MySQL  8.0.17
Source Code Documentation
sys_vars_resource_mgr.h
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1 #ifndef SYS_VARS_RESOURCE_MGR_INCLUDED
2 #define SYS_VARS_RESOURCE_MGR_INCLUDED
3 
4 /* Copyright (c) 2014, 2017, Oracle and/or its affiliates. All rights reserved.
5 
6  This program is free software; you can redistribute it and/or modify
7  it under the terms of the GNU General Public License, version 2.0,
8  as published by the Free Software Foundation.
9 
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13  documentation. The authors of MySQL hereby grant you an additional
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15  separately licensed software that they have included with MySQL.
16 
17  This program is distributed in the hope that it will be useful,
18  but WITHOUT ANY WARRANTY; without even the implied warranty of
19  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
20  GNU General Public License, version 2.0, for more details.
21 
22  You should have received a copy of the GNU General Public License
23  along with this program; if not, write to the Free Software
24  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
25 
26 /**
27  Session_sysvar_resource_manager
28  -------------------------------
29  When a session (THD) gets initialized, it receives a shallow copy of all
30  global system variables.
31  thd->variables= global_system_variables; (see plugin_thdvar_init())
32 
33  In case of Sys_var_charptr variables, we need to maintain a separate copy for
34  each session though so that global and session variables can be altered
35  independently.
36 
37  This class is responsible for alloc|dealloc-ating memory for Sys_var_charptr
38  variables for every session. It works in three steps :
39 
40  (1) init :
41  Creates a copy (memdup()) of global Sys_var_charptr system variable for
42  the respective session variable (passed as a parameter) & inserts it
43  into sysvar_string_alloc_hash (containing the alloced address) to infer
44  that memory has been allocated for the session. init() is called during
45  the initialization of session system variables. (plugin_thdvar_init())
46  (2) update :
47  When the session variable is updated, the old memory is freed and new
48  memory is allocated to hold the new value. The corresponding member in
49  sysvar_string_alloc_hash is also updated to hold the new alloced memory
50  address. (Sys_var_charptr::session_update())
51  (3) deinit :
52  Its a one-shot operation to free all the session Sys_var_charptr system
53  variables. It basically traverses down the sysvar_string_alloc_hash
54  hash and calls free() for all the addresses that it holds.
55 
56  Note, there should always be at most one node per Sys_var_charptr session
57  system variable.
58 
59 */
60 
61 #include <stddef.h>
62 #include <memory>
63 
64 #include "map_helpers.h"
65 #include "sql/psi_memory_key.h"
66 
68  private:
69  // The value always contains the string that the key points to.
73 
74  public:
75  /**
76  Allocates memory for Sys_var_charptr session variable during session
77  initialization.
78  */
79  bool init(char **var);
80 
81  /**
82  Frees the old alloced memory, memdup()'s the given val to a new memory
83  address & updated the session variable pointer.
84  */
85  bool update(char **var, char *val, size_t val_len);
86 
88 
89  /**
90  Frees the memory allocated for Sys_var_charptr session variables.
91  */
92  void deinit();
93 };
94 
95 #endif /* SYS_VARS_RESOURCE_MGR_INCLUDED */
bool init(char **var)
Allocates memory for Sys_var_charptr session variable during session initialization.
Definition: sys_vars_resource_mgr.cc:46
PSI_memory_key key_memory_THD_Session_sysvar_resource_manager
Definition: psi_memory_key.cc:89
malloc_unordered_map< char **, unique_ptr_my_free< char > > m_sysvar_string_alloc_hash
Definition: sys_vars_resource_mgr.h:71
std::unordered_map, but with my_malloc, so that you can track the memory used using PSI memory keys...
Definition: map_helpers.h:175
bool update(char **var, char *val, size_t val_len)
Frees the old alloced memory, memdup()&#39;s the given val to a new memory address & updated the session ...
Definition: sys_vars_resource_mgr.cc:72
Session_sysvar_resource_manager When a session (THD) gets initialized, it receives a shallow copy of ...
Definition: sys_vars_resource_mgr.h:67
void deinit()
Frees the memory allocated for Sys_var_charptr session variables.
Definition: sys_vars_resource_mgr.cc:110
void claim_memory_ownership()
Definition: sys_vars_resource_mgr.cc:99