plpgsql: handle multiple expressions when one expression is expected

Previously, the PLpgSQL parser could panic when the user supplied more
than one expression in a location where only one was expected, for example,
in a return statement. This was because the PLpgSQL parser delegated to
the SQL parser's `ParseExpr` function, which expects exactly one input
expression. This commit returns a syntax error instead of the panic by
switching to use `ParseExprs`, which can handle multiple input expressions.

Informs cockroachdb#109342

Release note: None

pkg/sql/plpgsql/parser/lexer.go

@@ -463,5 +463,14 @@ func (l *lexer) GetTypeFromValidSQLSyntax(sqlStr string) (tree.ResolvableTypeRef
 }
 
 func (l *lexer) ParseExpr(sqlStr string) (plpgsqltree.Expr, error) {
-	return parser.ParseExpr(sqlStr)
+	// Use ParseExprs instead of ParseExpr in order to correctly handle the case
+	// when multiple expressions are incorrectly passed.
+	exprs, err := parser.ParseExprs([]string{sqlStr})
+	if err != nil {
+		return nil, err
+	}
+	if len(exprs) != 1 {
+		return nil, pgerror.Newf(pgcode.Syntax, "query returned %d columns", len(exprs))
+	}
+	return exprs[0], nil
 }

pkg/sql/plpgsql/parser/testdata/stmt_assign

@@ -22,6 +22,21 @@ BEGIN
 a := NULL;
 END
 
+parse
+DECLARE
+BEGIN
+  a := 1, 'string';
+END
+----
+at or near ";": syntax error: query returned 2 columns
+
+parse
+DECLARE
+BEGIN
+  a := 1, (2, 3, 4, 5);
+END
+----
+at or near ";": syntax error: query returned 2 columns
 
 feature-count
 DECLARE