Support reproducible XZ Linux archives without changing runtime contents

tools/test_package_linux.py

@@ -99,6 +99,22 @@ class LinuxPackageTests(unittest.TestCase):
                 self.run_package(fixture)
             self.assertEqual(previous, fixture[-1].read_bytes())
 
+    def test_xz_preserves_members_checksums_modes_and_reproducibility(self):
+        with tempfile.TemporaryDirectory() as directory:
+            fixture = self.fixture(directory)
+            self.run_package(fixture)
+            with tarfile.open(fixture[-1]) as archive:
+                original = {m.name: (archive.extractfile(m).read(), m.mode) for m in archive}
+            with mock.patch.object(pack, "dynamic", side_effect=self.metadata):
+                pack.package(*fixture, compression="xz")
+                first = fixture[-1].read_bytes()
+                pack.package(*fixture, compression="xz")
+            self.assertEqual(first, fixture[-1].read_bytes())
+            self.assertTrue(first.startswith(b"\xfd7zXZ\x00"))
+            with tarfile.open(fixture[-1]) as archive:
+                compressed = {m.name: (archive.extractfile(m).read(), m.mode) for m in archive}
+            self.assertEqual(original, compressed)
+
     def test_media_helper_tamper_is_rejected_against_pinned_archive(self):
         with tempfile.TemporaryDirectory() as directory:
             fixture = self.fixture(directory)