Thread: [Patch] Fix bounds check in trim_array()

Hi,

while working on something else i encountered a bug in the trim_array() 
function. The bounds check fails for empty arrays without any 
dimensions. It reads the size of the non existing first dimension to 
determine the arrays length.

   select trim_array('{}'::int[], 10);
   ------------
    {}

   select trim_array('{}'::int[], 100);
   ERROR:  number of elements to trim must be between 0 and 64

The attached patch fixes that check.

Martin

On Mon, Jul 25, 2022 at 04:40:51PM +0200, Martin Kalcher wrote:
> +SELECT trim_array(ARRAY[]::int[], 1); -- fail
> +ERROR:  number of elements to trim must be between 0 and 0

Can we improve the error message?  Maybe it should look something like

    ERROR:  number of elements to trim must be 0

for this case.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

Nathan Bossart <nathandbossart@gmail.com> writes:
> On Mon, Jul 25, 2022 at 04:40:51PM +0200, Martin Kalcher wrote:
>> +SELECT trim_array(ARRAY[]::int[], 1); -- fail
>> +ERROR:  number of elements to trim must be between 0 and 0

> Can we improve the error message?  Maybe it should look something like
>     ERROR:  number of elements to trim must be 0
> for this case.

Hmm, I'm unexcited about making our long-suffering translators
deal with another translatable string for such a corner case.
I think it's fine as-is.

A bigger problem is that a little further down, there's an equally
unprotected reference to ARR_LBOUND(v)[0].  Now, the fact that that
expression computes garbage doesn't matter too much, because AFAICS
if the array is zero-D then array_get_slice is going to exit at

    if (ndim < nSubscripts || ndim <= 0 || ndim > MAXDIM)
        return PointerGetDatum(construct_empty_array(elemtype));

without ever examining its upperIndx[] argument.  However,
once we put in a test case covering this behavior, I bet that
valgrind-using buildfarm animals will start to bleat about the
invalid memory access.  I think the easiest fix is like

    if (ARR_NDIM(v) > 0)
    {
        upper[0] = ARR_LBOUND(v)[0] + array_length - n - 1;
        upperProvided[0] = true;
    }

It'd be good to get this fix into next week's minor releases,
so I'll go push it.

            regards, tom lane