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/

test/test_util_ostree.py

@@ -0,0 +1,92 @@
+import json
+import unittest
+import subprocess
+
+from osbuild.util import ostree
+
+
+def run(*args, check=True, encoding="utf-8", **kwargs):
+    res = subprocess.run(*args,
+                         encoding=encoding,
+                         check=check,
+                         **kwargs)
+    return res
+
+
+def have_rpm_ostree():
+    try:
+        r = run(["rpm-ostree", "--version"],
+                capture_output=True, check=False)
+    except FileNotFoundError:
+        return False
+    return r.returncode == 0 and "compose" in r.stdout
+
+
+class TestObjectStore(unittest.TestCase):
+
+    @unittest.skipIf(not have_rpm_ostree(), "rpm-ostree missing")
+    def test_treefile_empty(self):
+        # check we produce a valid treefile from an empty object
+        tf = ostree.Treefile()
+
+        with tf.as_tmp_file() as f:
+            run(["rpm-ostree", "compose", "tree", "--print-only", f])
+
+    def test_treefile_types(self):
+        tf = ostree.Treefile()
+
+        tf["repos"] = ["a", "b", "c"]    # valid list of strings
+        tf["selinux"] = True             # valid boolean
+        tf["ref"] = "ref/sample/tip"     # valid string
+
+        with self.assertRaises(ValueError):
+            tf["repos"] = "not a list"   # not a list
+
+        with self.assertRaises(ValueError):
+            tf["repos"] = [1, 2, 3]       # not a string list
+
+        with self.assertRaises(ValueError):
+            tf["selinux"] = "not a bool"  # not a boolean
+
+    def test_treefile_dump(self):
+        tf = ostree.Treefile()
+        test_ref = "a/sample/ref"
+        tf["ref"] = test_ref
+
+        with tf.as_tmp_file() as path:
+            with open(path, "r") as f:
+                js = json.load(f)
+                self.assertEqual(js["ref"], test_ref)
+                self.assertEqual(tf["ref"], test_ref)
+
+    @unittest.skipIf(not have_rpm_ostree(), "rpm-ostree missing")
+    def test_treefile_full(self):
+        params = {
+            "ref": "osbuild/ostree/devel",
+            "repos": ["fedora", "osbuild"],
+            "selinux": True,
+            "boot-location": "new",
+            "etc-group-members": ["wheel"],
+            "machineid-compat": True
+        }
+
+        tf = ostree.Treefile()
+        for p, v in params.items():
+            tf[p] = v
+
+        with tf.as_tmp_file() as path:
+            r = run(["rpm-ostree",
+                     "compose",
+                     "tree",
+                     "--print-only",
+                     path],
+                    capture_output=True)
+            self.assertEqual(r.returncode, 0)
+            js = json.loads(r.stdout)
+
+        for p, v in params.items():
+            self.assertEqual(v, js[p])
+
+
+if __name__ == "__main__":
+    unittest.main()