Shulou(Shulou.com)06/01 Report--

PostgreSQL will do checkpoint when shutdown. The process is as follows.

1. In the main process, a signal processing function reaper is registered first, which is used to send signals to child processes such as checkpoint. Send SIGUSR2 signals to the checkpoint process

PostmasterMain (int argc, char argv [])

Pqsignal_no_restart (SIGCHLD, reaper); / handle child termination * /

Reaper:

While ((pid = waitpid (- 1, & exitstatus, WNOHANG)) > 0) {

...

If (pid = = CheckpointerPID) {

...

SignalChildren (SIGUSR2)

}

...

}

...

PostmasterStateMachine (); / / send a SIGUSR2 signal to the checkpoint process

|

| | if (pmState = = PM_WAIT_BACKENDS) {|

| | if (CountChildren (BACKEND_TYPE_NORMAL | BACKEND_TYPE_WORKER)) = = 0 &

| | StartupPID = = 0 & &

| | WalReceiverPID = = 0 & &

| | BgWriterPID = = 0 & &

| (CheckpointerPID = = 0 | | (! FatalError & & Shutdown)

= ImmediateShutdown | | FatalError) {

| | pmState = PM_WAIT_DEAD_END |

| |} else {|

| | if (CheckpointerPID! = 0) {|

| | signal_child (CheckpointerPID, SIGUSR2) |

| | pmState = PM_SHUTDOWN |

| |}

}

| |}

| | -} |

2. The checkpoint process will also register a signal processing function ReqShutdownHandler, which is used to process the SIGUSR2 signal sent by the main process. When the signal is received, shutdown_requested is set to TRUE. In the for loop of the checkpoint process, if shutdown_requested is TRUE, enter the shutdown process: stop each sender process, and after all sender processes stop, checkpoint:CreateRestartPoint the latter CreateCheckPoint according to the conditions

CheckpointerMain (void)->

/ / install the signal processing function ReqShutdownHandler for the signal SIGUSR2

Pqsignal (SIGUSR2, ReqShutdownHandler); / request shutdown /

...

For (;;) {

If (shutdown_requested) {

/ Close down the database /

ShutdownXLOG (0,0)

| |-- WalSndInitStopping (); / / send a signal to each sender process to modify the stopping status |

| | WalSndWaitStopping () |

| | if (RecoveryInProgress ()) |

| | CreateRestartPoint (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE) |

| | else {|

| | if (XLogArchivingActive () & & XLogArchiveCommandSet ()) |

| | RequestXLogSwitch (false) |

| | CreateCheckPoint (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE) |

| |}

|--.

Proc_exit (0)

}

...

Checkpoint

}

3. The ReqShutdownHandler function sets shutdown_requested to TRUE.

ReqShutdownHandler (SIGNAL_ARGS)

{

Shutdown_requested = true

SetLatch (MyLatch)

}

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