Enforce RETURNING typmod for empty-set JSON_ARRAY(query)

Commit 8d829f5a0 introduced a COALESCE wrapper around the
JSON_ARRAYAGG subquery so that JSON_ARRAY(query) returns '[]' rather
than NULL when the subquery yields no rows, per the SQL/JSON standard.

The empty-array Const used as the COALESCE fallback was, however,
built with typmod -1 and the type input function was likewise invoked
with typmod -1.  As a result, any length restriction from the
RETURNING clause was silently bypassed on the empty-set path, while
the non-empty path enforced it via the JSON_ARRAYAGG coercion.

Build the empty-array Const using the typmod of the COALESCE's
non-empty argument, and pass that typmod to OidInputFunctionCall as
well so the value is length-checked at parse time.  This makes the
empty-set and non-empty-set paths behave consistently.

Reported-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAJTYsWXPYqa58YXrU+SQMVonsAhjLS46HNUMU=wO5zm9MgY3_g@mail.gmail.com

src/backend/parser/parse_expr.c

@@ -3826,6 +3826,7 @@ transformJsonArrayQueryConstructor(ParseState *pstate,
 	CoalesceExpr *coalesce;
 	Const	   *empty_const;
 	Oid			result_type;
+	int32		result_typmod;
 	Oid			typinput;
 	Oid			typioparam;
 	int16		typlen;
@@ -3904,19 +3905,22 @@ transformJsonArrayQueryConstructor(ParseState *pstate,
 
 	/*
 	 * Wrap in COALESCE so that an empty result set produces '[]' rather than
-	 * NULL.  The empty-array constant is created in the output type so that
-	 * the COALESCE arguments have consistent types.
+	 * NULL.  The empty-array constant is created in the output type and
+	 * typmod, so that the COALESCE arguments have consistent types and any
+	 * length restriction from the RETURNING clause is enforced uniformly
+	 * across the empty and non-empty paths.
 	 */
 	result_type = exprType(exec_expr);
+	result_typmod = exprTypmod(exec_expr);
 	getTypeInputInfo(result_type, &typinput, &typioparam);
 	get_typlenbyval(result_type, &typlen, &typbyval);
 
 	empty_const = makeConst(result_type,
-							-1,
+							result_typmod,
 							exprCollation(exec_expr),
 							(int) typlen,
 							OidInputFunctionCall(typinput, "[]",
-												 typioparam, -1),
+												 typioparam, result_typmod),
 							false,
 							typbyval);
 

src/test/regress/expected/sqljson.out

@@ -783,6 +783,25 @@ WHERE JSON_ARRAY(
  4
 (2 rows)
 
+-- JSON_ARRAY(subquery) RETURNING with a length-restricted output type
+-- Should fail
+SELECT JSON_ARRAY(SELECT 1 RETURNING varchar(1));
+ERROR:  value too long for type character varying(1)
+SELECT JSON_ARRAY(SELECT 1 WHERE FALSE RETURNING varchar(1));
+ERROR:  value too long for type character varying(1)
+-- Should work
+SELECT JSON_ARRAY(SELECT 1 RETURNING varchar(3));
+ json_array 
+------------
+ [1]
+(1 row)
+
+SELECT JSON_ARRAY(SELECT 1 WHERE FALSE RETURNING varchar(2));
+ json_array 
+------------
+ []
+(1 row)
+
 -- Should fail
 SELECT JSON_ARRAY(SELECT FROM (VALUES (1)) foo(i));
 ERROR:  subquery must return only one column

src/test/regress/sql/sqljson.sql

@@ -213,6 +213,14 @@ WHERE JSON_ARRAY(
     RETURNING jsonb
 ) = '[]'::jsonb;
 
+-- JSON_ARRAY(subquery) RETURNING with a length-restricted output type
+-- Should fail
+SELECT JSON_ARRAY(SELECT 1 RETURNING varchar(1));
+SELECT JSON_ARRAY(SELECT 1 WHERE FALSE RETURNING varchar(1));
+-- Should work
+SELECT JSON_ARRAY(SELECT 1 RETURNING varchar(3));
+SELECT JSON_ARRAY(SELECT 1 WHERE FALSE RETURNING varchar(2));
+
 -- Should fail
 SELECT JSON_ARRAY(SELECT FROM (VALUES (1)) foo(i));
 SELECT JSON_ARRAY(SELECT i, i FROM (VALUES (1)) foo(i));