BUG #19487: Error while executing SQL query involving XML parsing

The following bug has been logged on the website:

Bug reference: 19487
Logged by: Ilya Portnov
Email address: i(dot)portnov(at)compassplus(dot)com
PostgreSQL version: 18.3
Operating system: Ubuntu 24.04 LTS
Description:

Hello.

While testing our software product for compatibility with PostgreSQL 18, we
found a problem which appears when executing particular query. Initial query
we
were executing was quite complex and involved our custom SQL functions; we
tried to simplify it as far as we could, but it's still not very simple.

Environment which we used to reproduce this:

OS: Ubuntu 24.04 LTS
Architecture: x86_64
PostgreSQL version() output:
PostgreSQL 18.3 (Ubuntu 18.3-1.pgdg22.04+1) on x86_64-pc-linux-gnu,
compiled by gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0, 64-bit

We also tried on several different Linux distributions and PostgreSQL
builds, the problem still reproduces.

So, steps to reproduce are:

1. Create function:

create or replace function xml_to_text(pXml xml) returns text
as $$
select
case when pXml is document
then (xpath('/*/text()', pXml))[1] ::text
else pXml::text
end;
$$ language sql immutable;

2. Execute query:

select xml_to_text( (xpath('ns:rq/@Day', case
when tbl.strcolumn != '' then XMLParse(document tbl.strcolumn)
when tbl.clobcolumn != '' then XMLParse(DOCUMENT
replace(replace(replace(replace(replace(tbl.clobcolumn, '',
'?'),'', '?'),'', '?'),'', '?'), '', '?'))
else XMLParse(DOCUMENT '<rq></rq>')
end,
array[array['ns', 'http://example.com/schema.xsd']]
))[1] )
from
(
(select '<ns:rq xmlns:ns="http://example.com/schema.xsd"
Day="2019-12-16T00:00:00.000"/>' as strcolumn, null as clobcolumn)
union all
(select '<ns:rq xmlns:ns="http://example.com/schema.xsd"
Day="2019-12-16T00:00:00.000"></ns:rq>' as strcolumn, null as clobcolumn)
) tbl;

Expected result, which is what we get on PostgreSQL 17.9:

xml_to_text |
-----------------------+
2019-12-16T00:00:00.000|
2019-12-16T00:00:00.000|

Actual result on PostgreSQL 18.3:

SQL Error [2200M]: ERROR: could not parse XML document
Detail: line 1: Start tag expected, '<' not found
2019-12-16T00:00:00.000
^
Where: SQL function "xpath" statement 1
SQL function "xml_to_text" statement 1

It appears that all parts of problematic query are important for this
problem:
if I try to simplify it, for example if I try to remove one of five nested
replace() calls, the problem goes away.

This looks like a problem which was introduced somewhere between PostgreSQL
17 and 18. Any help in fixing it will be appreciated.

Best regards,
Ilya V. Portnov.