SQL/JSON: Fix coercion of constructor outputs to types with typmod

Ensure SQL/JSON constructor functions that allow specifying the
target type using the RETURNING clause perform implicit cast to
that type.  This ensures that output values that exceed the specified
length produce an error rather than being  silently truncated. This
behavior conforms to the SQL standard.

Reported-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Jian He <jian.universality@gmail.com>
Discussion: https://postgr.es/m/202405271326.5a5rprki64aw%40alvherre.pgsql

src/backend/parser/parse_expr.c

@@ -3583,6 +3583,7 @@ coerceJsonFuncExpr(ParseState *pstate, Node *expr,
 	Node	   *res;
 	int			location;
 	Oid			exprtype = exprType(expr);
+	int32		baseTypmod = returning->typmod;
 
 	/* if output type is not specified or equals to function type, return */
 	if (!OidIsValid(returning->typid) || returning->typid == exprtype)
@@ -3611,10 +3612,19 @@ coerceJsonFuncExpr(ParseState *pstate, Node *expr,
 		return (Node *) fexpr;
 	}
 
+	/*
+	 * For domains, consider the base type's typmod to decide whether to setup
+	 * an implicit or explicit cast.
+	 */
+	if (get_typtype(returning->typid) == TYPTYPE_DOMAIN)
+		(void) getBaseTypeAndTypmod(returning->typid, &baseTypmod);
+
 	/* try to coerce expression to the output type */
 	res = coerce_to_target_type(pstate, expr, exprtype,
-								returning->typid, returning->typmod,
+								returning->typid, baseTypmod,
+								baseTypmod > 0 ? COERCION_IMPLICIT :
 								COERCION_EXPLICIT,
+								baseTypmod > 0 ? COERCE_IMPLICIT_CAST :
 								COERCE_EXPLICIT_CAST,
 								location);
 
@@ -3640,6 +3650,7 @@ makeJsonConstructorExpr(ParseState *pstate, JsonConstructorType type,
 	JsonConstructorExpr *jsctor = makeNode(JsonConstructorExpr);
 	Node	   *placeholder;
 	Node	   *coercion;
+	int32		baseTypmod = returning->typmod;
 
 	jsctor->args = args;
 	jsctor->func = fexpr;
@@ -3677,6 +3688,17 @@ makeJsonConstructorExpr(ParseState *pstate, JsonConstructorType type,
 		placeholder = (Node *) cte;
 	}
 
+	/*
+	 * Convert the source expression to text, because coerceJsonFuncExpr()
+	 * will create an implicit cast to the RETURNING types with typmod and
+	 * there are no implicit casts from json(b) to such types.  For domains,
+	 * the base type's typmod will be considered, so do so here too.
+	 */
+	if (get_typtype(returning->typid) == TYPTYPE_DOMAIN)
+		(void) getBaseTypeAndTypmod(returning->typid, &baseTypmod);
+	if (baseTypmod > 0)
+		placeholder = coerce_to_specific_type(pstate, placeholder, TEXTOID,
+											  "JSON_CONSTRUCTOR()");
 	coercion = coerceJsonFuncExpr(pstate, placeholder, returning, true);
 
 	if (coercion != placeholder)

src/test/regress/expected/sqljson.out

@@ -1278,3 +1278,20 @@ CREATE OR REPLACE VIEW public.is_json_view AS
     '{}'::text IS JSON OBJECT WITH UNIQUE KEYS AS object
    FROM generate_series(1, 3) i(i)
 DROP VIEW is_json_view;
+-- Test implicit coercion to a fixed-length type specified in RETURNING
+SELECT JSON_SERIALIZE('{ "a" : 1 } ' RETURNING varchar(2));
+ERROR:  value too long for type character varying(2)
+SELECT JSON_OBJECT('a': JSON_OBJECT('b': 1 RETURNING varchar(2)));
+ERROR:  value too long for type character varying(2)
+SELECT JSON_ARRAY(JSON_ARRAY('{ "a" : 123 }' RETURNING varchar(2)));
+ERROR:  value too long for type character varying(2)
+SELECT JSON_ARRAYAGG(('111' || i)::bytea FORMAT JSON NULL ON NULL RETURNING varchar(2)) FROM generate_series(1,1) i;
+ERROR:  value too long for type character varying(2)
+SELECT JSON_OBJECTAGG(i: ('111' || i)::bytea FORMAT JSON WITH UNIQUE RETURNING varchar(2)) FROM generate_series(1, 1) i;
+ERROR:  value too long for type character varying(2)
+-- Now try domain over fixed-length type
+CREATE DOMAIN sqljson_char2 AS char(2) CHECK (VALUE NOT IN ('12'));
+SELECT JSON_SERIALIZE('123' RETURNING sqljson_char2);
+ERROR:  value too long for type character(2)
+SELECT JSON_SERIALIZE('12' RETURNING sqljson_char2);
+ERROR:  value for domain sqljson_char2 violates check constraint "sqljson_char2_check"

src/test/regress/sql/sqljson.sql

@@ -463,3 +463,15 @@ SELECT '1' IS JSON AS "any", ('1' || i) IS JSON SCALAR AS "scalar", '[]' IS NOT
 \sv is_json_view
 
 DROP VIEW is_json_view;
+
+-- Test implicit coercion to a fixed-length type specified in RETURNING
+SELECT JSON_SERIALIZE('{ "a" : 1 } ' RETURNING varchar(2));
+SELECT JSON_OBJECT('a': JSON_OBJECT('b': 1 RETURNING varchar(2)));
+SELECT JSON_ARRAY(JSON_ARRAY('{ "a" : 123 }' RETURNING varchar(2)));
+SELECT JSON_ARRAYAGG(('111' || i)::bytea FORMAT JSON NULL ON NULL RETURNING varchar(2)) FROM generate_series(1,1) i;
+SELECT JSON_OBJECTAGG(i: ('111' || i)::bytea FORMAT JSON WITH UNIQUE RETURNING varchar(2)) FROM generate_series(1, 1) i;
+
+-- Now try domain over fixed-length type
+CREATE DOMAIN sqljson_char2 AS char(2) CHECK (VALUE NOT IN ('12'));
+SELECT JSON_SERIALIZE('123' RETURNING sqljson_char2);
+SELECT JSON_SERIALIZE('12' RETURNING sqljson_char2);