6.10 Unix Signal Handling in MySQL

On Unix and Unix-like systems, a process can be the recipient of signals sent to it by the root system account or the system account that owns the process. Signals can be sent using the kill command. Some command interpreters associate certain key sequences with signals, such as Control+C to send a SIGINT signal. This section describes how the MySQL server and client programs respond to signals.

• Server Response to Signals

• Client Response to Signals

Server Response to Signals

mysqld responds to signals as follows:

• SIGTERM causes the server to shut down. This is like executing a SHUTDOWN statement without having to connect to the server (which for shutdown requires an account that has the SHUTDOWN privilege).

• SIGHUP causes the server to reload the grant tables and to flush tables, logs, the thread cache, and the host cache. These actions are like various forms of the FLUSH statement. Sending the signal enables the flush operations to be performed without having to connect to the server, which requires a MySQL account that has privileges sufficient for those operations. Prior to MySQL 8.0.20, the server also writes a status report to the error log that has this format:

  Status information:
  
  Current dir: /var/mysql/data/
  Running threads: 4  Stack size: 262144
  Current locks:
  lock: 0x7f742c02c0e0:
  
  lock: 0x2cee2a20:
  :
  lock: 0x207a080:
  
  Key caches:
  default
  Buffer_size:       8388608
  Block_size:           1024
  Division_limit:        100
  Age_limit:             300
  blocks used:             4
  not flushed:             0
  w_requests:              0
  writes:                  0
  r_requests:              8
  reads:                   4
  
  handler status:
  read_key:           13
  read_next:           4
  read_rnd             0
  read_first:         13
  write:               1
  delete               0
  update:              0
  
  Table status:
  Opened tables:        121
  Open tables:          114
  Open files:            18
  Open streams:           0
  
  Memory status:
  <malloc version="1">
  <heap nr="0">
  <sizes>
    <size from="17" to="32" total="32" count="1"/>
    <size from="33" to="48" total="96" count="2"/>
    <size from="33" to="33" total="33" count="1"/>
    <size from="97" to="97" total="6014" count="62"/>
    <size from="113" to="113" total="904" count="8"/>
    <size from="193" to="193" total="193" count="1"/>
    <size from="241" to="241" total="241" count="1"/>
    <size from="609" to="609" total="609" count="1"/>
    <size from="16369" to="16369" total="49107" count="3"/>
    <size from="24529" to="24529" total="98116" count="4"/>
    <size from="32689" to="32689" total="32689" count="1"/>
    <unsorted from="241" to="7505" total="7746" count="2"/>
  </sizes>
  <total type="fast" count="3" size="128"/>
  <total type="rest" count="84" size="195652"/>
  <system type="current" size="690774016"/>
  <system type="max" size="690774016"/>
  <aspace type="total" size="690774016"/>
  <aspace type="mprotect" size="690774016"/>
  </heap>
  :
  <total type="fast" count="85" size="5520"/>
  <total type="rest" count="116" size="316820"/>
  <total type="mmap" count="82" size="939954176"/>
  <system type="current" size="695717888"/>
  <system type="max" size="695717888"/>
  <aspace type="total" size="695717888"/>
  <aspace type="mprotect" size="695717888"/>
  </malloc>
  
  Events status:
  LLA = Last Locked At  LUA = Last Unlocked At
  WOC = Waiting On Condition  DL = Data Locked
  
  Event scheduler status:
  State      : INITIALIZED
  Thread id  : 0
  LLA        : n/a:0
  LUA        : n/a:0
  WOC        : NO
  Workers    : 0
  Executed   : 0
  Data locked: NO
  
  Event queue status:
  Element count   : 0
  Data locked     : NO
  Attempting lock : NO
  LLA             : init_queue:95
  LUA             : init_queue:103
  WOC             : NO
  Next activation : never

• As of MySQL 8.0.19, SIGUSR1 causes the server to flush the error log, general query log, and slow query log. One use for SIGUSR1 is to implement log rotation without having to connect to the server, which requires a MySQL account that has privileges sufficient for those operations. For information about log rotation, see Section 7.4.6, “Server Log Maintenance”.

  The server response to SIGUSR1 is a subset of the response to SIGHUP, enabling SIGUSR1 to be used as a more “lightweight” signal that flushes certain logs without the other SIGHUP effects such as flushing the thread and host caches and writing a status report to the error log.

• SIGINT normally is ignored by the server. Starting the server with the --gdb option installs an interrupt handler for SIGINT for debugging purposes. See Section 7.9.1.4, “Debugging mysqld under gdb”.

Client Response to Signals

MySQL client programs respond to signals as follows:

• The mysql client interprets SIGINT (typically the result of typing Control+C) as instruction to interrupt the current statement if there is one, or to cancel any partial input line otherwise. This behavior can be disabled using the --sigint-ignore option to ignore SIGINT signals.

• Client programs that use the MySQL client library block SIGPIPE signals by default. These variations are possible:

  • Client can install their own SIGPIPE handler to override the default behavior. See Writing C API Threaded Client Programs.

  • Clients can prevent installation of SIGPIPE handlers by specifying the CLIENT_IGNORE_SIGPIPE option to mysql_real_connect() at connect time. See mysql_real_connect().