MDEV-27690 Crash on `CHARACTER SET csname COLLATE DEFAULT` in column definition

This is a temporary fix for 10.2.
This problem was permanently fixed in 10.9 under terms of MDEV-27743.

This patch should propagate up to 10.8 then null-merged to 10.9.

mysql-test/r/ctype_collate.result

@@ -754,3 +754,20 @@ hex(b)
 
 61
 drop table t1;
+#
+# Start of 10.2 tests
+#
+#
+# MDEV-27690 Crash on `CHARACTER SET csname COLLATE DEFAULT` in column definition
+#
+CREATE TABLE t1 (a CHAR(10) CHARACTER SET latin1 COLLATE DEFAULT);
+DROP TABLE t1;
+SELECT CAST('a' AS CHAR(10) CHARACTER SET latin1 COLLATE DEFAULT);
+CAST('a' AS CHAR(10) CHARACTER SET latin1 COLLATE DEFAULT)
+a
+SELECT COLUMN_GET(COLUMN_CREATE(0, 'string'),0 AS CHAR CHARACTER SET latin1 COLLATE DEFAULT) AS c1;
+c1
+string
+#
+# End of 10.2 tests
+#

mysql-test/t/ctype_collate.test

@@ -339,3 +339,20 @@ select hex(b) from t1 where b<'zzz' order by b;
 
 drop table t1;
 
+--echo #
+--echo # Start of 10.2 tests
+--echo #
+
+--echo #
+--echo # MDEV-27690 Crash on `CHARACTER SET csname COLLATE DEFAULT` in column definition
+--echo #
+
+CREATE TABLE t1 (a CHAR(10) CHARACTER SET latin1 COLLATE DEFAULT);
+DROP TABLE t1;
+SELECT CAST('a' AS CHAR(10) CHARACTER SET latin1 COLLATE DEFAULT);
+SELECT COLUMN_GET(COLUMN_CREATE(0, 'string'),0 AS CHAR CHARACTER SET latin1 COLLATE DEFAULT) AS c1;
+
+
+--echo #
+--echo # End of 10.2 tests
+--echo #

sql/sql_yacc.yy

@@ -1916,6 +1916,7 @@ bool my_yyoverflow(short **a, YYSTYPE **b, size_t *yystacksize);
 
 %type <charset>
         opt_collate
+        collate
         charset_name
         charset_or_alias
         charset_name_or_default
@@ -6776,10 +6777,7 @@ charset_or_alias:
           }
         ;
 
-collate: COLLATE_SYM collation_name_or_default
-         {
-           Lex->charset= Lex->last_field->charset= $2;
-         }
+collate: COLLATE_SYM collation_name_or_default { $$= $2; }
        ;
 
 opt_binary:
@@ -6790,11 +6788,17 @@ opt_binary:
         | BINARY charset_or_alias { bincmp_collation($2, true); }
         | charset_or_alias collate
           {
-            if (!my_charset_same(Lex->charset, $1))
-              my_yyabort_error((ER_COLLATION_CHARSET_MISMATCH, MYF(0),
-                                Lex->charset->name, $1->csname));
+            if (!$2)
+              Lex->charset= $1; // CHARACTER SET cs COLLATE DEFAULT
+            else
+            {
+              if (!my_charset_same($2, $1))
+                my_yyabort_error((ER_COLLATION_CHARSET_MISMATCH, MYF(0),
+                                  $2->name, $1->csname));
+              Lex->charset= $2;
+            }
           }
-        | collate { }
+        | collate { Lex->charset= $1; }
         ;
 
 opt_bin_mod: