gh-91078: Return None from TarFile.next when the tarfile is empty (GH-91850)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Author: dignissimus

Lib/tarfile.py

@@ -2339,6 +2339,8 @@ def next(self):
 
         # Advance the file pointer.
         if self.offset != self.fileobj.tell():
+            if self.offset == 0:
+                return None
             self.fileobj.seek(self.offset - 1)
             if not self.fileobj.read(1):
                 raise ReadError("unexpected end of data")

Lib/test/test_tarfile.py

@@ -734,6 +734,18 @@ def test_zlib_error_does_not_leak(self):
             with self.assertRaises(tarfile.ReadError):
                 tarfile.open(self.tarname)
 
+    def test_next_on_empty_tarfile(self):
+        fd = io.BytesIO()
+        tf = tarfile.open(fileobj=fd, mode="w")
+        tf.close()
+
+        fd.seek(0)
+        with tarfile.open(fileobj=fd, mode="r|") as tf:
+            self.assertEqual(tf.next(), None)
+
+        fd.seek(0)
+        with tarfile.open(fileobj=fd, mode="r") as tf:
+            self.assertEqual(tf.next(), None)
 
 class MiscReadTest(MiscReadTestBase, unittest.TestCase):
     test_fail_comp = None

Misc/NEWS.d/next/Library/2022-04-23-03-46-37.gh-issue-91078.87-hkp.rst

@@ -0,0 +1 @@
+:meth:`TarFile.next` now returns ``None`` when called on an empty tarfile.