AuxiliaryProcessMain函数是后台一等公民进程的入口,我们来看一看哪里调用了该入口函数:
- src/backend/main/main.c main函数,如下图所示,argv第二个参数为–boot
- src/backend/postmaster/postmaster.c SubPostmasterMain函数 argv第二个参数为–forkboot
- src/backend/postmaster/postmaster.c StartChildProcess函数,我们以该函数为主要讲解点。主要看这个分支流程,上面两个分支流程不重要。
调用链:StartChildProcess --> AuxiliaryProcessMain
StartChildProcess函数
StartChildProcess函数为postmaster启动一个an auxiliary process。为AuxiliaryProcessMain拼接输入参数数组argv:“postgres -x%d”。fork一个子进程,调用函数AuxiliaryProcessMain(ac, av)。
static pid_t StartChildProcess(AuxProcType type) {
pid_t pid;
char *av[10]; int ac = 0;
char typebuf[32];
/* Set up command-line arguments for subprocess */
av[ac++] = "postgres";
snprintf(typebuf, sizeof(typebuf), "-x%d", type);
av[ac++] = typebuf;
av[ac] = NULL;
Assert(ac < lengthof(av));
pid = fork_process();
if (pid == 0) { /* child 子进程 */
InitPostmasterChild();
ClosePostmasterPorts(false); /* Close the postmaster's sockets */
/* Release postmaster's working memory context */
MemoryContextSwitchTo(TopMemoryContext);
MemoryContextDelete(PostmasterContext);
PostmasterContext = NULL;
AuxiliaryProcessMain(ac, av);
ExitPostmaster(0);
}
if (pid < 0) {
/* in parent, fork failed */
int save_errno = errno; errno = save_errno;
switch (type) {
case StartupProcess:
ereport(LOG,(errmsg("could not fork startup process: %m"))); break;
case BgWriterProcess:
ereport(LOG, (errmsg("could not fork background writer process: %m"))); break;
case CheckpointerProcess:
ereport(LOG, (errmsg("could not fork checkpointer process: %m"))); break;
case WalWriterProcess:
ereport(LOG, (errmsg("could not fork WAL writer process: %m"))); break;
case WalReceiverProcess:
ereport(LOG, (errmsg("could not fork WAL receiver process: %m"))); break;
default:
ereport(LOG, (errmsg("could not fork process: %m")));
break;
}
/* fork failure is fatal during startup, but there's no need to choke immediately if starting other child types fails. 如果是StartupProcess子进程fork失败,fatal级别的错误,需要调用ExitPostmaster(1) */
if (type == StartupProcess) ExitPostmaster(1);
return 0;
}
/*
* in parent, successful fork
*/
return pid;
}
AuxProcType的类型如下,下面就是后台一等公民进程,和PG的执行逻辑紧密耦合。
后台一等公民进程
第一组一等公民:StartupProcess、BgWriterProcess、CheckpointerProcess、WalWriterProcess、WalReceiverProcess。调用流程如下:
- PostmasterMain --> StartupDataBase
- pmdie/reaper/sigusr1_handler -> PostmasterStateMachine --> StartupDataBase
- ServerLoop(postmaster.c)/reaper/sigusr1_handler --> StartBackgroundWriter
- ServerLoop(postmaster.c)/reaper/PostmasterStateMachine/sigusr1_handler --> StartCheckpointer
- ServerLoop(postmaster.c)/reaper --> StartWalWriter
- ServerLoop(postmaster.c)/sigusr1_handler --> MaybeStartWalReceiver --> StartWalReceiver
第二组一等公民:CheckerProcess、BootstrapProcess
- AuxiliaryProcessMain -> CheckerProcess
- AuxiliaryProcessMain -> BootstrapProcess