Fix erroneous hash calculations in gin_extract_jsonb_path().

The jsonb_path_ops code calculated hash values inconsistently in some cases
involving nested arrays and objects.  This would result in queries possibly
not finding entries that they should find, when using a jsonb_path_ops GIN
index for the search.  The problem cases involve JSONB values that contain
both scalars and sub-objects at the same nesting level, for example an
array containing both scalars and sub-arrays.  To fix, reset the current
stack->hash after processing each value or sub-object, not before; and
don't try to be cute about the outermost level's initial hash.

Correcting this means that existing jsonb_path_ops indexes may now be
inconsistent with the new hash calculation code.  The symptom is the same
--- searches not finding entries they should find --- but the specific
rows affected are likely to be different.  Users will need to REINDEX
jsonb_path_ops indexes to make sure that all searches work as expected.

Per bug #13756 from Daniel Cheng.  Back-patch to 9.4 where the faulty
logic was introduced.

src/test/regress/expected/jsonb.out

@@ -2420,6 +2420,56 @@ SELECT '{"a":[1,2,{"c":3,"x":4}],"c":"b"}'::jsonb @> '{"a":[{"x":4},1]}';
  t
 (1 row)
 
+-- check some corner cases for indexed nested containment (bug #13756)
+create temp table nestjsonb (j jsonb);
+insert into nestjsonb (j) values ('{"a":[["b",{"x":1}],["b",{"x":2}]],"c":3}');
+insert into nestjsonb (j) values ('[[14,2,3]]');
+insert into nestjsonb (j) values ('[1,[14,2,3]]');
+create index on nestjsonb using gin(j jsonb_path_ops);
+set enable_seqscan = on;
+set enable_bitmapscan = off;
+select * from nestjsonb where j @> '{"a":[[{"x":2}]]}'::jsonb;
+                         j                         
+---------------------------------------------------
+ {"a": [["b", {"x": 1}], ["b", {"x": 2}]], "c": 3}
+(1 row)
+
+select * from nestjsonb where j @> '{"c":3}';
+                         j                         
+---------------------------------------------------
+ {"a": [["b", {"x": 1}], ["b", {"x": 2}]], "c": 3}
+(1 row)
+
+select * from nestjsonb where j @> '[[14]]';
+        j        
+-----------------
+ [[14, 2, 3]]
+ [1, [14, 2, 3]]
+(2 rows)
+
+set enable_seqscan = off;
+set enable_bitmapscan = on;
+select * from nestjsonb where j @> '{"a":[[{"x":2}]]}'::jsonb;
+                         j                         
+---------------------------------------------------
+ {"a": [["b", {"x": 1}], ["b", {"x": 2}]], "c": 3}
+(1 row)
+
+select * from nestjsonb where j @> '{"c":3}';
+                         j                         
+---------------------------------------------------
+ {"a": [["b", {"x": 1}], ["b", {"x": 2}]], "c": 3}
+(1 row)
+
+select * from nestjsonb where j @> '[[14]]';
+        j        
+-----------------
+ [[14, 2, 3]]
+ [1, [14, 2, 3]]
+(2 rows)
+
+reset enable_seqscan;
+reset enable_bitmapscan;
 -- nested object field / array index lookup
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb -> 'n';
  ?column? 

src/test/regress/sql/jsonb.sql

@@ -618,6 +618,26 @@ SELECT '{"a":[1,2,{"c":3,"x":4}],"c":"b"}'::jsonb @> '{"a":[{"x":4}]}';
 SELECT '{"a":[1,2,{"c":3,"x":4}],"c":"b"}'::jsonb @> '{"a":[{"x":4},3]}';
 SELECT '{"a":[1,2,{"c":3,"x":4}],"c":"b"}'::jsonb @> '{"a":[{"x":4},1]}';
 
+-- check some corner cases for indexed nested containment (bug #13756)
+create temp table nestjsonb (j jsonb);
+insert into nestjsonb (j) values ('{"a":[["b",{"x":1}],["b",{"x":2}]],"c":3}');
+insert into nestjsonb (j) values ('[[14,2,3]]');
+insert into nestjsonb (j) values ('[1,[14,2,3]]');
+create index on nestjsonb using gin(j jsonb_path_ops);
+
+set enable_seqscan = on;
+set enable_bitmapscan = off;
+select * from nestjsonb where j @> '{"a":[[{"x":2}]]}'::jsonb;
+select * from nestjsonb where j @> '{"c":3}';
+select * from nestjsonb where j @> '[[14]]';
+set enable_seqscan = off;
+set enable_bitmapscan = on;
+select * from nestjsonb where j @> '{"a":[[{"x":2}]]}'::jsonb;
+select * from nestjsonb where j @> '{"c":3}';
+select * from nestjsonb where j @> '[[14]]';
+reset enable_seqscan;
+reset enable_bitmapscan;
+
 -- nested object field / array index lookup
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb -> 'n';
 SELECT '{"n":null,"a":1,"b":[1,2],"c":{"1":2},"d":{"1":[2,3]}}'::jsonb -> 'a';