MDEV-6068 Upgrade removes all changes to 'mysql' database

mysql_upgrade overrode some of the custom changes made to system tables
in order to increase user name length. 
Make ALTER statements involving the affected columns take into account
length changes, as per https://mariadb.com/kb/en/create-user/#user-names

scripts/mysql_system_tables_fix.sql

@@ -21,6 +21,18 @@
 # because these just mean that your tables are already up to date.
 # This script is safe to run even if your tables are already up to date!
 
+# To facilitate custom changes to system tables that might have been done 
+# (as per MDEV-4332, MDEV-6068 and https://mariadb.com/kb/en/create-user/#user-names),
+# we need to preserve non-default column length for user name fields.
+# These variables will be further used to alter system tables.
+# We presume that the set of ALTERs, if it was run, was run in full,
+# so either all columns were modified, or none was. 
+
+SELECT character_maximum_length, IF(character_maximum_length = 16,77,141) 
+  FROM INFORMATION_SCHEMA.COLUMNS
+  WHERE table_schema = 'mysql' AND table_name = 'user' AND column_name = 'user'
+  INTO @user_name_length, @definer_name_length;
+
 set sql_mode='';
 set storage_engine=MyISAM;
 
@@ -56,14 +68,18 @@ ALTER TABLE user MODIFY ssl_type enum('','ANY','X509', 'SPECIFIED') NOT NULL;
 ALTER TABLE tables_priv
   ADD KEY Grantor (Grantor);
 
+SET @alter_statement = CONCAT("
 ALTER TABLE tables_priv
   MODIFY Host char(60) NOT NULL default '',
   MODIFY Db char(64) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
+  MODIFY User char(", @user_name_length, ") NOT NULL default '',
   MODIFY Table_name char(64) NOT NULL default '',
-  MODIFY Grantor char(77) NOT NULL default '',
+  MODIFY Grantor char(", @definer_name_length, ") NOT NULL default '',
   ENGINE=MyISAM,
-  CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
+  CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin
+");
+PREPARE alter_stmt FROM @alter_statement;
+EXECUTE alter_stmt;
 
 ALTER TABLE tables_priv
   MODIFY Column_priv set('Select','Insert','Update','References')
@@ -84,15 +100,19 @@ ALTER TABLE columns_priv
   CHANGE Type Column_priv set('Select','Insert','Update','References')
     COLLATE utf8_general_ci DEFAULT '' NOT NULL;
 
+SET @alter_statement = CONCAT("
 ALTER TABLE columns_priv
   MODIFY Host char(60) NOT NULL default '',
   MODIFY Db char(64) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
+  MODIFY User char(", @user_name_length, ") NOT NULL default '',
   MODIFY Table_name char(64) NOT NULL default '',
   MODIFY Column_name char(64) NOT NULL default '',
   ENGINE=MyISAM,
   CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin,
-  COMMENT='Column privileges';
+  COMMENT='Column privileges'
+");
+PREPARE alter_stmt FROM @alter_statement;
+EXECUTE alter_stmt;
 
 ALTER TABLE columns_priv
   MODIFY Column_priv set('Select','Insert','Update','References')
@@ -156,10 +176,16 @@ alter table func comment='User defined functions';
 
 # Convert all tables to UTF-8 with binary collation
 # and reset all char columns to correct width
+
+SET @alter_statement = CONCAT("
 ALTER TABLE user
   MODIFY Host char(60) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
-  ENGINE=MyISAM, CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
+  MODIFY User char(", @user_name_length, ") NOT NULL default '',
+  ENGINE=MyISAM, CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin
+");
+PREPARE alter_stmt FROM @alter_statement;
+EXECUTE alter_stmt;
+
 ALTER TABLE user
   MODIFY Password char(41) character set latin1 collate latin1_bin NOT NULL default '',
   MODIFY Select_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL,
@@ -185,11 +211,16 @@ ALTER TABLE user
   MODIFY Repl_client_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL,
   MODIFY ssl_type enum('','ANY','X509', 'SPECIFIED') COLLATE utf8_general_ci DEFAULT '' NOT NULL;
 
+SET @alter_statement = CONCAT("
 ALTER TABLE db
   MODIFY Host char(60) NOT NULL default '',
   MODIFY Db char(64) NOT NULL default '',
-  MODIFY User char(16) NOT NULL default '',
-  ENGINE=MyISAM, CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;
+  MODIFY User char(", @user_name_length, ") NOT NULL default '',
+  ENGINE=MyISAM, CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin
+");
+PREPARE alter_stmt FROM @alter_statement;
+EXECUTE alter_stmt;
+
 ALTER TABLE db
   MODIFY  Select_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL,
   MODIFY  Insert_priv enum('N','Y') COLLATE utf8_general_ci DEFAULT 'N' NOT NULL,
@@ -439,12 +470,16 @@ ALTER TABLE proc MODIFY name char(64) DEFAULT '' NOT NULL,
 # Correct the character set and collation
 ALTER TABLE proc CONVERT TO CHARACTER SET utf8;
 # Reset some fields after the conversion
+SET @alter_statement = CONCAT("
 ALTER TABLE proc  MODIFY db
                          char(64) collate utf8_bin DEFAULT '' NOT NULL,
                   MODIFY definer
-                         char(77) collate utf8_bin DEFAULT '' NOT NULL,
+                         char(", @definer_name_length, ") collate utf8_bin DEFAULT '' NOT NULL,
                   MODIFY comment
-                         char(64) collate utf8_bin DEFAULT '' NOT NULL;
+                         char(64) collate utf8_bin DEFAULT '' NOT NULL
+");
+PREPARE alter_stmt FROM @alter_statement;
+EXECUTE alter_stmt;
 
 ALTER TABLE proc ADD character_set_client
                      char(32) collate utf8_bin DEFAULT NULL
@@ -641,3 +676,5 @@ UPDATE user SET host=LOWER( host ) WHERE LOWER( host ) <> host;
 # changes was correct
 
 flush privileges;
+DEALLOCATE PREPARE alter_stmt;
+