bpo-33578: Add getstate/setstate for CJK codec (GH-6984)

This implements getstate and setstate for the cjkcodecs multibyte incremental encoders/decoders, primarily to fix issues with seek/tell.

The encoder getstate/setstate is slightly tricky as the "state" is pending bytes + MultibyteCodec_State but only an integer can be returned. The approach I've taken is to encode this data into a long, similar to how .tell() encodes a "cookie_type" as a long.


https://bugs.python.org/issue33578

Lib/test/test_io.py

@@ -2971,6 +2971,34 @@ class TextIOWrapperTest(unittest.TestCase):
         finally:
             StatefulIncrementalDecoder.codecEnabled = 0
 
+    def test_multibyte_seek_and_tell(self):
+        f = self.open(support.TESTFN, "w", encoding="euc_jp")
+        f.write("AB\n\u3046\u3048\n")
+        f.close()
+
+        f = self.open(support.TESTFN, "r", encoding="euc_jp")
+        self.assertEqual(f.readline(), "AB\n")
+        p0 = f.tell()
+        self.assertEqual(f.readline(), "\u3046\u3048\n")
+        p1 = f.tell()
+        f.seek(p0)
+        self.assertEqual(f.readline(), "\u3046\u3048\n")
+        self.assertEqual(f.tell(), p1)
+        f.close()
+
+    def test_seek_with_encoder_state(self):
+        f = self.open(support.TESTFN, "w", encoding="euc_jis_2004")
+        f.write("\u00e6\u0300")
+        p0 = f.tell()
+        f.write("\u00e6")
+        f.seek(p0)
+        f.write("\u0300")
+        f.close()
+
+        f = self.open(support.TESTFN, "r", encoding="euc_jis_2004")
+        self.assertEqual(f.readline(), "\u00e6\u0300\u0300")
+        f.close()
+
     def test_encoded_writes(self):
         data = "1234567890"
         tests = ("utf-16",