Re: quoting psql varible as identifier

On Mon, Jan 18, 2010 at 1:52 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
> 2010/1/18 Robert Haas <robertmhaas(at)gmail(dot)com>:
>> On Sun, Jan 17, 2010 at 2:04 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>>> I rewrote patch so now interface for PQescapeIdentConn is same as
>>> PQescapeStringConn
>>>
>>> @3. I though so the protection under incomplete multibyte chars are
>>> enought - missing bytes are replaced by space - like
>>> PQescapeStringConn does.
>>
>> That much is fine, but the output buffer is only guaranteed to be of
>> size 2n+1.  Imagine the input is two double-quotes followed by a byte
>> for which pg_encoding_mblen() returns 4.  The input is 3 characters
>> long so the user was responsible to provide 7 bytes of output space,
>> but you'll try to write 9 bytes to it (including the terminating NUL).
>>
> I don't understand. The "length" is number of bytes, not number of
> chars. It is maybe bad documented only. If your input string has 6
> bytes, then buffer have to allocated to 13 bytes. Nobody knows how
> much is chars there.

Right, but the point is we can't assume that the input is validly
encoded. If the input ends with a garbage character that looks like
the start of a multi-byte character, we can't assume that there's
enough space in the output buffer to store the required number of
padding spaces.

To take an extreme example, suppose there were an encoding where any
time the first byte of a multi-byte character has the high-bit set,
the character is 100 bytes long. Then suppose someone call
PQescapeStringConn(), or this new function we're adding, with a length
argument of 1, and the first byte of the input buffer has the high-bit
set. The caller is only required to provide a 3-byte output buffer,
and the third byte is needed for the terminating NUL. That means that
after we copy that first character we only have room to insert one
padding space. The way you had it coded, since we were expecting a
character 100 bytes long, we'd always try to insert 99 padding spaces.

>> Let me take a crack at this and post a patch.  We're making this
>> harder than it needs to be.
>
> sure, please.

Working on it...

...Robert