PostgreSQL在shutdown时会进行checkpoint。其流程如下。

1、在主进程中，会首先注册一个信号处理函数reaper，用于向checkpoint等子进程发送信号。向checkpoint进程发送SIGUSR2信号

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进程发送SIGUSR2信号
|-----------------------
| 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不会向checkpoint进程发送信号，即不会做checkpoint
| if (Shutdown >= ImmediateShutdown || FatalError){
| pmState = PM_WAIT_DEAD_END;
| }else{
| if (CheckpointerPID != 0){
| signal_child(CheckpointerPID, SIGUSR2);
| pmState = PM_SHUTDOWN;
| }
}
| }
|-------}
2、checkpoint进程，也会注册一个信号处理函数ReqShutdownHandler，用于处理主进程发送过来的SIGUSR2信号。接收到该信号后将shutdown_requested置为TRUE。在checkpoint进程的for循环中，如果shutdown_requested为TRUE，则进入shutdown流程：stop每个sender进程，所有sender进程stop后，根据条件进行checkpoint：CreateRestartPoint后者CreateCheckPoint

CheckpointerMain(void)->
//为信号SIGUSR2安装信号处理函数ReqShutdownHandler
pqsignal(SIGUSR2, ReqShutdownHandler); / request shutdown /
...
for (;;){
if (shutdown_requested){
/ Close down the database /
ShutdownXLOG(0, 0);
|-- WalSndInitStopping();//向每个sender进程发送信号修改到stopping状态
| 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函数将shutdown_requested置为TRUE

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