bug#870: Repeatable instance of bug#870

[Jason Rumney <jasonr@gnu.org>]

Juanma Barranquero wrote:
>    emacs -Q --eval "(desktop-save-mode 1)" ChangeLog.870
>   

I can also reproduce the bug with C-x RET r utf-8-dos after visiting the 
file normally.

It appears that there is a bug in all the decode_coding_* functions when 
a CR lies on a CHARBUF_SIZE (0x4000) boundary with a matching LF on the 
other side of the boundary.

They all do something like:

      if (eol_crlf && c1 == '\r')
        ONE_MORE_BYTE (byte_after_cr);

but ONE_MORE_BYTE will abort the decode if it reaches the end of the 
buffer, leaving the CR in limbo between having been read and being added 
to the buffer. Then on decoding the subsequent block, the initial LF 
does not trip the normal CRLF decoding, so it is put into the buffer.