Re: BUG #17912: Invalid memory access when converting plpython' array containing empty array

Hello Tom,

28.04.2023 18:14, Tom Lane wrote:
> PG Bug reporting form <noreply(at)postgresql(dot)org> writes:
>> CREATE EXTENSION plpython3u;
>> CREATE OR REPLACE FUNCTION test() RETURNS text[] AS $$
>> return [[], "a"]
>> $$ LANGUAGE plpython3u;
>> SELECT test();
>> As I can see, for the first case we get len = 0 in PLySequence_ToArray();
>> elems, nulls palloc'ed with zero elements, but PLyObject_ToScalar() tries
>> to write a value into nulls[0]...
> Yeah. The calculation of the array size is being done in the wrong
> place, so that we may update len to zero before breaking out of the
> loop. But really it's poor coding for this function to be doing
> its own calculation of the array size at all, rather than consulting
> the authoritative ArrayGetNItems(). I think we need something like
> the attached.

Thank you! I've read the message of the commit you just pushed,
and I confirm that there are another oddities in that area. For example:
CREATE OR REPLACE FUNCTION test() RETURNS text[] AS $$
return [1, [2]]
$$ LANGUAGE plpython3u;
SELECT test();
---------
 {1,[2]}

But:
CREATE OR REPLACE FUNCTION test() RETURNS text[] AS $$
return [[1], 2]
$$ LANGUAGE plpython3u;
SELECT test();
ERROR:  wrong length of inner sequence: has length -1, but 1 was expected
DETAIL:  To construct a multidimensional array, the inner sequences must all have the same length.
CONTEXT:  while creating return value
PL/Python function "test"

Or:
CREATE OR REPLACE FUNCTION test() RETURNS text[] AS $$
return [["1"], "abc"]
$$ LANGUAGE plpython3u;
SELECT test();
ERROR:  wrong length of inner sequence: has length 3, but 1 was expected
DETAIL:  To construct a multidimensional array, the inner sequences must all have the same length.
CONTEXT:  while creating return value
PL/Python function "test"

Though may be it's okay, considering python's concepts of
strings/arrays/sequences, or at least deserves a separate discussion.

Best regards,
Alexander