Bring back --default to allow OBS builds to select an entry point among many

When building a mkosi repository called 'particleos', OBS calls mkosi as such:

mkosi --directory /usr/src/packages/SOURCES/particleos --default mkosi.conf

with mkosi.conf being the source file extracted at source time, ie: defined
by the user, among the many files available in the repository.
So this can be used to select the entry point, that defines what to build,
when a repository provides many build types, profiles, images, etc.

Right now --default is a no-op, so make it add the passed conf file as a
high priority config file, so that the entry point can have precedence
over anything else.

The file is placed at:

/usr/src/packages/SOURCES/mkosi.conf

This cannot be controlled by the user, and it's fixed.

Config file parsing requires absolute paths, and aborts otherwise.
Because it is called with a filename only, but mkosi executes from the
--directory location, special case it and if a non-absolute path is passed,
look in the parent of --directory so that the use case of providing an
entry point can be enabled.

Author: bluca

mkosi/config.py

@@ -1768,6 +1768,7 @@ class Args:
     cmdline: list[str]
     force: int
     directory: Optional[Path]
+    default_config: Optional[Path]
     debug: bool
     debug_shell: bool
     debug_workspace: bool
@@ -4328,6 +4329,14 @@ def create_argument_parser(chdir: bool = True) -> argparse.ArgumentParser:
         help="Change to specified directory before doing anything",
         metavar="PATH",
     )
+    parser.add_argument(
+        "--default",
+        dest="default_config",
+        help="Use a specific configuration file as the entry point, allowing to set various defaults",
+        type=Path,
+        default=None,
+        metavar="PATH",
+    )
     parser.add_argument(
         "--debug",
         help="Turn on debugging output",
@@ -4412,10 +4421,6 @@ def create_argument_parser(chdir: bool = True) -> argparse.ArgumentParser:
         nargs=0,
         action=IgnoreAction,
     )
-    parser.add_argument(
-        "--default",
-        action=IgnoreAction,
-    )
     parser.add_argument(
         "--cache",
         action=IgnoreAction,
@@ -4531,6 +4536,7 @@ def __call__(
 
 class ParseContext:
     def __init__(self, resources: Path = Path("/")) -> None:
+        self.default_config: Optional[Path] = None
         self.resources = resources
         # We keep two namespaces around, one for the settings specified on the CLI and one for
         # the settings specified in configuration files. This is required to implement both [Match]
@@ -4810,10 +4816,14 @@ def parse_config_one(self, path: Path, parse_profiles: bool = False, parse_local
 
         if extras:
             if parse_local:
-                for localpath in (
-                    *([p] if (p := path.parent / "mkosi.local").is_dir() else []),
-                    *([p] if (p := path.parent / "mkosi.local.conf").is_file() else []),
-                ):
+                localpaths = []
+                if (p := path.parent / "mkosi.local").is_dir():
+                    localpaths.append(p)
+                if (p := path.parent / "mkosi.local.conf").is_file():
+                    localpaths.append(p)
+                if self.default_config is not None and self.default_config.is_file():
+                    localpaths.append(self.default_config)
+                for localpath in localpaths:
                     with chdir(localpath if localpath.is_dir() else Path.cwd()):
                         self.parse_config_one(localpath if localpath.is_file() else Path.cwd())
 
@@ -5146,6 +5156,15 @@ def parse_config(
     # One of the specifiers needs access to the directory, so make sure it is available.
     context.config["directory"] = args.directory
 
+    # Config paths need to be absolute - as a special case, if it is not, look in the parent
+    # of --directory. This works nicely on OBS, where the --directory is the git repository
+    # and the --default is the mkosi.conf selected at source service time, placed in the parent.
+    if args.default_config is not None:
+        if args.directory is not None and not args.default_config.is_absolute():
+            context.default_config = args.directory.parent / args.default_config
+        else:
+            context.default_config = args.default_config
+
     context.parse_new_includes()
 
     context.config["files"] = []

mkosi/resources/man/mkosi.1.md

@@ -256,6 +256,12 @@ Those settings cannot be configured in the configuration files.
     working directory. If the empty string is specified, all configuration in
     the current working directory will be ignored.
 
+`--default=`
+:   Takes an absolute path to a configuration file. **mkosi** parses this file
+    before doing anything. This allows a user to select an entry point to
+    programmatically decide among various available options (e.g.: profiles)
+    without having to select them on the command line.
+
 `--debug`
 :   Enable additional debugging output.
 

tests/test_json.py

@@ -61,6 +61,7 @@ def test_args(path: Optional[Path]) -> None:
             "DebugSandbox": false,
             "DebugShell": false,
             "DebugWorkspace": false,
+            "DefaultConfig": null,
             "Directory": {f'"{os.fspath(path)}"' if path is not None else "null"},
             "DocFormat": "auto",
             "Force": 9001,
@@ -82,6 +83,7 @@ def test_args(path: Optional[Path]) -> None:
         debug_sandbox=False,
         debug_shell=False,
         debug_workspace=False,
+        default_config=None,
         directory=Path(path) if path is not None else None,
         doc_format=DocFormat.auto,
         force=9001,