Fix unique key checks in JSON object constructors

When building a JSON object, the code builds a hash table of keys, to
allow checking if the keys are unique. The uniqueness check and adding
the new key happens in json_unique_check_key(), but this assumes the
pointer to the key remains valid.

Unfortunately, two places passed pointers to keys in a buffer, while
also appending more data (additional key/value pairs) to the buffer.
With enough data the buffer is resized by enlargeStringInfo(), which
calls repalloc(), invalidating the earlier key pointers.

Due to this the uniqueness check may fail with both false negatives and
false positives, producing JSON objects with duplicate keys or failing
to produce a perfectly valid JSON object.

This affects multiple functions that enforce uniqueness of keys, all
introduced in PG16 with the new SQL/JSON:

- json_object_agg_unique / jsonb_object_agg_unique
- json_object / jsonb_objectagg

Existing regression tests did not detect the issue, simply because the
initial buffer size is 1024 and the objects were small enough not to
require the repalloc.

With a sufficiently large object, AddressSanitizer reported the access
to invalid memory immediately. So would valgrind, of course.

Fixed by copying the key into the hash table memory context, and adding
regression tests with enough data to repalloc the buffer. Backpatch to
16, where the functions were introduced.

Reported by Alexander Lakhin. Investigation and initial fix by Junwang
Zhao, with various improvements and tests by me.

Reported-by: Alexander Lakhin
Author: Junwang Zhao, Tomas Vondra
Backpatch-through: 16
Discussion: https://postgr.es/m/18598-3279ed972a2347c7@postgresql.org
Discussion: https://postgr.es/m/CAEG8a3JjH0ReJF2_O7-8LuEbO69BxPhYeXs95_x7+H9AMWF1gw@mail.gmail.com

src/backend/utils/adt/json.c

@@ -1111,7 +1111,14 @@ json_object_agg_transfn_worker(FunctionCallInfo fcinfo,
 
 	if (unique_keys)
 	{
-		const char *key = &out->data[key_offset];
+		/*
+		 * Copy the key first, instead of pointing into the buffer. It will be
+		 * added to the hash table, but the buffer may get reallocated as
+		 * we're appending more data to it. That would invalidate pointers to
+		 * keys in the current buffer.
+		 */
+		const char *key = MemoryContextStrdup(aggcontext,
+											  &out->data[key_offset]);
 
 		if (!json_unique_check_key(&state->unique_check.check, key, 0))
 			ereport(ERROR,
@@ -1274,8 +1281,15 @@ json_build_object_worker(int nargs, const Datum *args, const bool *nulls, const
 
 		if (unique_keys)
 		{
-			/* check key uniqueness after key appending */
-			const char *key = &out->data[key_offset];
+			/*
+			 * check key uniqueness after key appending
+			 *
+			 * Copy the key first, instead of pointing into the buffer. It
+			 * will be added to the hash table, but the buffer may get
+			 * reallocated as we're appending more data to it. That would
+			 * invalidate pointers to keys in the current buffer.
+			 */
+			const char *key = pstrdup(&out->data[key_offset]);
 
 			if (!json_unique_check_key(&unique_check.check, key, 0))
 				ereport(ERROR,

src/test/regress/expected/json.out

@@ -2330,6 +2330,9 @@ select json_object('{a,b,"","d e f"}','{1,2,3,"a b c"}');
  {"a" : "1", "b" : "2", "" : "3", "d e f" : "a b c"}
 (1 row)
 
+-- json_object_agg_unique requires unique keys
+select json_object_agg_unique(mod(i,100), i) from generate_series(0, 199) i;
+ERROR:  duplicate JSON object key value: "0"
 -- json_to_record and json_to_recordset
 select * from json_to_record('{"a":1,"b":"foo","c":"bar"}')
     as x(a int, b text, d text);

src/test/regress/expected/sqljson.out

@@ -537,6 +537,8 @@ SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2 ABSENT ON NULL);
  {"a" : "1", "c" : 2}
 (1 row)
 
+SELECT JSON_OBJECT(1: 1, '2': NULL, '3': 1, repeat('x', 1000): 1, 2: repeat('a', 100) WITH UNIQUE);
+ERROR:  duplicate JSON object key value: "2"
 SELECT JSON_OBJECT(1: 1, '1': NULL WITH UNIQUE);
 ERROR:  duplicate JSON object key value: "1"
 SELECT JSON_OBJECT(1: 1, '1': NULL ABSENT ON NULL WITH UNIQUE);
@@ -921,6 +923,9 @@ FROM (VALUES (1, 1), (0, NULL),(4, null), (5, null),(6, null),(2, 2)) foo(k, v);
  {"1": 1, "2": 2}
 (1 row)
 
+SELECT JSON_OBJECTAGG(mod(i,100): (i)::text FORMAT JSON WITH UNIQUE)
+FROM generate_series(0, 199) i;
+ERROR:  duplicate JSON object key value: "0"
 -- Test JSON_OBJECT deparsing
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT JSON_OBJECT('foo' : '1' FORMAT JSON, 'bar' : 'baz' RETURNING json);

src/test/regress/sql/json.sql

@@ -755,6 +755,8 @@ select json_object('{a,b,NULL,"d e f"}','{1,2,3,"a b c"}');
 
 select json_object('{a,b,"","d e f"}','{1,2,3,"a b c"}');
 
+-- json_object_agg_unique requires unique keys
+select json_object_agg_unique(mod(i,100), i) from generate_series(0, 199) i;
 
 -- json_to_record and json_to_recordset
 

src/test/regress/sql/sqljson.sql

@@ -138,6 +138,8 @@ SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2);
 SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2 NULL ON NULL);
 SELECT JSON_OBJECT('a': '1', 'b': NULL, 'c': 2 ABSENT ON NULL);
 
+SELECT JSON_OBJECT(1: 1, '2': NULL, '3': 1, repeat('x', 1000): 1, 2: repeat('a', 100) WITH UNIQUE);
+
 SELECT JSON_OBJECT(1: 1, '1': NULL WITH UNIQUE);
 SELECT JSON_OBJECT(1: 1, '1': NULL ABSENT ON NULL WITH UNIQUE);
 SELECT JSON_OBJECT(1: 1, '1': NULL NULL ON NULL WITH UNIQUE RETURNING jsonb);
@@ -283,6 +285,9 @@ FROM (VALUES (1, 1), (1, NULL), (2, 2)) foo(k, v);
 SELECT JSON_OBJECTAGG(k: v ABSENT ON NULL WITH UNIQUE KEYS RETURNING jsonb)
 FROM (VALUES (1, 1), (0, NULL),(4, null), (5, null),(6, null),(2, 2)) foo(k, v);
 
+SELECT JSON_OBJECTAGG(mod(i,100): (i)::text FORMAT JSON WITH UNIQUE)
+FROM generate_series(0, 199) i;
+
 -- Test JSON_OBJECT deparsing
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT JSON_OBJECT('foo' : '1' FORMAT JSON, 'bar' : 'baz' RETURNING json);