bug#46933: Possible bugs in filepos-to-bufferpos / bufferpos-to-filepos

[Gregory Heytings <gregory@heytings.org>]

>> Kenichi, why are these 6 bytes inserted by encode-coding-region, but 
>> not when we encode the same text as part of saving the buffer to its 
>> file?  And why does it happen near the end of the text, between those 2 
>> particular letters?
>
> There surely exists a bug.  Could you please try the attached patch?
>
> The reason why that bug did not happen on file writing is that the code 
> in write_region calls encoding routine repeatedly without 
> CODING_MODE_LAST_BLOCK flag, and only in the case that flushing is 
> required (e.g. the case of iso-2022-jp), just for flushing, it calls 
> enoding routine again with CODING_MODE_LAST_BLOCK flag.  In that case, 
> carryover does not happen in encode_coding ().
>

Thank you.  I tried the patch, and it seems to fix the 
bufferpos-to-filepos bug, but not the filepos-to-bufferpos one.  On the 
example file,

(bufferpos-to-filepos (- (point-max) POS) 'exact)

gives the expected results:

POS = 0: 2997
POS = 1: 2995
POS = 2: 2993
POS = 3 (IDEOGRAPHIC FULL STOP): 2991
POS = 4 (HIRAGANA LETTER RU): 2989

But (goto-char (filepos-to-bufferpos POS 'exact)) gives:

POS = 2985, 2986: last but one visible character (HIRAGANA LETTER RU)
POS = 2987, 2988: last visible character (IDEOGRAPHIC FULL STOP)
POS = 2989, 2990: first CRLF
POS = 2991: second CRLF
POS = 2992: point-max
POS = 2993: first CRLF
POS = 2994, 2995: second CRLF
POS >= 2996: point-max

where I would have expected:

POS = 2989, 2990: last but one visible character (HIRAGANA LETTER RU)
POS = 2991, 2992: last visible character (IDEOGRAPHIC FULL STOP)
POS = 2993, 2994: first CRLF
POS = 2995, 2996: second CRLF
POS >= 2997: point-max