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