MDEV-30164 System variable for default collationsbb-10.11-bar-collations

This patch adds a way to override default collations
(or "character set collations") for desired character sets.

The SQL standard says:
> Each collation known in an SQL-environment is applicable to one
> or more character sets, and for each character set, one or more
> collations are applicable to it, one of which is associated with
> it as its character set collation.

In MariaDB, character set collations has been hard-coded so far,
e.g. utf8mb4_general_ci has been a hard-coded character set collation
for utf8mb4.

This patch allows to override (globally per server, or per session)
character set collations, so for example, uca1400_ai_ci can be set as a
character set collation for Unicode character sets
(instead of compiled xxx_general_ci).

The array of overridden character set collations is stored in a new
(session and global) system variable @@character_set_collations and
can be set as a comma separated list of charset=collation pairs, e.g.:

SET @@character_set_collations='utf8mb3=uca1400_ai_ci,utf8mb4=uca1400_ai_ci';

The variable is empty by default, which mean use the hard-coded
character set collations (e.g. utf8mb4_general_ci for utf8mb4).

The variable can also be set globally by passing to the server startup command
line, and/or in my.cnf.

1 files changed, 22 insertions, 0 deletions

diff --git a/sql/mysqld.cc b/sql/mysqld.cc
index 23f34dd0d84..f62f0dc1515 100644
--- a/sql/mysqld.cc
+++ b/sql/mysqld.cc
@@ -342,6 +342,7 @@ char *enforced_storage_engine=NULL;
 char *gtid_pos_auto_engines;
 plugin_ref *opt_gtid_pos_auto_plugins;
 static char compiled_default_collation_name[]= MYSQL_DEFAULT_COLLATION_NAME;
+static const char *character_set_collations_str= "";
 Thread_cache thread_cache;
 static bool binlog_format_used= false;
 LEX_STRING opt_init_connect, opt_init_slave;
@@ -4271,6 +4272,18 @@ static int init_common_variables()
   */
   myf utf8_flag= global_system_variables.old_behavior &
                  OLD_MODE_UTF8_IS_UTF8MB3 ? MY_UTF8_IS_UTF8MB3 : 0;
+
+  if (character_set_collations_str[0])
+  {
+    Lex_cstring_strlen str(character_set_collations_str);
+    if (global_system_variables.character_set_collations.
+                                  from_text(str, utf8_flag))
+    {
+      sql_print_error(ER_DEFAULT(ER_WRONG_VALUE_FOR_VAR),
+                      "character_set_collations", character_set_collations_str);
+    }
+  }
+
   for (;;)
   {
     char *next_character_set_name= strchr(default_character_set_name, ',');
@@ -4289,7 +4302,13 @@ static int init_common_variables()
         return 1;                           // Eof of the list
     }
     else
+    {
+      Charset_collation_map_st::Used used;
+      default_charset_info= global_system_variables.character_set_collations.
+                              get_collation_for_charset(&used,
+                                                        default_charset_info);
       break;
+    }
   }
 
   if (default_collation_name)
@@ -6469,6 +6488,9 @@ struct my_option my_long_options[]=
   {"collation-server", 0, "Set the default collation.",
    &default_collation_name, &default_collation_name,
    0, GET_STR, REQUIRED_ARG, 0, 0, 0, 0, 0, 0 },
+  {"character-set-collations", 0, "Set default collations for character sets.",
+   &character_set_collations_str, &character_set_collations_str,
+   0, GET_STR, REQUIRED_ARG, 0, 0, 0, 0, 0, 0 },
   {"console", OPT_CONSOLE, "Write error output on screen; don't remove the console window on windows.",
    &opt_console, &opt_console, 0, GET_BOOL, NO_ARG, 0, 0, 0,
    0, 0, 0},