Sort the dependent objects before recursing in findDependentObjects().

Historically, the notices output by DROP CASCADE tended to come out
in uncertain order, and in some cases you might get different claims
about which object depends on which other one.  This is because we
just traversed the dependency tree in the order in which pg_depend
entries are seen, and nbtree has never promised anything about the
order of equal-keyed index entries.  We've put up with that for years,
hacking regression tests when necessary to prevent them from emitting
unstable output.  However, it's a problem for pending work that will
change nbtree's behavior for equal keys, as that causes unexpected
changes in the regression test results.

Hence, adjust findDependentObjects to sort the results of each
indexscan before processing them.  The sort is on descending OID of
the dependent objects, hence more or less reverse creation order.
While this rule could still result in bogus regression test failures
if an OID wraparound occurred mid-test, that seems unlikely to happen
in any plausible development or packaging-test scenario.

This is enough to ensure output stability for ordinary DROP CASCADE
commands, but not for DROP OWNED BY, because that has a different
code path with the same problem.  We might later choose to sort in
the DROP OWNED BY code as well, but this patch doesn't do so.

I've also not done anything about reverting the existing hacks to
suppress unstable DROP CASCADE output in specific regression tests.
It might be worth undoing those, but it seems like a distinct question.

The first indexscan loop in findDependentObjects is not touched,
meaning there is a hazard of unstable error reports from that too.
However, said hazard is not the fault of that code: it was designed
on the assumption that there could be at most one "owning" object
to complain about, and that assumption does not seem unreasonable.
The recent patch that added the possibility of multiple
DEPENDENCY_INTERNAL_AUTO links broke that assumption, but we should
fix that situation not band-aid around it.  That's a matter for
another patch, though.

Discussion: https://postgr.es/m/12244.1547854440@sss.pgh.pa.us

src/test/regress/expected/alter_table.out

@@ -2583,10 +2583,10 @@ DETAIL:  drop cascades to table alter2.t1
 drop cascades to view alter2.v1
 drop cascades to function alter2.plus1(integer)
 drop cascades to type alter2.posint
-drop cascades to operator family alter2.ctype_hash_ops for access method hash
 drop cascades to type alter2.ctype
 drop cascades to function alter2.same(alter2.ctype,alter2.ctype)
 drop cascades to operator alter2.=(alter2.ctype,alter2.ctype)
+drop cascades to operator family alter2.ctype_hash_ops for access method hash
 drop cascades to conversion alter2.ascii_to_utf8
 drop cascades to text search parser alter2.prs
 drop cascades to text search configuration alter2.cfg

src/test/regress/expected/create_type.out

@@ -161,13 +161,13 @@ DROP FUNCTION base_fn_out(opaque); -- error
 ERROR:  function base_fn_out(opaque) does not exist
 DROP TYPE base_type; -- error
 ERROR:  cannot drop type base_type because other objects depend on it
-DETAIL:  function base_fn_out(base_type) depends on type base_type
-function base_fn_in(cstring) depends on type base_type
+DETAIL:  function base_fn_in(cstring) depends on type base_type
+function base_fn_out(base_type) depends on type base_type
 HINT:  Use DROP ... CASCADE to drop the dependent objects too.
 DROP TYPE base_type CASCADE;
 NOTICE:  drop cascades to 2 other objects
-DETAIL:  drop cascades to function base_fn_out(base_type)
-drop cascades to function base_fn_in(cstring)
+DETAIL:  drop cascades to function base_fn_in(cstring)
+drop cascades to function base_fn_out(base_type)
 -- Check usage of typmod with a user-defined type
 -- (we have borrowed numeric's typmod functions)
 CREATE TEMP TABLE mytab (foo widget(42,13,7));     -- should fail

src/test/regress/expected/domain.out

@@ -645,8 +645,8 @@ alter domain dnotnulltest drop not null;
 update domnotnull set col1 = null;
 drop domain dnotnulltest cascade;
 NOTICE:  drop cascades to 2 other objects
-DETAIL:  drop cascades to column col1 of table domnotnull
-drop cascades to column col2 of table domnotnull
+DETAIL:  drop cascades to column col2 of table domnotnull
+drop cascades to column col1 of table domnotnull
 -- Test ALTER DOMAIN .. DEFAULT ..
 create table domdeftest (col1 ddef1);
 insert into domdeftest default values;

src/test/regress/expected/matview.out

@@ -311,12 +311,12 @@ SELECT type, m.totamt AS mtot, v.totamt AS vtot FROM mvtest_tm m LEFT JOIN mvtes
 DROP TABLE mvtest_t;
 ERROR:  cannot drop table mvtest_t because other objects depend on it
 DETAIL:  view mvtest_tv depends on table mvtest_t
+materialized view mvtest_mvschema.mvtest_tvm depends on view mvtest_tv
+materialized view mvtest_tvmm depends on materialized view mvtest_mvschema.mvtest_tvm
 view mvtest_tvv depends on view mvtest_tv
 materialized view mvtest_tvvm depends on view mvtest_tvv
 view mvtest_tvvmv depends on materialized view mvtest_tvvm
 materialized view mvtest_bb depends on view mvtest_tvvmv
-materialized view mvtest_mvschema.mvtest_tvm depends on view mvtest_tv
-materialized view mvtest_tvmm depends on materialized view mvtest_mvschema.mvtest_tvm
 materialized view mvtest_tm depends on table mvtest_t
 materialized view mvtest_tmm depends on materialized view mvtest_tm
 HINT:  Use DROP ... CASCADE to drop the dependent objects too.
@@ -327,12 +327,12 @@ BEGIN;
 DROP TABLE mvtest_t CASCADE;
 NOTICE:  drop cascades to 9 other objects
 DETAIL:  drop cascades to view mvtest_tv
+drop cascades to materialized view mvtest_mvschema.mvtest_tvm
+drop cascades to materialized view mvtest_tvmm
 drop cascades to view mvtest_tvv
 drop cascades to materialized view mvtest_tvvm
 drop cascades to view mvtest_tvvmv
 drop cascades to materialized view mvtest_bb
-drop cascades to materialized view mvtest_mvschema.mvtest_tvm
-drop cascades to materialized view mvtest_tvmm
 drop cascades to materialized view mvtest_tm
 drop cascades to materialized view mvtest_tmm
 ROLLBACK;

src/test/regress/expected/updatable_views.out

@@ -334,6 +334,7 @@ DETAIL:  drop cascades to view ro_view1
 drop cascades to view ro_view17
 drop cascades to view ro_view2
 drop cascades to view ro_view3
+drop cascades to view ro_view4
 drop cascades to view ro_view5
 drop cascades to view ro_view6
 drop cascades to view ro_view7
@@ -341,11 +342,10 @@ drop cascades to view ro_view8
 drop cascades to view ro_view9
 drop cascades to view ro_view11
 drop cascades to view ro_view13
+drop cascades to view rw_view14
 drop cascades to view rw_view15
 drop cascades to view rw_view16
 drop cascades to view ro_view20
-drop cascades to view ro_view4
-drop cascades to view rw_view14
 DROP VIEW ro_view10, ro_view12, ro_view18;
 DROP SEQUENCE uv_seq CASCADE;
 NOTICE:  drop cascades to view ro_view19