BUG #15869: Custom aggregation returns null when parallelized

The following bug has been logged on the website:

Bug reference: 15869
Logged by: Kassym Dorsel
Email address: kdorsel(at)gmail(dot)com
PostgreSQL version: 11.4
Operating system: Windows 10 / Ubuntu 18
Description:

I have a custom aggregate setup. It run and returns the correct results when
run sequentially. Once I set parallel = safe it returns null values. I have
tested and gotten the same results on Windows 10 / postgres 11.4 and Ubuntu
18 / postgres 11.3.

Here's the aggregate:
CREATE OR REPLACE FUNCTION array_sort(ANYARRAY)
RETURNS ANYARRAY LANGUAGE SQL
AS $$
SELECT ARRAY(SELECT unnest($1) ORDER BY 1)
$$;

create type _stats_agg_accum_type AS (
cnt bigint,
q double precision[],
n double precision[],
np double precision[],
dn double precision[]
);

create type _stats_agg_result_type AS (
count bigint,
q25 double precision,
q50 double precision,
q75 double precision
);

create or replace function _stats_agg_p2_parabolic(_stats_agg_accum_type,
double precision, double precision)
returns double precision AS '
DECLARE
a alias for $1;
i alias for $2;
d alias for $3;
BEGIN
RETURN a.q[i] + d / (a.n[i + 1] - a.n[i - 1]) * ((a.n[i] - a.n[i - 1] +
d) * (a.q[i + 1] - a.q[i]) / (a.n[i + 1] - a.n[i]) + (a.n[i + 1] - a.n[i] -
d) * (a.q[i] - a.q[i - 1]) / (a.n[i] - a.n[i - 1]));
END;
'
language plpgsql;

create or replace function _stats_agg_p2_linear(_stats_agg_accum_type,
double precision, double precision)
returns double precision AS '
DECLARE
a alias for $1;
i alias for $2;
d alias for $3;
BEGIN
return a.q[i] + d * (a.q[i + d] - a.q[i]) / (a.n[i + d] - a.n[i]);
END;
'
language plpgsql;

create or replace function _stats_agg_accumulator(_stats_agg_accum_type,
double precision)
returns _stats_agg_accum_type AS '
DECLARE
a ALIAS FOR $1;
x alias for $2;
k int;
d double precision;
qp double precision;
BEGIN
a.cnt = a.cnt + 1;

if a.cnt <= 5 then
a.q = array_append(a.q, x);
if a.cnt = 5 then
a.q = array_sort(a.q);
end if;
return a;
end if;

case
when x < a.q[1] then
a.q[1] = x;
k = 1;
when x >= a.q[1] and x < a.q[2] then
k = 1;
when x >= a.q[2] and x < a.q[3] then
k = 2;
when x >= a.q[3] and x < a.q[4] then
k = 3;
when x >= a.q[4] and x <= a.q[5] then
k = 4;
when x > a.q[5] then
a.q[5] = x;
k = 4;
end case;

for ii in 1..5 loop
if ii > k then
a.n[ii] = a.n[ii] + 1;
end if;
a.np[ii] = a.np[ii] + a.dn[ii];
end loop;

for ii in 2..4 loop
d = a.np[ii] - a.n[ii];
if (d >= 1 and a.n[ii+1] - a.n[ii] > 1) or (d <= -1 and a.n[ii-1] -
a.n[ii] < -1) then
d = sign(d);
qp = _stats_agg_p2_parabolic(a, ii, d);
if qp > a.q[ii-1] and qp < a.q[ii+1] then
a.q[ii] = qp;
else
a.q[ii] = _stats_agg_p2_linear(a, ii, d);
end if;
a.n[ii] = a.n[ii] + d;
end if;
end loop;

return a;
END;
'
language plpgsql;

create or replace function _stats_agg_combiner(_stats_agg_accum_type,
_stats_agg_accum_type)
returns _stats_agg_accum_type AS '
DECLARE
a alias for $1;
b alias for $2;
c _stats_agg_accum_type;
BEGIN
c.cnt = a.cnt + b.cnt;
c.q[2] = (a.q[2] + b.q[2]) / 2;
c.q[3] = (a.q[3] + b.q[3]) / 2;
c.q[4] = (a.q[4] + b.q[4]) / 2;
RETURN c;
END;
'
strict language plpgsql;

create or replace function _stats_agg_finalizer(_stats_agg_accum_type)
returns _stats_agg_result_type AS '
BEGIN
RETURN row(
$1.cnt,
$1.q[2],
$1.q[3],
$1.q[4]
);
END;
'
language plpgsql;

create aggregate stats_agg(double precision) (
sfunc = _stats_agg_accumulator,
stype = _stats_agg_accum_type,
finalfunc = _stats_agg_finalizer,
combinefunc = _stats_agg_combiner,
--parallel = safe,
initcond = '(0, {}, "{1,2,3,4,5}", "{1,2,3,4,5}",
"{0,0.25,0.5,0.75,1}")'
);

Here's the setup code:
--CREATE TABLE temp (val double precision);
--insert into temp (val) select i from generate_series(0, 150000) as t(i);
--set force_parallel_mode = on;
select (stats_agg(val)).* from temp;

Expected results:
150001, 37500, 75000, 112500

Results when run in parallel:
150001, null, null, null