PostgreSQL checkpoint--shutdown

PostgreSQL在shutdown時會進行checkpoint。其流程如下。

1、在主進程中，會首先註冊一個信號處理函數reaper，用於向checkpoint等子進程發送信號。向checkpoint進程發送SIGUSR2信號

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();//向checkpoint進程發送SIGUSR2信號
|-----------------------
| 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)) &&
| WalWriterPID == 0 &&
| AutoVacPID == 0){
| //pg_ctl stop -m immediate不會向checkpoint進程發送信號，即不會做checkpoint
| if (Shutdown >= ImmediateShutdown || FatalError){
| pmState = PM_WAIT_DEAD_END;
| }else{
| if (CheckpointerPID != 0){
| signal_child(CheckpointerPID, SIGUSR2);
| pmState = PM_SHUTDOWN;
| }
}
| }
|-------}
2、checkpoint進程，也會註冊一個信號處理函數ReqShutdownHandler，用於處理主進程發送過來的SIGUSR2信號。接收到該信號後將shutdown_requested置爲TRUE。在checkpoint進程的for循環中，如果shutdown_requested爲TRUE，則進入shutdown流程：stop每個sender進程，所有sender進程stop後，根據條件進行checkpoint：CreateRestartPoint後者CreateCheckPoint

CheckpointerMain(void)->
//爲信號SIGUSR2安裝信號處理函數ReqShutdownHandler
pqsignal(SIGUSR2, ReqShutdownHandler); / request shutdown /
...
for (;;){
if (shutdown_requested){
/ Close down the database /
ShutdownXLOG(0, 0);
|-- WalSndInitStopping();//向每個sender進程發送信號修改到stopping狀態
| 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、ReqShutdownHandler函數將shutdown_requested置爲TRUE

ReqShutdownHandler(SIGNAL_ARGS)
{
shutdown_requested = true;
SetLatch(MyLatch);
}