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, 28 insertions, 1 deletions

diff --git a/sql/sql_class.h b/sql/sql_class.h
index 54a213d8553..57eb31a3542 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -887,6 +887,8 @@ typedef struct system_variables
   vers_asof_timestamp_t vers_asof_timestamp;
   ulong vers_alter_history;
   my_bool binlog_alter_two_phase;
+
+  Charset_collation_map_st character_set_collations;
 } SV;
 
 /**
@@ -2925,7 +2927,9 @@ public:
 
   typedef uint used_t;
   enum { RAND_USED=1, TIME_ZONE_USED=2, QUERY_START_SEC_PART_USED=4,
-         THREAD_SPECIFIC_USED=8 };
+         THREAD_SPECIFIC_USED=8,
+         CHARACTER_SET_COLLATIONS_USED= 16
+       };
 
   used_t used;
 
@@ -5620,6 +5624,29 @@ public:
 };
 
 
+class Character_set_collations_used: public Charset_collation_map_st::Used
+{
+  THD *m_thd;
+public:
+  Character_set_collations_used(THD *thd)
+   :m_thd(thd)
+  { }
+  ~Character_set_collations_used()
+  {
+    /*
+      Mark THD that the collation map was used,
+      no matter if a compiled or a mapped collation was
+      found during charset->collation resolution.
+      Even if the map was empty, we still need to print
+        SET @@session.character_set_collations='';
+      in mariadb-binlog output.
+    */
+    if (m_used)
+      m_thd->used|= THD::CHARACTER_SET_COLLATIONS_USED;
+  }
+};
+
+
 /*
   Start a new independent transaction for the THD.
   The old one is stored in this object and restored when calling