BUG #15485: Order by of inlineable stable function incorrect

The following bug has been logged on the website:

Bug reference: 15485
Logged by: Tommas Factorylab
Email address: tommas(at)factorylab(dot)nl
PostgreSQL version: 9.6.10
Operating system: ubuntu 14.04 x64
Description:

Hi,

I am encountering a problem where the resultset is not correctly ordered
(and that results in incorrect behaviour of my application). The ordering is
done by a table function that is referenced in the "from clause" (function
"fA"). The query also left joins various other table functions. These table
functions do not perform any ordering.

I expect column "fA_c6" to be ordered ascending, but instead it has an
undefined order, or even a descending order.
What I have tried/discovered so far:
- The order is consistent over multiple runs
- The order is not ignored (as in, the order is different than a query
without any order specification would return)
- If I mark the functions as "volatile", the resultset is ordered
correctly
- If I remove certain columns from the "select clause", the resultset is
ordered correctly. I have not found any pattern, it is not like one specific
column causes the behaviour.
- If I only execute "fA", the resultset is ordered correctly

The way I understand the function volatility categories, my functions are
allowed to be "stable". From the documentation I understand that using table
functions in left joins like this is allowed. I am aware of the existence of
"row from", but am unable to pass arguments to the functions using that. If
using table functions in left joins like this is not allowed, I would expect
more defined behaviour.

The obfuscated SQL that reproduces the behaviour:
CREATE TYPE public.datatype AS ENUM (
'a',
'b',
'c'
);

CREATE TABLE public.tA (
c1 integer NOT NULL,
c2 integer,
c3 integer NOT NULL,
c4 integer NOT NULL,
c5 integer NOT NULL,
c6 integer NOT NULL,
c7 integer NOT NULL,
c8 double precision,
CONSTRAINT check1 CHECK ((((c7 = 1) AND (c8 IS NULL)) OR ((c7 > 1) AND
(c8 IS NOT NULL)))),
CONSTRAINT check2 CHECK ((c7 <> 0))
);

CREATE TABLE public.tB (
c1 integer NOT NULL,
c2 integer NOT NULL,
c3 character varying(200) NOT NULL,
c4 integer NOT NULL
);

CREATE TABLE public.tC (
c1 character varying(200) NOT NULL,
c2 text
);

CREATE TABLE public.tD (
c1 integer NOT NULL,
c2 character varying(200) NOT NULL,
c3 character varying(200) NOT NULL,
c4 character varying(200) NOT NULL
);

CREATE TABLE public.tE (
c1 character varying(200) NOT NULL,
c2 text
);

CREATE TABLE public.tF (
c1 character varying(200) NOT NULL,
c2 character varying(20) NOT NULL
);

CREATE TABLE public.tG (
c1 integer NOT NULL,
c2 character varying(200) NOT NULL,
c3 character varying(200) NOT NULL,
c4 text
);

CREATE TABLE public.tH (
c1 integer NOT NULL,
c2 character varying(200) NOT NULL,
c3 character varying(200) NOT NULL,
c4 character varying(200) NOT NULL,
c5 public.datatype NOT NULL,
c6 character varying(200) NOT NULL
);

CREATE OR REPLACE FUNCTION public.fA(p1 integer) RETURNS TABLE(c1 integer,
c2 integer, c3 integer, c4 integer, c5 integer, c6 integer, c7 integer, c8
double precision)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2, c3, c4, c5, c6, c7, c8
FROM tA
WHERE c3 = p1
ORDER BY c6 ASC
$$;

CREATE OR REPLACE FUNCTION public.fB(p1 integer) RETURNS TABLE(c1 integer,
c2 integer, c3 character varying, c4 integer)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2, c3, c4
FROM tB
WHERE c1 = p1
$$;

CREATE OR REPLACE FUNCTION public.fC(p1 character varying) RETURNS TABLE(c1
character varying, c2 text)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2
FROM tC
WHERE c1 = p1
$$;

CREATE OR REPLACE FUNCTION public.fD(p1 integer) RETURNS TABLE(c1 integer,
c2 character varying, c3 character varying, c4 character varying)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2, c3, c4
FROM tD
WHERE c1 = p1
$$;

CREATE OR REPLACE FUNCTION public.fE(p1 character varying) RETURNS TABLE(c1
character varying, c2 text)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2
FROM tE
WHERE c1 = p1
$$;

CREATE OR REPLACE FUNCTION public.fF(p1 character varying) RETURNS TABLE(c1
character varying, c2 character varying)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2
FROM tF
WHERE c1 = p1
$$;

CREATE OR REPLACE FUNCTION public.fG(p1 integer) RETURNS TABLE(c1 integer,
c2 character varying, c3 character varying, c4 text)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2, c3, c4
FROM tG
WHERE c1 = p1
$$;

CREATE OR REPLACE FUNCTION public.fH(p1 integer) RETURNS TABLE(c1 integer,
c2 character varying, c3 character varying, c4 character varying, c5
public.datatype, c6 character varying)
LANGUAGE sql STABLE
AS $$
SELECT c1, c2, c3, c4, c5, c6
FROM tH
WHERE c1 = p1
$$;

INSERT INTO public.ta (c1, c2, c3, c4, c5, c6, c7, c8) VALUES (1, 1, 1, 1,
1, 0, 1, NULL);
INSERT INTO public.ta (c1, c2, c3, c4, c5, c6, c7, c8) VALUES (2, 2, 1, 1,
2, 1, 1, NULL);
INSERT INTO public.ta (c1, c2, c3, c4, c5, c6, c7, c8) VALUES (3, 3, 1, 1,
1, 2, 1, NULL);
INSERT INTO public.ta (c1, c2, c3, c4, c5, c6, c7, c8) VALUES (4, 4, 1, 1,
2, 3, 1, NULL);

INSERT INTO public.tb (c1, c2, c3, c4) VALUES (1, 1, 'nA', 1);
INSERT INTO public.tb (c1, c2, c3, c4) VALUES (2, 2, 'nA', 1);
INSERT INTO public.tb (c1, c2, c3, c4) VALUES (3, 1, 'nA', 2);
INSERT INTO public.tb (c1, c2, c3, c4) VALUES (4, 2, 'nA', 2);

INSERT INTO public.tc (c1, c2) VALUES ('nA', '');
INSERT INTO public.tc (c1, c2) VALUES ('nB', '');

INSERT INTO public.td (c1, c2, c3, c4) VALUES (1, 'msA', 'qA', '');
INSERT INTO public.td (c1, c2, c3, c4) VALUES (2, 'msA', 'qB', '');
INSERT INTO public.td (c1, c2, c3, c4) VALUES (3, 'msB', 'qA', '');
INSERT INTO public.td (c1, c2, c3, c4) VALUES (4, 'msB', 'qB', '');

INSERT INTO public.te (c1, c2) VALUES ('msA', '');
INSERT INTO public.te (c1, c2) VALUES ('msB', '');

INSERT INTO public.tf (c1, c2) VALUES ('qA', 'uA');
INSERT INTO public.tf (c1, c2) VALUES ('qB', 'uB');

INSERT INTO public.tg (c1, c2, c3, c4) VALUES (1, 'nA', 'nsA', '');

INSERT INTO public.th (c1, c2, c3, c4, c5, c6) VALUES (1, 'mA', 'mB', 'qA',
'a', 'fA');
INSERT INTO public.th (c1, c2, c3, c4, c5, c6) VALUES (2, 'mA', 'tB', 'qB',
'b', 'fB');

SELECT
fA.c1 AS fA_c1, fA.c4 AS fA_c4, fA.c6 AS fA_c6, fA.c7 AS fA_c7, fA.c8 AS
fA_c8,
fB.c1 AS fB_c1, fB.c4 AS fB_c4,
fC.c1 AS fC_c1, fC.c2 AS fC_c2,
fD.c1 AS fD_c1, fD.c4 AS fD_c4,
fE.c1 AS fE_c1, fE.c2 AS fE_c2,
fF.c1 AS fF_c1, fF.c2 AS fF_c2,
fG.c1 AS fG_c1, fG.c2 AS fG_c2, fG.c3 AS fG_c3, fG.c4 AS fG_c4,
fH.c1 AS fH_c1, fH.c2 AS fH_c2, fH.c3 AS fH_c3, fH.c4 AS fH_c4, fH.c5 AS
fH_c5, fH.c6 AS fH_c6
FROM fA(1) AS fA
LEFT OUTER JOIN fB(fA.c2) AS fB ON true
LEFT OUTER JOIN fC(fB.c3) AS fC ON true
LEFT OUTER JOIN fD(fB.c2) AS fD ON true
LEFT OUTER JOIN fE(fD.c2) AS fE ON true
LEFT OUTER JOIN fF(fD.c3) AS fF ON true
LEFT OUTER JOIN fG(fA.c3) AS fG ON true
LEFT OUTER JOIN fH(fA.c5) AS fH ON true