stages: extend org.osbuild.systemd to create .service unit drop-ins

Extend the `org.osbuild.systemd` stage to create drop-in configuration
files for Systemd `.service` units under `/usr/lib/systemd/system`.
Currently only the `Environment` option in the `Service` section can be
configured.

Update the `org.osbuild.systemd` stage test case to create drop-in
configuration `10-rh-enable-for-ec2.conf` for `nm-cloud-setup.service`
unit, as used in RHEL AMI images.

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

stages/org.osbuild.systemd

@@ -1,25 +1,38 @@
 #!/usr/bin/python3
 """
-Enable or disable systemd services
+Configure Systemd services.
 
-Enable or disable systemd units (service, socket, path, etc.)
+Enable, disable or mask systemd units (service, socket, path, etc.) by running
+`systemctl` from the buildhost.
 
 This stage runs `systemctl enable` for all `enabled_services` items, which may
 create symlinks under `/etc/systemd/system`.
 After enabling units, it runs `systemctl disable` for all `disabled_services`
 items, which will delete _all_ symlinks to the named services.
 
-Uses `systemctl` from the buildhost.
+The 'default_target' option allows to configure the default Systemd target.
+
+The 'unit_dropins' option allows to create Systemd unit drop-in configuration
+files in `/usr/lib/systemd/system/<unit_name>.d/`. Its properties are names of
+'.service' files to be modified using drop-ins. These names are validated using
+the same rules as specified by systemd.unit(5) and they must contain the
+'.service' suffix (other types of unit files are not supported). Value of each
+specified '.service' file is an object, which properties are names of drop-in
+configuration '.conf' files. Drop-in configuration files can currently specify
+the following subset of options:
+  - 'Service' section
+    - 'Environment' option
 """
 
-
+import os
 import subprocess
 import sys
+import configparser
 
 import osbuild.api
 
 
-SCHEMA = """
+SCHEMA = r"""
 "additionalProperties": false,
 "properties": {
   "enabled_services": {
@@ -40,16 +53,73 @@ SCHEMA = """
   "default_target": {
     "type": "string",
     "description": "The default target to boot into"
+  },
+  "unit_dropins": {
+    "additionalProperties": false,
+    "type": "object",
+    "description": "Systemd unit drop-in configurations.",
+    "patternProperties": {
+      "^[\\w:.\\\\-]+[@]{0,1}[\\w:.\\\\-]*\\.service$": {
+        "additionalProperties": false,
+        "type": "object",
+        "description": "Drop-in configurations for a '.service' unit.",
+        "patternProperties": {
+          "^[\\w.-]{1,250}\\.conf$": {
+            "additionalProperties": false,
+            "type": "object",
+            "description": "Drop-in configuration for a '.service' unit.",
+            "properties": {
+              "Service": {
+                "additionalProperties": false,
+                "type": "object",
+                "description": "'Service' configuration section of a unit file.",
+                "properties": {
+                  "Environment": {
+                    "type": "string",
+                    "description": "Sets environment variables for executed process."
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    }
   }
 }
 """
 
 
+def configure_unit_dropins(tree, unit_dropins_options):
+    for unit, unit_dropins in unit_dropins_options.items():
+        # ensure the unit name + ".d" does not exceed maximum filename length
+        if len(unit+".d") > 255:
+            raise ValueError(f"Error: the {unit} unit drop-in directory exceeds the maximum filename length.")
+
+        unit_dropins_dir = f"{tree}/usr/lib/systemd/system/{unit}.d"
+        os.makedirs(unit_dropins_dir, exist_ok=True)
+
+        for dropin_file, dropin_config in unit_dropins.items():
+            config = configparser.ConfigParser()
+            # prevent conversion of the option name to lowercase
+            config.optionxform = lambda option: option
+
+            for section, options in dropin_config.items():
+                if not config.has_section(section):
+                    config.add_section(section)
+                for option, value in options.items():
+                    config.set(section, option, str(value))
+
+            with open(f"{unit_dropins_dir}/{dropin_file}", "w") as f:
+                config.write(f, space_around_delimiters=False)
+
+
 def main(tree, options):
     enabled_services = options.get("enabled_services", [])
     disabled_services = options.get("disabled_services", [])
     masked_services = options.get("masked_services", [])
     default_target = options.get("default_target")
+    unit_dropins_options = options.get("unit_dropins", {})
 
     for service in enabled_services:
         subprocess.run(["systemctl", "--root", tree, "enable", service], check=True)
@@ -63,6 +133,8 @@ def main(tree, options):
     if default_target:
         subprocess.run(["systemctl", "--root", tree, "set-default", default_target], check=True)
 
+    configure_unit_dropins(tree, unit_dropins_options)
+
     return 0
 
 

test/data/stages/systemd/b.json

@@ -456,7 +456,16 @@
           ],
           "disabled_services": [
             "sshd"
-          ]
+          ],
+          "unit_dropins": {
+            "nm-cloud-setup.service": {
+              "10-rh-enable-for-ec2.conf": {
+                "Service": {
+                  "Environment": "NM_CLOUD_SETUP_EC2=yes"
+                }
+              }
+            }
+          }
         }
       }
     ]

test/data/stages/systemd/b.mpp.json

@@ -42,7 +42,16 @@
           ],
           "disabled_services": [
             "sshd"
-          ]
+          ],
+          "unit_dropins": {
+            "nm-cloud-setup.service": {
+              "10-rh-enable-for-ec2.conf": {
+                "Service": {
+                  "Environment": "NM_CLOUD_SETUP_EC2=yes"
+                }
+              }
+            }
+          }
         }
       }
     ]

test/data/stages/systemd/diff.json

@@ -1,7 +1,9 @@
 {
   "added_files": [
     "/etc/systemd/system/ldconfig.service",
-    "/etc/systemd/system/multi-user.target.wants/nftables.service"
+    "/etc/systemd/system/multi-user.target.wants/nftables.service",
+    "/usr/lib/systemd/system/nm-cloud-setup.service.d",
+    "/usr/lib/systemd/system/nm-cloud-setup.service.d/10-rh-enable-for-ec2.conf"
   ],
   "deleted_files": [
     "/etc/systemd/system/multi-user.target.wants/sshd.service"