Two comments:

 * I like the new *-getarg parameters, but it would be better to test
   for '#' first since this is still by far the most used getarg
   parameter.

 * Antoine, I think your codecs.c patch has a glitch:

+    decoded = PyUnicode_DecodeMBCSStateful(pbuf.buf, pbuf.len, errors,
+					   final ? NULL : &consumed);
+	PyBuffer_Release(&pbuf);
+    if (decoded == NULL)
 	return NULL;
-    return codec_tuple(decoded, final ? size : consumed);
+    return codec_tuple(decoded, consumed);
 }
 
   You dropped the "final ? size : " for no apparent reason.