MDEV-14123: .rocksdb folder may break workflow which re-create data directory

Part2: make MyRocks add its directory into @@ignore_db_dirs when starting.

This is necessary because apparently not everybody are using plugin's my.cnf
So load ha_rocksdb.{so,dll} manually and then hit MDEV-12451, MDEV-14461
etc.

sql/sql_show.cc

@@ -787,6 +787,57 @@ static void dispose_db_dir(void *ptr)
 }
 
 
+/*
+  Append an element into @@ignore_db_dirs
+
+  This is a function to be called after regular option processing has been
+  finalized.
+*/
+
+void ignore_db_dirs_append(const char *dirname_arg)
+{
+  char *new_entry_buf;
+  LEX_STRING *new_entry;
+  size_t len= strlen(dirname_arg);
+
+  if (!my_multi_malloc(0,
+                       &new_entry, sizeof(LEX_STRING),
+                       &new_entry_buf, len + 1,
+                       NullS))
+    return;
+
+  memcpy(new_entry_buf, dirname_arg, len+1);
+  new_entry->str = new_entry_buf;
+  new_entry->length= len;
+
+  if (my_hash_insert(&ignore_db_dirs_hash, (uchar *)new_entry))
+  {
+    // Either the name is already there or out-of-memory.
+    my_free(new_entry);
+    return;
+  }
+
+  // Append the name to the option string.
+  size_t curlen= strlen(opt_ignore_db_dirs);
+  // Add one for comma and one for \0.
+  size_t newlen= curlen + len + 1 + 1;
+  char *new_db_dirs;
+  if (!(new_db_dirs= (char*)my_malloc(newlen ,MYF(0))))
+  {
+    // This is not a critical condition
+    return;
+  }
+
+  memcpy(new_db_dirs, opt_ignore_db_dirs, curlen);
+  if (curlen != 0)
+    new_db_dirs[curlen]=',';
+  memcpy(new_db_dirs + (curlen + ((curlen!=0)?1:0)), dirname_arg, len+1);
+
+  if (opt_ignore_db_dirs)
+    my_free(opt_ignore_db_dirs);
+  opt_ignore_db_dirs= new_db_dirs;
+}
+
 bool
 ignore_db_dirs_process_additions()
 {

storage/rocksdb/ha_rocksdb.cc

@@ -111,6 +111,10 @@ bool thd_binlog_filter_ok(const MYSQL_THD thd);
 
 MYSQL_PLUGIN_IMPORT bool my_disable_leak_check;
 
+// Needed in rocksdb_init_func
+void ignore_db_dirs_append(const char *dirname_arg);
+
+
 namespace myrocks {
 
 static st_global_stats global_stats;
@@ -3934,6 +3938,7 @@ static rocksdb::Status check_rocksdb_options_compatibility(
   return status;
 }
 
+
 /*
   Storage Engine initialization function, invoked when plugin is loaded.
 */
@@ -3962,6 +3967,11 @@ static int rocksdb_init_func(void *const p) {
   mysql_mutex_init(rdb_mem_cmp_space_mutex_key, &rdb_mem_cmp_space_mutex,
                    MY_MUTEX_INIT_FAST);
 
+  const char* initial_rocksdb_datadir_for_ignore_dirs= rocksdb_datadir;
+  if (!strncmp(rocksdb_datadir, "./", 2))
+    initial_rocksdb_datadir_for_ignore_dirs += 2;
+  ignore_db_dirs_append(initial_rocksdb_datadir_for_ignore_dirs);
+
 #if defined(HAVE_PSI_INTERFACE)
   rdb_collation_exceptions =
       new Regex_list_handler(key_rwlock_collation_exception_list);

storage/rocksdb/mysql-test/rocksdb/r/mariadb_ignore_dirs.result

@@ -0,0 +1,9 @@
+# 
+# RocksDB plugin adds #rocksdb to ignore_db_dirs
+# 
+select @@ignore_db_dirs;
+@@ignore_db_dirs
+#rocksdb
+select @@ignore_db_dirs;
+@@ignore_db_dirs
+aa,bbb,#rocksdb

storage/rocksdb/mysql-test/rocksdb/suite.opt

@@ -1,2 +1,2 @@
---ignore-db-dirs=#rocksdb --plugin-load=$HA_ROCKSDB_SO --default-storage-engine=rocksdb
+--plugin-load=$HA_ROCKSDB_SO --default-storage-engine=rocksdb
 

storage/rocksdb/mysql-test/rocksdb/t/mariadb_ignore_dirs.test

@@ -0,0 +1,17 @@
+--source include/have_rocksdb.inc
+
+
+--echo # 
+--echo # RocksDB plugin adds #rocksdb to ignore_db_dirs
+--echo # 
+
+select @@ignore_db_dirs;
+
+--let $_mysqld_option=--ignore-db-dirs=aa --ignore-db-dirs=bbb
+--source include/restart_mysqld_with_option.inc
+
+select @@ignore_db_dirs;
+
+--let $_mysqld_option=--ignore-db-dirs=#rocksdb
+--source include/restart_mysqld_with_option.inc
+