PG守护进程（Postmaster）——Postmaster总体流程

PG守护进程（Postmaster）——Postmaster总体流程

Postmaster启动

第一阶段启动

Postgres可以以一下三种模式启动：

Bootstrap模式：从无到有创建数据库的模式，postgres --boot -x1 -k -FSingle模式：单用户模式，只允许单个用户执行SQL命令。Bootstrap创建了最核心的元数据之后使用single模式创建其他数据，对应着postbootstrap阶段Normal模式：多用户的正常模式

/src/backend/main/main.c/* Any Postgres server process begins execution here. */int main(int argc, char *argv[]) { ... if (argc > 1 && strcmp(argv[1], "--boot") == 0) AuxiliaryProcessMain(argc, argv); /* does not return */ else if (argc > 1 && strcmp(argv[1], "--describe-config") == 0) GucInfoMain(); /* does not return */ else if (argc > 1 && strcmp(argv[1], "--single") == 0) PostgresMain(argc, argv, NULL, /* no dbname */strdup(get_user_name_or_exit(progname)));/* does not return */ else PostmasterMain(argc, argv); /* does not return */ abort(); /* should not get here */}

第二阶段Normal模式

Normal模式调用在PostmasterMain(argc, argv)

/src/backend/postmaster/postmaster.c/* Postmaster main entry point */void PostmasterMain(int argc, char *argv[]){ ... /* We're ready to rock and roll... */ StartupPID = StartupDataBase(); Assert(StartupPID != 0); StartupStatus = STARTUP_RUNNING; pmState = PM_STARTUP; /* Some workers may be scheduled to start now */ maybe_start_bgworkers(); status = ServerLoop(); /* ServerLoop probably shouldn't ever return, but if it does, close down. */ ExitPostmaster(status != STATUS_OK); abort(); /* not reached */}

第三阶段服务循环

ServerLoop循环等待客户端连接，调用select函数阻塞等待客户端连接

static int ServerLoop(void) { fd_set readmask; int nSockets; time_t last_lockfile_recheck_time, last_touch_time; last_lockfile_recheck_time = last_touch_time = time(NULL); nSockets = initMasks(&readmask); for (;;) { fd_set rmask; int selres; time_t now; /* Wait for a connection request to arrive. * We block all signals except while sleeping. That makes it safe for * signal handlers, which again block all signals while executing, to * do nontrivial work. * If we are in PM_WAIT_DEAD_END state, then we don't want to accept * any new connections, so we don't call select(), and just sleep. */ memcpy((char *) &rmask, (char *) &readmask, sizeof(fd_set)); if (pmState == PM_WAIT_DEAD_END) { PG_SETMASK(&UnBlockSig); pg_usleep(100000L); /* 100 msec seems reasonable */ selres = 0; PG_SETMASK(&BlockSig); }else{ /* must set timeout each time; some OSes change it! */ struct timeval timeout; /* Needs to run with blocked signals! */ DetermineSleepTime(&timeout); PG_SETMASK(&UnBlockSig); selres = select(nSockets, &rmask, NULL, NULL, &timeout); PG_SETMASK(&BlockSig); } /* Now check the select() result */ if (selres < 0){ if (errno != EINTR && errno != EWOULDBLOCK){ ereport(LOG,(errcode_for_socket_access(),errmsg("select() failed in postmaster: %m"))); return STATUS_ERROR; } }

如果有客户端连接进来，fork子进程来处理该连接。

/* New connection pending on any of our sockets? If so, fork a child process to deal with it. */ if (selres > 0) { int i; for (i = 0; i < MAXLISTEN; i++) { if (ListenSocket[i] == PGINVALID_SOCKET) break; if (FD_ISSET(ListenSocket[i], &rmask)) { Port *port; port = ConnCreate(ListenSocket[i]); if (port) { BackendStartup(port); /* We no longer need the open socket or port structure in this process */ StreamClose(port->sock); ConnFree(port); } } } } ... }}

第四阶段启动postgres子进程

/* BackendStartup -- start backend process * returns: STATUS_ERROR if the fork failed, STATUS_OK otherwise. * Note: if you change this code, also consider StartAutovacuumWorker. */static int BackendStartup(Port *port) { Backend *bn; /* for backend cleanup */ pid_t pid; /* Create backend data structure. Better before the fork() so we can handle failure cleanly. */ bn = (Backend *) malloc(sizeof(Backend)); if (!bn){ ereport(LOG,(errcode(ERRCODE_OUT_OF_MEMORY),errmsg("out of memory"))); return STATUS_ERROR; } /* Compute the cancel key that will be assigned to this backend. The * backend will have its own copy in the forked-off process' value of * MyCancelKey, so that it can transmit the key to the frontend. */ if (!RandomCancelKey(&MyCancelKey)){ free(bn); ereport(LOG,(errcode(ERRCODE_INTERNAL_ERROR),errmsg("could not generate random cancel key"))); return STATUS_ERROR; } bn->cancel_key = MyCancelKey; /* Pass down canAcceptConnections state */ port->canAcceptConnections = canAcceptConnections(BACKEND_TYPE_NORMAL); bn->dead_end = (port->canAcceptConnections != CAC_OK && port->canAcceptConnections != CAC_SUPERUSER); /* Unless it's a dead_end child, assign it a child slot number */ if (!bn->dead_end) bn->child_slot = MyPMChildSlot = AssignPostmasterChildSlot(); else bn->child_slot = 0; /* Hasn't asked to be notified about any bgworkers yet */ bn->bgworker_notify = false;#ifdef EXEC_BACKEND pid = backend_forkexec(port);#else /* !EXEC_BACKEND */ pid = fork_process(); if (pid == 0) /* child */ { free(bn); /* Detangle from postmaster */ InitPostmasterChild(); /* Close the postmaster's sockets */ ClosePostmasterPorts(false); /* Perform additional initialization and collect startup packet */ BackendInitialize(port); /* And run the backend */ BackendRun(port); }#endif /* EXEC_BACKEND */ if (pid < 0){ /* in parent, fork failed */ int save_errno = errno; if (!bn->dead_end) (void) ReleasePostmasterChildSlot(bn->child_slot); free(bn); errno = save_errno; ereport(LOG, (errmsg("could not fork new process for connection: %m"))); report_fork_failure_to_client(port, save_errno); return STATUS_ERROR; } /* in parent, successful fork */ ereport(DEBUG2,(errmsg_internal("forked new backend, pid=%d socket=%d",(int) pid, (int) port->sock))); /* Everything's been successful, it's safe to add this backend to our list of backends. */ bn->pid = pid; bn->bkend_type = BACKEND_TYPE_NORMAL; /* Can change later to WALSND */ dlist_push_head(&BackendList, &bn->elem);#ifdef EXEC_BACKEND if (!bn->dead_end) ShmemBackendArrayAdd(bn);#endif return STATUS_OK;}

第五阶段启动postgres子进程执行PostgresMain

/* BackendRun -- set up the backend's argument list and invoke PostgresMain() * returns: * Shouldn't return at all. * If PostgresMain() fails, return status. */static void BackendRun(Port *port) { char **av; int maxac; int ac; int i; /* Now, build the argv vector that will be given to PostgresMain. * The maximum possible number of commandline arguments that could come * from ExtraOptions is (strlen(ExtraOptions) + 1) / 2; see * pg_split_opts(). */ maxac = 2; /* for fixed args supplied below */ maxac += (strlen(ExtraOptions) + 1) / 2; av = (char **) MemoryContextAlloc(TopMemoryContext, maxac * sizeof(char *)); ac = 0; av[ac++] = "postgres"; /* Pass any backend switches specified with -o on the postmaster's own * command line. We assume these are secure. */ pg_split_opts(av, &ac, ExtraOptions); av[ac] = NULL; Assert(ac < maxac); /* Debug: print arguments being passed to backend */ ereport(DEBUG3,(errmsg_internal("%s child[%d]: starting with (",progname, (int) getpid()))); for (i = 0; i < ac; ++i) ereport(DEBUG3,(errmsg_internal("\t%s", av[i]))); ereport(DEBUG3, (errmsg_internal(")"))); /* Make sure we aren't in PostmasterContext anymore. (We can't delete it * just yet, though, because InitPostgres will need the HBA data.) */ MemoryContextSwitchTo(TopMemoryContext); PostgresMain(ac, av, port->database_name, port->user_name);}

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