Add a new stage for configuring pam_limits module

Add a new stage `org.osbuild.pam_limits.conf`, which created
configuration files for `pam_limits` module in /etc/security/limits.d.

Add unit test for the new stage.

Fix #788

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

stages/org.osbuild.pam_limits.conf

@@ -0,0 +1,115 @@
+#!/usr/bin/python3
+"""
+Create pam_limits module configuration.
+
+This stage creates a pam_limits module configuration file with the given name in
+/etc/security/limits.d. Provided list of configuration directives is written
+as separate lines into the configuration file. At least one configuration
+directive must be specified.
+"""
+
+
+import sys
+
+import osbuild.api
+
+
+SCHEMA = r"""
+"definitions": {
+  "configuration": {
+    "type": "object",
+    "additionalProperties": false,
+    "required": ["domain", "type", "item", "value"],
+    "description": "pam_limits module configuration directive representing one line in the configuration.",
+    "properties": {
+      "domain": {
+        "type": "string",
+        "description": "Domain to which the limit applies. E.g. username, groupname, etc."
+      },
+      "type": {
+        "type": "string",
+        "description": "Type of the limit.",
+        "enum": ["hard", "soft", "-"]
+      },
+      "item": {
+        "type": "string",
+        "description": "The resource type, which is being limited.",
+        "enum": [
+          "core",
+          "data",
+          "fsize",
+          "memlock",
+          "nofile",
+          "rss",
+          "stack",
+          "cpu",
+          "nproc",
+          "as",
+          "maxlogins",
+          "maxsyslogins",
+          "nonewprivs",
+          "priority",
+          "locks",
+          "sigpending",
+          "msgqueue",
+          "nice",
+          "rtprio"
+        ]
+      },
+      "value": {
+        "oneOf": [
+          {
+            "type": "string",
+            "enum": ["unlimited", "infinity"]
+          },
+          {
+            "type": "integer"
+          }
+        ],
+        "description": "The limit value."
+      }
+    }
+  }
+},
+"additionalProperties": false,
+"required": ["filename", "config"],
+"properties": {
+  "filename": {
+    "type": "string",
+    "description": "Name of the pam_limits module configuration file to create.",
+    "pattern": "^[\\w.-]{1,250}\\.conf$"
+  },
+  "config": {
+    "additionalProperties": false,
+    "type": "array",
+    "description": "List of configuration directives written into the configuration file.",
+    "minItems": 1,
+    "items": {
+      "$ref": "#/definitions/configuration"
+    }
+  }
+}
+"""
+
+
+def main(tree, options):
+    filename = options["filename"]
+    cfg = options["config"]
+
+    limitsd_config_dir = f"{tree}/etc/security/limits.d"
+
+    cfg_lines = []
+    for cfg_item in cfg:
+        cfg_line = f'{cfg_item["domain"]} {cfg_item["type"]} {cfg_item["item"]} {cfg_item["value"]}\n'
+        cfg_lines.append(cfg_line)
+
+    with open(f"{limitsd_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)