parameter": {
+    "type": "object",
+    "additionalProperties": false,
+    "required": ["key", "value"],
+    "description": "Kernel parameter to set.",
+    "properties": {
+      "key": {
+        "type": "string",
+        "description": "Kernel parameter name."
+      },
+      "value": {
+        "type": "string",
+        "description": "Kernel parameter value."
+      }
+    }
+  },
+  "parameter-excluded": {
+    "type": "object",
+    "additionalProperties": false,
+    "required": ["key"],
+    "description": "Parameter to exclude from being set by a matching glob",
+    "properties": {
+      "key": {
+        "type": "string",
+        "pattern": "^-.+$",
+        "description": "Key to exclude from being set by a matching glob"
+      }
+    }
+  }
+},
+"additionalProperties": false,
+"required": ["filename", "config"],
+"properties": {
+  "filename": {
+    "type": "string",
+    "description": "Name of the sysctl.d configuration file to create.",
+    "pattern": "^[\\w.-]{1,250}\\.conf$"
+  },
+  "config": {
+    "additionalProperties": false,
+    "type": "array",
+    "description": "List of kernel parameters to write into the configuration file.",
+    "minItems": 1,
+    "items": {
+      "anyOf": [
+        {"$ref": "#/definitions/parameter"},
+        {"$ref": "#/definitions/parameter-excluded"}
+      ]
+    }
+  }
+}
+"""
+
+
+def main(tree, options):
+    filename = options["filename"]
+    cfg = options["config"]
+
+    sysctld_config_dir = f"{tree}/usr/lib/sysctl.d"
+
+    cfg_lines = []
+    for cfg_item in cfg:
+        key = cfg_item["key"]
+        value = cfg_item.get("value")
+        cfg_line = f"{key} = {value}\n" if value else f"{key}\n"
+        cfg_lines.append(cfg_line)
+
+    with open(f"{sysctld_config_dir}/{filename}", "w") as f:
+        f.writelines(cfg_lines)
+
+    return 0
+
+
+if __name__ == '__main__':
+    args = osbuild.api.arguments()
+    r = main(args["tree"], args["options"])
+    sys.exit(r)