bpo-39039: tarfile raises descriptive exception from zlib.error (GH-27766)

* during tarfile parsing, a zlib error indicates invalid data
* tarfile.open now raises a descriptive exception from the zlib error
* this makes it clear to the user that they may be trying to open a
  corrupted tar file

Lib/test/test_tarfile.py

@@ -19,6 +19,10 @@ try:
     import gzip
 except ImportError:
     gzip = None
+try:
+    import zlib
+except ImportError:
+    zlib = None
 try:
     import bz2
 except ImportError:
@@ -687,6 +691,16 @@ class MiscReadTestBase(CommonReadTest):
                 self.assertEqual(m1.offset, m2.offset)
                 self.assertEqual(m1.get_info(), m2.get_info())
 
+    @unittest.skipIf(zlib is None, "requires zlib")
+    def test_zlib_error_does_not_leak(self):
+        # bpo-39039: tarfile.open allowed zlib exceptions to bubble up when
+        # parsing certain types of invalid data
+        with unittest.mock.patch("tarfile.TarInfo.fromtarfile") as mock:
+            mock.side_effect = zlib.error
+            with self.assertRaises(tarfile.ReadError):
+                tarfile.open(self.tarname)
+
+
 class MiscReadTest(MiscReadTestBase, unittest.TestCase):
     test_fail_comp = None