Different error decoding Shift-JIS sequence in JDK8

[Charles Oliver Nutter]

Apologies if this is not the correct place to post this, bit i18n
seemed more focused on languages and localization than the mechanics
of transcoding.

I have noticed a behavioral difference in JDK8 decoding a two-byte
Shift-JIS sequence. Specifically, JDK8 appears to report malformed
input for what should be a valid Shift-JIS sequence, where JDK7
reported that the character was unmappable.

The code to reproduce is fairly simple:

byte[] bytes = {(byte)0xEF, 0x40};
CharsetDecoder decoder = Charset.forName("Shift-JIS").newDecoder();
System.out.println(decoder.decode(ByteBuffer.wrap(bytes),
CharBuffer.allocate(2), false));

Note that this is pumping the decoder directly and specifying partial
input (false). We use this mechanism in JRuby for transcoding
arbitrary byte[] from one encoding to another.

The result of running this on JDK7 is "UNMAPPABLE[2]" while the result
on JDK8 is "MALFORMED[1]".

Information online is spotty as to whether this sequence is valid. It
does appear on the table for [JIS X
203](http://x0213.org/codetable/sjis-0213-2004-std.txt) and several
articles on Shift-JIS claim that it is at worst undefined and at best
valid. So I'm leaning toward this being a bug in JDK8's Shift-JIS
decoder.

Note that on JDK7 it is "unmappable", which may mean this code
represents a character with no equivalent in Unicode.

I have uploaded my code to github here:
https://github.com/headius/jdk8_utf8_decoding_bug

Thoughts?

- Charlie