util: tweak bls.options_append() support no/multiple options

The BLS specification [0] says the `options` field is optional and
can also appear multiple times. This commit tweaks the code to
deal with these corner cases and also adds tests that ensure that
this works correctly.

It also tweaks the file handling to be atomic.

[0] https://uapi-group.org/specifications/specs/boot_loader_specification/

test/mod/test_util_bls.py

@@ -0,0 +1,56 @@
+#
+# Test for the util/bls.py
+#
+
+import textwrap
+
+from osbuild.util import bls
+
+
+def make_fake_bls_file(rootdir, kernel_opts):
+    if kernel_opts:
+        options = f"options {kernel_opts}"
+    else:
+        options = ""
+    bls_path = rootdir / "loader/entries/test.conf"
+    bls_path.parent.mkdir(exist_ok=True, parents=True)
+    bls_path.write_text(textwrap.dedent(f"""\
+    title        Fedora 19 (Rawhide)
+    sort-key     fedora
+    machine-id   6a9857a393724b7a981ebb5b8495b9ea
+    version      3.8.0-2.fc19.x86_64
+    {options}
+    architecture x64
+    linux        /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/linux
+    initrd       /6a9857a393724b7a981ebb5b8495b9ea/3.8.0-2.fc19.x86_64/initrd
+    """), encoding="utf8")
+    return bls_path
+
+
+def test_options_append_one(tmp_path):
+    bls_path = make_fake_bls_file(tmp_path, "root=/dev/sda1 quiet")
+    bls.options_append(tmp_path, ["splash", "console=ttyS0"])
+    new_content = bls_path.read_text(encoding="utf8")
+    assert "\noptions root=/dev/sda1 quiet splash console=ttyS0\n" in new_content
+
+
+def test_options_append_none(tmp_path):
+    bls_path = make_fake_bls_file(tmp_path, "")
+    bls.options_append(tmp_path, ["splash", "console=ttyS0"])
+    new_content = bls_path.read_text(encoding="utf8")
+    assert "\noptions splash console=ttyS0\n" in new_content
+
+
+def test_options_append_multiple(tmp_path):
+    bls_path = make_fake_bls_file(tmp_path, "root=/dev/sda1")
+    with bls_path.open("a") as fp:
+        fp.write("options quiet")
+    bls.options_append(tmp_path, ["splash", "console=ttyS0"])
+    new_content = bls_path.read_text(encoding="utf8")
+    options = [line for line in new_content.split("\n")
+               if line.startswith("options ")]
+    # note that the new options only got added once
+    assert options == [
+        "options root=/dev/sda1 splash console=ttyS0",
+        "options quiet",
+    ]