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进程概览

当执行完pg_ctl -l logfile start后，postgres将运行以下进程

• /usr/local/pgsql/bin/postgres
• postgres: checkpointer
• postgres: background writer
• postgres: walwriter
• postgres: autovacuum launcher
• postgres: logical replication launcher
  查询进程状态如下：
  
  其中第一个进程/usr/local/pgsql/bin/postgres为master，后文中会用master指代该进程，其会监听5432端口及本地socket接收新的连接；
  参考postgres文档
  

接受新的连接

新增进程

使用psql test连接后，将会有一个新的进程产生，进程状态如下：

• /usr/local/pgsql/bin/postgres
• postgres: checkpointer
• postgres: background writer
• postgres: walwriter
• postgres: autovacuum launcher
• postgres: logical replication launcher
• postgres: postgres test [local] idle
  

代码示例

通过gdb -p 54382 master进程的调用栈如下

从调用栈中不难看出，master的会在ServerLoop函数中进行循环接收新的连接，并创建新的进程处理该连接。
代码如下：

for (;;){time_t		now;nevents = WaitEventSetWait(pm_wait_set,DetermineSleepTime(),events,lengthof(events),0 /* postmaster posts no wait_events */ );/** Latch set by signal handler, or new connection pending on any of* our sockets? If the latter, fork a child process to deal with it.*/for (int i = 0; i < nevents; i++){// ...if (events[i].events & WL_SOCKET_ACCEPT){ClientSocket s;if (AcceptConnection(events[i].fd, &s) == STATUS_OK)BackendStartup(&s);		// 此函数会创建新的进程并处理}}

BackenStartup函数内容如下：

static int BackendStartup(ClientSocket *client_sock)
{// ...pid = postmaster_child_launch(B_BACKEND,(char *) &startup_data, sizeof(startup_data),client_sock);// ..

忽略其中的细节，不难看出实际上调用了postmaster_child_launch函数，其主要内容如下

pid_t postmaster_child_launch(BackendType child_type,char *startup_data, size_t startup_data_len,ClientSocket *client_sock)
{pid_t		pid;pid = fork_process();if (pid == 0)				/* child */{child_process_kinds[child_type].main_fn(startup_data, startup_data_len);pg_unreachable();		/* main_fn never returns */}
#endif							/* EXEC_BACKEND */return pid;
}

其中child_proccess_kinds定义如下：

child_process_kind child_process_kinds[] = {[B_INVALID] = {"invalid", NULL, false},[B_BACKEND] = {"backend", BackendMain, true},[B_AUTOVAC_LAUNCHER] = {"autovacuum launcher", AutoVacLauncherMain, true},[B_AUTOVAC_WORKER] = {"autovacuum worker", AutoVacWorkerMain, true},[B_BG_WORKER] = {"bgworker", BackgroundWorkerMain, true},/** WAL senders start their life as regular backend processes, and change* their type after authenticating the client for replication.  We list it* here for PostmasterChildName() but cannot launch them directly.*/[B_WAL_SENDER] = {"wal sender", NULL, true},[B_SLOTSYNC_WORKER] = {"slot sync worker", ReplSlotSyncWorkerMain, true},[B_STANDALONE_BACKEND] = {"standalone backend", NULL, false},[B_ARCHIVER] = {"archiver", PgArchiverMain, true},[B_BG_WRITER] = {"bgwriter", BackgroundWriterMain, true},[B_CHECKPOINTER] = {"checkpointer", CheckpointerMain, true},[B_STARTUP] = {"startup", StartupProcessMain, true},[B_WAL_RECEIVER] = {"wal_receiver", WalReceiverMain, true},[B_WAL_SUMMARIZER] = {"wal_summarizer", WalSummarizerMain, true},[B_WAL_WRITER] = {"wal_writer", WalWriterMain, true},[B_LOGGER] = {"syslogger", SysLoggerMain, false},
};

postmaster_child_launch函数调用传入的第一个参数为B_BACKEND，所以新进程将进入BackendMain函数进行处理。从函数调用fork_process，可以看出新的进程为master的子进程。

子进程处理逻辑

新的子进程启动后，对该连接进行权限认证，如果认证通过后，将等待客户端输入sql语句。

权限认证

代码调用的逻辑为
BackendMain->PostgresMain->InitPostgres->PerformAuthentication
代码片段如下：

void BackendMain(char *startup_data, size_t startup_data_len){// ...PostgresMain(MyProcPort->database_name, MyProcPort->user_name);
}
void PostgresMain(const char *dbname, const char *username){sigjmp_buf	local_sigjmp_buf;// .../* Early initialization */BaseInit();InitPostgres(dbname, InvalidOid,	/* database to connect to */username, InvalidOid,	/* role to connect as */(!am_walsender) ? INIT_PG_LOAD_SESSION_LIBS : 0,NULL);			/* no out_dbname */// ...}void InitPostgres(const char *in_dbname, Oid dboid,const char *username, Oid useroid,bits32 flags,char *out_dbname) {bool		bootstrap = IsBootstrapProcessingMode();bool		am_superuser;char	   *fullpath;char		dbname[NAMEDATALEN];int			nfree = 0;elog(DEBUG3, "InitPostgres");InitProcessPhase2();SharedInvalBackendInit(false);ProcSignalInit();/** Perform client authentication if necessary, then figure out our* postgres user ID, and see if we are a superuser.** In standalone mode, autovacuum worker processes and slot sync worker* process, we use a fixed ID, otherwise we figure it out from the* authenticated user name.*/if (bootstrap || AmAutoVacuumWorkerProcess() || AmLogicalSlotSyncWorkerProcess()){...}else if (!IsUnderPostmaster){...}else if (AmBackgroundWorkerProcess()){...}else{/* normal multiuser case */Assert(MyProcPort != NULL);PerformAuthentication(MyProcPort);}...

其中PerformAuthentication为远端客户端登录进行权限认证的函数体。

等待sql输入

完成权限认证后，子进程将进入idle状态，等待sql输入。其调用栈如下：

接收sql后及其执行过程，在下一节我们在进行说明。