gh-127847: Fix position in the special-cased zipfile seek (#127856)

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>

Lib/test/test_zipfile/test_core.py

@@ -2333,6 +2333,18 @@ def test_read_after_seek(self):
                 fp.seek(1, os.SEEK_CUR)
                 self.assertEqual(fp.read(-1), b'men!')
 
+    def test_uncompressed_interleaved_seek_read(self):
+        # gh-127847: Make sure the position in the archive is correct
+        # in the special case of seeking in a ZIP_STORED entry.
+        with zipfile.ZipFile(TESTFN, "w") as zipf:
+            zipf.writestr("a.txt", "123")
+            zipf.writestr("b.txt", "456")
+        with zipfile.ZipFile(TESTFN, "r") as zipf:
+            with zipf.open("a.txt", "r") as a, zipf.open("b.txt", "r") as b:
+                self.assertEqual(a.read(1), b"1")
+                self.assertEqual(b.seek(1), 1)
+                self.assertEqual(b.read(1), b"5")
+
     @requires_bz2()
     def test_decompress_without_3rd_party_library(self):
         data = b'PK\x05\x06\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'

Lib/zipfile/__init__.py

@@ -819,6 +819,9 @@ def seek(self, offset, whence=0):
                 raise ValueError("Can't reposition in the ZIP file while "
                         "there is an open writing handle on it. "
                         "Close the writing handle before trying to read.")
+            if whence == os.SEEK_CUR:
+                self._file.seek(self._pos + offset)
+            else:
                 self._file.seek(offset, whence)
             self._pos = self._file.tell()
             return self._pos

Misc/NEWS.d/next/Library/2024-12-12-07-27-51.gh-issue-127847.ksfNKM.rst

@@ -0,0 +1 @@
+Fix the position when doing interleaved seeks and reads in uncompressed, unencrypted zip files returned by :meth:`zipfile.ZipFile.open`.