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, 21 insertions, 0 deletions

diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index b5d93e6fd99..0e35beed8f7 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -6049,6 +6049,27 @@ finish:
       }
       thd->reset_kill_query();
     }
+
+    /*
+      If a non-default collation (in @@character_set_collations)
+      was used during the statement, the mysqlbinlog output for
+      the current statement will contain a sequence like this:
+
+        SET character_set_collations='utf8mb3=utf8mb3_bin';
+        INSERT INTO t1 VALUES (_utf8mb3'test');
+        COMMIT;
+
+      The statment (INSERT in this example) is already in binlog at this point, and the
+      and the "SET character_set_collations" is written inside a
+      Q_CHARACTER_SET_COLLATIONS chunk in its log entry header.
+      The flag CHARACTER_SET_COLLATIONS_USED is not needed any more.
+
+      Let's suppress the flag to avoid a Q_CHARACTER_SET_COLLATIONS chunk
+      inside the COMMIT log entry header - it would be useless and would
+      only waste space in the binary log.
+    */
+    thd->used&= ~THD::CHARACTER_SET_COLLATIONS_USED;
+
     if (unlikely(thd->is_error()) ||
         (thd->variables.option_bits & OPTION_MASTER_SQL_ERROR))
     {