sys_vars_resource_mgr.h

#ifndef SYS_VARS_RESOURCE_MGR_INCLUDED
#define SYS_VARS_RESOURCE_MGR_INCLUDED

/* Copyright (c) 2014, 2024, Oracle and/or its affiliates.

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   it under the terms of the GNU General Public License, version 2.0,
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   This program is distributed in the hope that it will be useful,
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   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License, version 2.0, for more details.

   You should have received a copy of the GNU General Public License
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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */

/**
  Session_sysvar_resource_manager
  -------------------------------
  When a session (THD) gets initialized, it receives a shallow copy of all
  global system variables.
  thd->variables= global_system_variables; (see plugin_thdvar_init())

  In case of Sys_var_charptr variables, we need to maintain a separate copy for
  each session though so that global and session variables can be altered
  independently.

  This class is responsible for alloc|dealloc-ating memory for Sys_var_charptr
  variables for every session. It works in three steps :

  (1) init :
        Creates a copy (memdup()) of global Sys_var_charptr system variable for
        the respective session variable (passed as a  parameter) & inserts it
        into sysvar_string_alloc_hash (containing the allocated address) to
        infer that memory has been allocated for the session. init() is called
        during the initialization of session system variables.
        (plugin_thdvar_init())
  (2) update :
        When the session variable is updated, the old memory is freed and new
        memory is allocated to hold the new value. The corresponding member in
        sysvar_string_alloc_hash is also updated to hold the new allocated
        memory address.
        (Sys_var_charptr::session_update())
  (3) deinit :
        Its a one-shot operation to free all the session Sys_var_charptr system
        variables. It basically traverses down the sysvar_string_alloc_hash hash
        and calls free() for all the addresses that it holds.

  Note, there should always be at most one node per Sys_var_charptr session
  system variable.

*/

#include <stddef.h>
#include <memory>

#include "map_helpers.h"
#include "sql/psi_memory_key.h"

class Session_sysvar_resource_manager {
 private:
  // The value always contains the string that the key points to.
  malloc_unordered_map<char **, unique_ptr_my_free<char>>
      m_sysvar_string_alloc_hash{
          key_memory_THD_Session_sysvar_resource_manager};

 public:
  /**
    Allocates memory for Sys_var_charptr session variable during session
    initialization.
  */
  bool init(char **var);

  /**
    Frees the old allocated memory, memdup()'s the given val to a new memory
    address & updates the session variable pointer.
  */
  bool update(char **var, char *val, size_t val_len);

  void claim_memory_ownership(bool claim);

  /**
    Frees the memory allocated for Sys_var_charptr session variables.
  */
  void deinit();
};

#endif /* SYS_VARS_RESOURCE_MGR_INCLUDED */