util: new ostree module with TreeFile helper

When using rpm-ostree compose, a Treefile[1] controls various
aspects of its behaviour. Since rpm-ostree will, at least in
the beginning, be used to post-process and committing the tree
add a helper class to ease the creation of correct Treefiles.
The docstring of the Treefile contains the information in which
phases ('install', 'postprocess', 'commit') the option is used,
as of rpm-ostree commit 1cf0d557ae8059e689b1fed670022727e9842288

Add basic checks for the ostree.Treefile helper. Some of the
tests require rpm-ostree to be installed.

[1] https://rpm-ostree.readthedocs.io/en/stable/manual/treefile/

osbuild/util/ostree.py

@@ -0,0 +1,104 @@
+import contextlib
+import json
+import os
+import tempfile
+
+from typing import List
+
+
+class Param:
+    """rpm-ostree Treefile parameter"""
+
+    def __init__(self, value_type, mandatory=False):
+        self.type = value_type
+        self.mandatory = mandatory
+
+    def check(self, value):
+        origin = getattr(self.type, "__origin__", None)
+        if origin:
+            self.typecheck(value, origin)
+            if origin is list:
+                self.check_list(value, self.type)
+            else:
+                raise NotImplementedError(origin)
+        else:
+            self.typecheck(value, self.type)
+
+    @staticmethod
+    def check_list(value, tp):
+        inner = tp.__args__
+        for x in value:
+            Param.typecheck(x, inner)
+
+    @staticmethod
+    def typecheck(value, tp):
+        if isinstance(value, tp):
+            return
+        raise ValueError(f"{value} is not of {tp}")
+
+
+class Treefile:
+    """Representation of an rpm-ostree Treefile
+
+    The following parameters are currently supported,
+    presented together with the rpm-ostree compose
+    phase that they are used in.
+      - ref: commit
+      - repos: install
+      - selinux: install, postprocess, commit
+      - boot-location: postprocess
+      - etc-group-members: postprocess
+      - machineid-compat
+
+    NB: 'ref' and 'repos' are mandatory and must be
+    present, even if they are not used in the given
+    phase; they therefore have defaults preset.
+    """
+
+    parameters = {
+        "ref": Param(str, True),
+        "repos": Param(List[str], True),
+        "selinux": Param(bool),
+        "boot-location": Param(str),
+        "etc-group-members": Param(List[str]),
+        "machineid-compat": Param(bool)
+    }
+
+    def __init__(self):
+        self._data = {}
+        self["ref"] = "osbuild/devel"
+        self["repos"] = ["osbuild"]
+
+    def __getitem__(self, key):
+        param = self.parameters.get(key)
+        if not param:
+            raise ValueError(f"Unknown param: {key}")
+        return self._data[key]
+
+    def __setitem__(self, key, value):
+        param = self.parameters.get(key)
+        if not param:
+            raise ValueError(f"Unknown param: {key}")
+        param.check(value)
+        self._data[key] = value
+
+    def dumps(self):
+        return json.dumps(self._data)
+
+    def dump(self, fp):
+        return json.dump(self._data, fp)
+
+    @contextlib.contextmanager
+    def as_tmp_file(self):
+        name = None
+        try:
+            fd, name = tempfile.mkstemp(suffix=".json",
+                                        text=True)
+
+            with os.fdopen(fd, "w+") as f:
+                self.dump(f)
+
+            yield name
+        finally:
+            if name:
+                os.unlink(name)