bpo-38334: Fix seeking backward on an encrypted zipfile.ZipExtFile. (GH-16937)

Test by Daniel Hillier.

Lib/test/test_zipfile.py

@@ -1934,6 +1934,44 @@ class DecryptionTests(unittest.TestCase):
         self.assertRaises(TypeError, self.zip.open, "test.txt", pwd="python")
         self.assertRaises(TypeError, self.zip.extract, "test.txt", pwd="python")
 
+    def test_seek_tell(self):
+        self.zip.setpassword(b"python")
+        txt = self.plain
+        test_word = b'encryption'
+        bloc = txt.find(test_word)
+        bloc_len = len(test_word)
+        with self.zip.open("test.txt", "r") as fp:
+            fp.seek(bloc, os.SEEK_SET)
+            self.assertEqual(fp.tell(), bloc)
+            fp.seek(-bloc, os.SEEK_CUR)
+            self.assertEqual(fp.tell(), 0)
+            fp.seek(bloc, os.SEEK_CUR)
+            self.assertEqual(fp.tell(), bloc)
+            self.assertEqual(fp.read(bloc_len), txt[bloc:bloc+bloc_len])
+
+            # Make sure that the second read after seeking back beyond
+            # _readbuffer returns the same content (ie. rewind to the start of
+            # the file to read forward to the required position).
+            old_read_size = fp.MIN_READ_SIZE
+            fp.MIN_READ_SIZE = 1
+            fp._readbuffer = b''
+            fp._offset = 0
+            fp.seek(0, os.SEEK_SET)
+            self.assertEqual(fp.tell(), 0)
+            fp.seek(bloc, os.SEEK_CUR)
+            self.assertEqual(fp.read(bloc_len), txt[bloc:bloc+bloc_len])
+            fp.MIN_READ_SIZE = old_read_size
+
+            fp.seek(0, os.SEEK_END)
+            self.assertEqual(fp.tell(), len(txt))
+            fp.seek(0, os.SEEK_SET)
+            self.assertEqual(fp.tell(), 0)
+
+            # Read the file completely to definitely call any eof integrity
+            # checks (crc) and make sure they still pass.
+            fp.read()
+
+
 class AbstractTestsWithRandomBinaryFiles:
     @classmethod
     def setUpClass(cls):