对IsUnderPostmaster变量初步学习

开始

在postmaster.c 中的 BackendStartup 中，有如下的代码：

其中定义了 IsUnderPostmaster=true。

而bgwriter 作为 postmaster 的子进程，它的 IsUnderPostmaster 也是为真。

* 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.
*/
MyCancelKey = PostmasterRandom();
bn->cancel_key = MyCancelKey;

/* Pass down canAcceptConnections state */
port->canAcceptConnections = canAcceptConnections();
bn->dead_end = (port->canAcceptConnections != CAC_OK &&
port->canAcceptConnections != CAC_WAITBACKUP);

/*
* 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;

#ifdef EXEC_BACKEND
pid = backend_forkexec(port);
#else                            /* !EXEC_BACKEND */
pid = fork_process();
if (pid == 0)                /* child */
{
free(bn);

/*
* Let's clean up ourselves as the postmaster child, and close the
* postmaster's listen sockets.  (In EXEC_BACKEND case this is all
* done in SubPostmasterMain.)
*/
IsUnderPostmaster = true;        /* we are a postmaster subprocess now */

MyProcPid = getpid();    /* reset MyProcPid */

MyStartTime = time(NULL);

/* We don't want the postmaster's proc_exit() handlers */
on_exit_reset();

/* Close the postmaster's sockets */
ClosePostmasterPorts(false);

/* Perform additional initialization and collect startup packet */
BackendInitialize(port);

/* And run the backend */
proc_exit(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->is_autovacuum = false;
DLInitElem(&bn->elem, bn);
DLAddHead(BackendList, &bn->elem);
#ifdef EXEC_BACKEND
if (!bn->dead_end)
ShmemBackendArrayAdd(bn);
#endif

return STATUS_OK;
}

结束