MDEV-15686: Loading MyRocks plugin back after it has been unloaded causes a crash

- Disallow loading of MyRocks (or any auxilary) plugins after it has been
  unloaded.
- Do it carefully - Plugin's system variables may be accesssed (e.g. default
  value is set) after the first rocksdb_done_func() call but before
  the secon rocksdb_init_func() call.

storage/rocksdb/ha_rocksdb.cc

@@ -3957,14 +3957,25 @@ static rocksdb::Status check_rocksdb_options_compatibility(
   return status;
 }
 
+bool prevent_myrocks_loading= false;
+
 
 /*
   Storage Engine initialization function, invoked when plugin is loaded.
 */
 
 static int rocksdb_init_func(void *const p) {
+
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+  {
+    my_error(ER_INTERNAL_ERROR, MYF(0),
+             "Loading MyRocks plugin after it has been unloaded is not "
+             "supported. Please restart mysqld");
+    DBUG_RETURN(1);
+  }
+
   // Validate the assumption about the size of ROCKSDB_SIZEOF_HIDDEN_PK_COLUMN.
   static_assert(sizeof(longlong) == 8, "Assuming that longlong is 8 bytes.");
 
@@ -4505,12 +4516,27 @@ static int rocksdb_done_func(void *const p) {
   }
 #endif /* HAVE_purify */
 
-  rocksdb_db_options = nullptr;
-  rocksdb_tbl_options = nullptr;
+  /*
+    MariaDB: don't clear rocksdb_db_options and rocksdb_tbl_options.
+    MyRocks' plugin variables refer to them.
+
+    The plugin cannot be loaded again (see prevent_myrocks_loading) but plugin
+    variables are processed before myrocks::rocksdb_init_func is invoked, so
+    they must point to valid memory.
+  */
+  //rocksdb_db_options = nullptr;
+  rocksdb_db_options->statistics = nullptr;
+  //rocksdb_tbl_options = nullptr;
   rocksdb_stats = nullptr;
 
   my_error_unregister(HA_ERR_ROCKSDB_FIRST, HA_ERR_ROCKSDB_LAST);
 
+  /*
+    Prevent loading the plugin after it has been loaded and then unloaded. This
+    doesn't work currently.
+  */
+  prevent_myrocks_loading= true;
+
   DBUG_RETURN(error);
 }
 

storage/rocksdb/ha_rocksdb.h

@@ -1414,4 +1414,5 @@ struct Rdb_inplace_alter_ctx : public my_core::inplace_alter_handler_ctx {
 
 const int MYROCKS_MARIADB_PLUGIN_MATURITY_LEVEL= MariaDB_PLUGIN_MATURITY_GAMMA;
 
+extern bool prevent_myrocks_loading;
 } // namespace myrocks

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

@@ -10,3 +10,11 @@ insert into test.t1 values (1);
 connection default;
 DROP TABLE t1;
 UNINSTALL SONAME 'ha_rocksdb';
+#
+# MDEV-15686: Loading MyRocks plugin back after it has been unloaded causes a crash
+#
+call mtr.add_suppression("Plugin 'ROCKSDB.*' init function returned error.");
+call mtr.add_suppression("Plugin 'ROCKSDB.*' registration as a INFORMATION SCHEMA failed.");
+call mtr.add_suppression("Plugin 'ROCKSDB' registration as a STORAGE ENGINE failed");
+INSTALL SONAME 'ha_rocksdb';
+ERROR HY000: Internal error: Loading MyRocks plugin after it has been unloaded is not supported. Please restart mysqld

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

@@ -26,3 +26,13 @@ connection default;
 # Cleanup
 DROP TABLE t1;
 UNINSTALL SONAME 'ha_rocksdb';
+
+--echo #
+--echo # MDEV-15686: Loading MyRocks plugin back after it has been unloaded causes a crash
+--echo #
+call mtr.add_suppression("Plugin 'ROCKSDB.*' init function returned error.");
+call mtr.add_suppression("Plugin 'ROCKSDB.*' registration as a INFORMATION SCHEMA failed.");
+call mtr.add_suppression("Plugin 'ROCKSDB' registration as a STORAGE ENGINE failed");
+
+--error ER_INTERNAL_ERROR
+INSTALL SONAME 'ha_rocksdb';

storage/rocksdb/rdb_i_s.cc

@@ -142,6 +142,9 @@ static int rdb_i_s_cfstats_fill_table(
 static int rdb_i_s_cfstats_init(void *p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -235,6 +238,9 @@ static int rdb_i_s_dbstats_fill_table(
 static int rdb_i_s_dbstats_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -336,6 +342,8 @@ static int rdb_i_s_perf_context_fill_table(
 static int rdb_i_s_perf_context_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -403,6 +411,9 @@ static int rdb_i_s_perf_context_global_fill_table(
 static int rdb_i_s_perf_context_global_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -1017,6 +1028,9 @@ static int rdb_i_s_ddl_fill_table(my_core::THD *const thd,
 static int rdb_i_s_ddl_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   my_core::ST_SCHEMA_TABLE *schema;
 
   DBUG_ASSERT(p != nullptr);
@@ -1032,6 +1046,9 @@ static int rdb_i_s_ddl_init(void *const p) {
 static int rdb_i_s_cfoptions_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -1047,6 +1064,9 @@ static int rdb_i_s_cfoptions_init(void *const p) {
 static int rdb_i_s_global_info_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -1063,6 +1083,10 @@ static int rdb_i_s_compact_stats_init(void *p) {
   my_core::ST_SCHEMA_TABLE *schema;
 
   DBUG_ENTER_FUNC();
+
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   schema = reinterpret_cast<my_core::ST_SCHEMA_TABLE *>(p);
@@ -1237,6 +1261,9 @@ static int rdb_i_s_index_file_map_fill_table(
 static int rdb_i_s_index_file_map_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -1320,6 +1347,9 @@ static int rdb_i_s_lock_info_fill_table(
 static int rdb_i_s_lock_info_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;
@@ -1450,6 +1480,9 @@ static int rdb_i_s_trx_info_fill_table(
 static int rdb_i_s_trx_info_init(void *const p) {
   DBUG_ENTER_FUNC();
 
+  if (prevent_myrocks_loading)
+    DBUG_RETURN(1);
+
   DBUG_ASSERT(p != nullptr);
 
   my_core::ST_SCHEMA_TABLE *schema;