MDEV-32043 mariadb-upgrade should remove bundled plugins from mysql.plugin

Also in the startup, lets not "Error" on attempting to install a
mysql.plugin that is already there. We use the 'if_not_exists'
parameter to true to downgrade this to a "Note".

Also corrects: MDEV-32041 "plugin already loaded" should be a Warning, not an Error

mysql-test/main/mysql_upgrade.result

@@ -1058,4 +1058,12 @@ test
 Phase 7/7: Running 'FLUSH PRIVILEGES'
 OK
 set global sql_safe_updates=@orig_sql_safe_updates;
+#
+# MDEV-32043 Remove plugins previously external that are now built in (unix_socket)
+#
+INSERT INTO mysql.plugin SELECT 'unix_socket', 'auth_socket.so'
+    FROM dual WHERE convert(@@version_compile_os using latin1) not in ('Win32', 'Win64', 'Windows');
+# mariadb-upgrade --force --silent 2>&1
+SELECT * FROM mysql.plugin WHERE name='unix_socket';
+name	dl
 # End of 10.4 tests

mysql-test/main/mysql_upgrade.test

@@ -495,4 +495,15 @@ set global sql_safe_updates=ON;
 --remove_file $MYSQLD_DATADIR/mysql_upgrade_info
 set global sql_safe_updates=@orig_sql_safe_updates;
 
+--echo #
+--echo # MDEV-32043 Remove plugins previously external that are now built in (unix_socket)
+--echo #
+
+INSERT INTO mysql.plugin SELECT 'unix_socket', 'auth_socket.so'
+    FROM dual WHERE convert(@@version_compile_os using latin1) not in ('Win32', 'Win64', 'Windows');
+--echo # mariadb-upgrade --force --silent 2>&1
+--exec $MYSQL_UPGRADE --force --silent 2>&1
+SELECT * FROM mysql.plugin WHERE name='unix_socket';
+--remove_file $MYSQLD_DATADIR/mysql_upgrade_info
+
 --echo # End of 10.4 tests

scripts/mysql_system_tables_fix.sql

@@ -180,6 +180,11 @@ UPDATE user
   SET plugin='unix_socket' WHERE plugin='auth_socket';
 DELETE FROM plugin
   WHERE name='auth_socket';
+# Delete plugins that are now inbuilt but might not have been before (MDEV-32043)
+DELETE plugin
+  FROM information_schema.PLUGINS is_p
+  JOIN plugin ON plugin.name = is_p.PLUGIN_NAME
+  WHERE is_p.PLUGIN_LIBRARY IS NULL;
 
 ALTER TABLE user
   MODIFY Password char(41) character set latin1 collate latin1_bin NOT NULL default '',

sql/sql_plugin.cc

@@ -1929,7 +1929,7 @@ static void plugin_load(MEM_ROOT *tmp_root)
       the mutex here to satisfy the assert
     */
     mysql_mutex_lock(&LOCK_plugin);
-    plugin_add(tmp_root, false, &name, &dl, MYF(ME_ERROR_LOG));
+    plugin_add(tmp_root, true, &name, &dl, MYF(ME_ERROR_LOG));
     free_root(tmp_root, MYF(MY_MARK_BLOCKS_FREE));
     mysql_mutex_unlock(&LOCK_plugin);
   }