Fixed bug #29417.

An assertion abort could occur for some grouping queries that employed 
decimal user variables with assignments to them.

The problem appeared the constructors of the class Field_new_decimal
because the function my_decimal_length_to_precision did not guarantee
returning decimal precision not greater than DECIMAL_MAX_PRECISION.

mysql-test/r/type_newdecimal.result

@@ -1471,4 +1471,12 @@ drop table t1;
 SELECT 1.000000000000 * 99.999999999998 / 100 a,1.000000000000 * (99.999999999998 / 100) b;
 a	b
 0.9999999999999800000000000000	0.9999999999999800000000000000
+CREATE TABLE t1 (a int DEFAULT NULL, b int DEFAULT NULL);
+INSERT INTO t1 VALUES (3,30), (1,10), (2,10);
+SET @a= CAST(1 AS decimal);
+SELECT 1 FROM t1 GROUP BY @b := @a, @b;
+1
+1
+1
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/type_newdecimal.test

@@ -1162,4 +1162,17 @@ drop table t1;
 #
 
 SELECT 1.000000000000 * 99.999999999998 / 100 a,1.000000000000 * (99.999999999998 / 100) b;
+
+#
+# Bug #29417: assertion abort for a grouping query with decimal user variable
+#
+
+CREATE TABLE t1 (a int DEFAULT NULL, b int DEFAULT NULL);
+INSERT INTO t1 VALUES (3,30), (1,10), (2,10);
+
+SET @a= CAST(1 AS decimal);
+SELECT 1 FROM t1 GROUP BY @b := @a, @b;
+
+DROP TABLE t1;
+
 --echo End of 5.0 tests

sql/field.cc

@@ -2267,6 +2267,7 @@ Field_new_decimal::Field_new_decimal(char *ptr_arg,
              dec_arg, zero_arg, unsigned_arg)
 {
   precision= my_decimal_length_to_precision(len_arg, dec_arg, unsigned_arg);
+  set_if_smaller(precision, DECIMAL_MAX_PRECISION);
   DBUG_ASSERT((precision <= DECIMAL_MAX_PRECISION) &&
               (dec <= DECIMAL_MAX_SCALE));
   bin_size= my_decimal_get_binary_size(precision, dec);
@@ -2286,6 +2287,7 @@ Field_new_decimal::Field_new_decimal(uint32 len_arg,
              0, unsigned_arg)
 {
   precision= my_decimal_length_to_precision(len_arg, dec_arg, unsigned_arg);
+  set_if_smaller(precision, DECIMAL_MAX_PRECISION);
   DBUG_ASSERT((precision <= DECIMAL_MAX_PRECISION) &&
               (dec <= DECIMAL_MAX_SCALE));
   bin_size= my_decimal_get_binary_size(precision, dec);