bpo-40105: ZipFile truncate in append mode with shorter comment (GH-19337)

Lib/test/test_zipfile.py

@@ -1856,11 +1856,14 @@ class OtherTests(unittest.TestCase):
             self.assertEqual(zipf.comment, b"an updated comment")
 
         # check that comments are correctly shortened in append mode
+        # and the file is indeed truncated
         with zipfile.ZipFile(TESTFN,mode="w") as zipf:
             zipf.comment = b"original comment that's longer"
             zipf.writestr("foo.txt", "O, for a Muse of Fire!")
+        original_zip_size = os.path.getsize(TESTFN)
         with zipfile.ZipFile(TESTFN,mode="a") as zipf:
             zipf.comment = b"shorter comment"
+        self.assertTrue(original_zip_size > os.path.getsize(TESTFN))
         with zipfile.ZipFile(TESTFN,mode="r") as zipf:
             self.assertEqual(zipf.comment, b"shorter comment")
 

Lib/zipfile.py

@@ -1918,6 +1918,8 @@ class ZipFile:
                              centDirSize, centDirOffset, len(self._comment))
         self.fp.write(endrec)
         self.fp.write(self._comment)
+        if self.mode == "a":
+            self.fp.truncate()
         self.fp.flush()
 
     def _fpclose(self, fp):

Misc/NEWS.d/next/Library/2020-04-03-16-13-59.bpo-40105.hfM2c0.rst

@@ -0,0 +1,2 @@
+ZipFile truncates files to avoid corruption when a shorter comment is provided
+in append ("a") mode. Patch by Jan Mazur.