Fix internal error from CollateExpr in SQL/JSON DEFAULT expressions

SQL/JSON functions such as JSON_VALUE could fail with "unrecognized
node type" errors when a DEFAULT clause contained an explicit COLLATE
expression. That happened because assign_collations_walker() could
invoke exprSetCollation() on a JsonBehavior expression whose DEFAULT
still contained a CollateExpr, which exprSetCollation() does not
handle.

For example:

  SELECT JSON_VALUE('{"a":1}', '$.c' RETURNING text
                    DEFAULT 'A' COLLATE "C" ON EMPTY);

Fix by validating in transformJsonBehavior() that the DEFAULT
expression's collation matches the enclosing JSON expression’s
collation. In exprSetCollation(), replace the recursive call on the
JsonBehavior expression with an assertion that its collation already
matches the target, since the parser now enforces that condition.

Reported-by: Jian He <jian.universality@gmail.com>
Author: Jian He <jian.universality@gmail.com>
Reviewed-by: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CACJufxHVwYYSyiVQ6o+PsRX6zQ7rAFinh_fv1kCfTsT1xG4Zeg@mail.gmail.com
Backpatch-through: 17

1 files changed, 49 insertions, 8 deletions

diff --git a/src/backend/parser/parse_expr.c b/src/backend/parser/parse_expr.c
index 9d95c7140ee..12119f147fc 100644
--- a/src/backend/parser/parse_expr.c
+++ b/src/backend/parser/parse_expr.c
@@ -94,7 +94,8 @@ static Node *transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func);
 static void transformJsonPassingArgs(ParseState *pstate, const char *constructName,
 									 JsonFormatType format, List *args,
 									 List **passing_values, List **passing_names);
-static JsonBehavior *transformJsonBehavior(ParseState *pstate, JsonBehavior *behavior,
+static JsonBehavior *transformJsonBehavior(ParseState *pstate, JsonExpr *jsexpr,
+										   JsonBehavior *behavior,
 										   JsonBehaviorType default_behavior,
 										   JsonReturning *returning);
 static Node *GetJsonBehaviorConst(JsonBehaviorType btype, int location);
@@ -4529,13 +4530,16 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 			{
 				jsexpr->returning->typid = BOOLOID;
 				jsexpr->returning->typmod = -1;
+				jsexpr->collation = InvalidOid;
 			}
 
 			/* JSON_TABLE() COLUMNS can specify a non-boolean type. */
 			if (jsexpr->returning->typid != BOOLOID)
 				jsexpr->use_json_coercion = true;
 
-			jsexpr->on_error = transformJsonBehavior(pstate, func->on_error,
+			jsexpr->on_error = transformJsonBehavior(pstate,
+													 jsexpr,
+													 func->on_error,
 													 JSON_BEHAVIOR_FALSE,
 													 jsexpr->returning);
 			break;
@@ -4550,6 +4554,8 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 				ret->typmod = -1;
 			}
 
+			jsexpr->collation = get_typcollation(jsexpr->returning->typid);
+
 			/*
 			 * Keep quotes on scalar strings by default, omitting them only if
 			 * OMIT QUOTES is specified.
@@ -4566,11 +4572,15 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 				jsexpr->use_json_coercion = true;
 
 			/* Assume NULL ON EMPTY when ON EMPTY is not specified. */
-			jsexpr->on_empty = transformJsonBehavior(pstate, func->on_empty,
+			jsexpr->on_empty = transformJsonBehavior(pstate,
+													 jsexpr,
+													 func->on_empty,
 													 JSON_BEHAVIOR_NULL,
 													 jsexpr->returning);
 			/* Assume NULL ON ERROR when ON ERROR is not specified. */
-			jsexpr->on_error = transformJsonBehavior(pstate, func->on_error,
+			jsexpr->on_error = transformJsonBehavior(pstate,
+													 jsexpr,
+													 func->on_error,
 													 JSON_BEHAVIOR_NULL,
 													 jsexpr->returning);
 			break;
@@ -4582,6 +4592,7 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 				jsexpr->returning->typid = TEXTOID;
 				jsexpr->returning->typmod = -1;
 			}
+			jsexpr->collation = get_typcollation(jsexpr->returning->typid);
 
 			/*
 			 * Override whatever transformJsonOutput() set these to, which
@@ -4607,11 +4618,15 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 			}
 
 			/* Assume NULL ON EMPTY when ON EMPTY is not specified. */
-			jsexpr->on_empty = transformJsonBehavior(pstate, func->on_empty,
+			jsexpr->on_empty = transformJsonBehavior(pstate,
+													 jsexpr,
+													 func->on_empty,
 													 JSON_BEHAVIOR_NULL,
 													 jsexpr->returning);
 			/* Assume NULL ON ERROR when ON ERROR is not specified. */
-			jsexpr->on_error = transformJsonBehavior(pstate, func->on_error,
+			jsexpr->on_error = transformJsonBehavior(pstate,
+													 jsexpr,
+													 func->on_error,
 													 JSON_BEHAVIOR_NULL,
 													 jsexpr->returning);
 			break;
@@ -4622,6 +4637,7 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 				jsexpr->returning->typid = exprType(jsexpr->formatted_expr);
 				jsexpr->returning->typmod = -1;
 			}
+			jsexpr->collation = get_typcollation(jsexpr->returning->typid);
 
 			/*
 			 * Assume EMPTY ARRAY ON ERROR when ON ERROR is not specified.
@@ -4629,7 +4645,9 @@ transformJsonFuncExpr(ParseState *pstate, JsonFuncExpr *func)
 			 * ON EMPTY cannot be specified at the top level but it can be for
 			 * the individual columns.
 			 */
-			jsexpr->on_error = transformJsonBehavior(pstate, func->on_error,
+			jsexpr->on_error = transformJsonBehavior(pstate,
+													 jsexpr,
+													 func->on_error,
 													 JSON_BEHAVIOR_EMPTY_ARRAY,
 													 jsexpr->returning);
 			break;
@@ -4705,7 +4723,8 @@ ValidJsonBehaviorDefaultExpr(Node *expr, void *context)
  * Transform a JSON BEHAVIOR clause.
  */
 static JsonBehavior *
-transformJsonBehavior(ParseState *pstate, JsonBehavior *behavior,
+transformJsonBehavior(ParseState *pstate, JsonExpr *jsexpr,
+					  JsonBehavior *behavior,
 					  JsonBehaviorType default_behavior,
 					  JsonReturning *returning)
 {
@@ -4720,7 +4739,11 @@ transformJsonBehavior(ParseState *pstate, JsonBehavior *behavior,
 		location = behavior->location;
 		if (btype == JSON_BEHAVIOR_DEFAULT)
 		{
+			Oid			targetcoll = jsexpr->collation;
+			Oid			exprcoll;
+
 			expr = transformExprRecurse(pstate, behavior->expr);
+
 			if (!ValidJsonBehaviorDefaultExpr(expr, NULL))
 				ereport(ERROR,
 						(errcode(ERRCODE_DATATYPE_MISMATCH),
@@ -4736,6 +4759,24 @@ transformJsonBehavior(ParseState *pstate, JsonBehavior *behavior,
 						(errcode(ERRCODE_DATATYPE_MISMATCH),
 						 errmsg("DEFAULT expression must not return a set"),
 						 parser_errposition(pstate, exprLocation(expr))));
+
+			/*
+			 * Reject a DEFAULT expression whose collation differs from the
+			 * enclosing JSON expression's result collation
+			 * (jsexpr->collation), as chosen by the RETURNING clause.
+			 */
+			exprcoll = exprCollation(expr);
+			if (!OidIsValid(exprcoll))
+				exprcoll = get_typcollation(exprType(expr));
+			if (OidIsValid(targetcoll) && OidIsValid(exprcoll) &&
+				targetcoll != exprcoll)
+				ereport(ERROR,
+						errcode(ERRCODE_COLLATION_MISMATCH),
+						errmsg("the collation of DEFAULT expression conflicts with RETURNING clause"),
+						errdetail("\"%s\" versus \"%s\"",
+								  get_collation_name(exprcoll),
+								  get_collation_name(targetcoll)),
+						parser_errposition(pstate, exprLocation(expr)));
 		}
 	}