gh-129255: tarfile: Update tarfile.offset with the remainder on close

Lib/tarfile.py

@@ -2088,6 +2088,7 @@ def close(self):
                 blocks, remainder = divmod(self.offset, RECORDSIZE)
                 if remainder > 0:
                     self.fileobj.write(NUL * (RECORDSIZE - remainder))
+                    self.offset += (RECORDSIZE - remainder)
         finally:
             if not self._extfileobj:
                 self.fileobj.close()

Lib/test/test_tarfile.py

@@ -1358,6 +1358,17 @@ def test_eof_marker(self):
         with self.open(tmpname, "rb") as fobj:
             self.assertEqual(len(fobj.read()), tarfile.RECORDSIZE * 2)
 
+    def test_offset_on_close(self):
+        # Check the offset after calling close matches the total number of
+        # bytes written.
+        tar = tarfile.open(tmpname, self.mode)
+        t = tarfile.TarInfo("foo")
+        tar.addfile(t)
+        tar.close()
+
+        with self.open(tmpname, "rb") as fobj:
+            self.assertEqual(len(fobj.read()), tar.offset)
+
 
 class WriteTest(WriteTestBase, unittest.TestCase):
 

Misc/NEWS.d/next/Library/2025-05-12-09-59-49.gh-issue-129255._bZvNr.rst

@@ -0,0 +1 @@
+Update the ``offset`` with the remainder when closing a :func:`tarfile.TarFile`.