gh-127847: Fix position in the special-cased zipfile seek

Author: dimaryaz

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):
+        # Issue 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:
+                a.read(1)
+                b.seek(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

@@ -1167,7 +1167,7 @@ def seek(self, offset, whence=os.SEEK_SET):
             self._expected_crc = None
             # seek actual file taking already buffered data into account
             read_offset -= len(self._readbuffer) - self._offset
-            self._fileobj.seek(read_offset, os.SEEK_CUR)
+            self._fileobj.seek(self._orig_compress_start + read_offset)
             self._left -= read_offset
             read_offset = 0
             # flush read buffer