Line data    Source code

       1              : /*
       2              :  * fork_process.c
       3              :  *   A simple wrapper on top of fork(). This does not handle the
       4              :  *   EXEC_BACKEND case; it might be extended to do so, but it would be
       5              :  *   considerably more complex.
       6              :  *
       7              :  * Copyright (c) 1996-2026, PostgreSQL Global Development Group
       8              :  *
       9              :  * IDENTIFICATION
      10              :  *    src/backend/postmaster/fork_process.c
      11              :  */
      12              : #include "postgres.h"
      13              : 
      14              : #include <fcntl.h>
      15              : #include <signal.h>
      16              : #include <time.h>
      17              : #include <sys/stat.h>
      18              : #include <sys/time.h>
      19              : #include <unistd.h>
      20              : 
      21              : #include "libpq/pqsignal.h"
      22              : #include "miscadmin.h"
      23              : #include "postmaster/fork_process.h"
      24              : 
      25              : #ifndef WIN32
      26              : /*
      27              :  * Wrapper for fork(). Return values are the same as those for fork():
      28              :  * -1 if the fork failed, 0 in the child process, and the PID of the
      29              :  * child in the parent process.  Signals are blocked while forking, so
      30              :  * the child must unblock.
      31              :  */
      32              : pid_t
      33        26768 : fork_process(void)
      34              : {
      35              :     pid_t       result;
      36              :     const char *oomfilename;
      37              :     sigset_t    save_mask;
      38              : 
      39              : #ifdef LINUX_PROFILE
      40              :     struct itimerval prof_itimer;
      41              : #endif
      42              : 
      43              :     /*
      44              :      * Flush stdio channels just before fork, to avoid double-output problems.
      45              :      */
      46        26768 :     fflush(NULL);
      47              : 
      48              : #ifdef LINUX_PROFILE
      49              : 
      50              :     /*
      51              :      * Linux's fork() resets the profiling timer in the child process. If we
      52              :      * want to profile child processes then we need to save and restore the
      53              :      * timer setting.  This is a waste of time if not profiling, however, so
      54              :      * only do it if commanded by specific -DLINUX_PROFILE switch.
      55              :      */
      56              :     getitimer(ITIMER_PROF, &prof_itimer);
      57              : #endif
      58              : 
      59              :     /*
      60              :      * We start postmaster children with signals blocked.  This allows them to
      61              :      * install their own handlers before unblocking, to avoid races where they
      62              :      * might run the postmaster's handler and miss an important control
      63              :      * signal. With more analysis this could potentially be relaxed.
      64              :      */
      65        26768 :     sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
      66        26768 :     result = fork();
      67        50473 :     if (result == 0)
      68              :     {
      69              :         /* fork succeeded, in child */
      70        23705 :         MyProcPid = getpid();
      71              : #ifdef LINUX_PROFILE
      72              :         setitimer(ITIMER_PROF, &prof_itimer, NULL);
      73              : #endif
      74              : 
      75              :         /*
      76              :          * By default, Linux tends to kill the postmaster in out-of-memory
      77              :          * situations, because it blames the postmaster for the sum of child
      78              :          * process sizes *including shared memory*.  (This is unbelievably
      79              :          * stupid, but the kernel hackers seem uninterested in improving it.)
      80              :          * Therefore it's often a good idea to protect the postmaster by
      81              :          * setting its OOM score adjustment negative (which has to be done in
      82              :          * a root-owned startup script).  Since the adjustment is inherited by
      83              :          * child processes, this would ordinarily mean that all the
      84              :          * postmaster's children are equally protected against OOM kill, which
      85              :          * is not such a good idea.  So we provide this code to allow the
      86              :          * children to change their OOM score adjustments again.  Both the
      87              :          * file name to write to and the value to write are controlled by
      88              :          * environment variables, which can be set by the same startup script
      89              :          * that did the original adjustment.
      90              :          */
      91        23705 :         oomfilename = getenv("PG_OOM_ADJUST_FILE");
      92              : 
      93        23705 :         if (oomfilename != NULL)
      94              :         {
      95              :             /*
      96              :              * Use open() not stdio, to ensure we control the open flags. Some
      97              :              * Linux security environments reject anything but O_WRONLY.
      98              :              */
      99            0 :             int         fd = open(oomfilename, O_WRONLY, 0);
     100              : 
     101              :             /* We ignore all errors */
     102            0 :             if (fd >= 0)
     103              :             {
     104            0 :                 const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
     105              :                 int         rc;
     106              : 
     107            0 :                 if (oomvalue == NULL)   /* supply a useful default */
     108            0 :                     oomvalue = "0";
     109              : 
     110            0 :                 rc = write(fd, oomvalue, strlen(oomvalue));
     111              :                 (void) rc;
     112            0 :                 close(fd);
     113              :             }
     114              :         }
     115              : 
     116              :         /* do post-fork initialization for random number generation */
     117        23705 :         pg_strong_random_init();
     118              :     }
     119              :     else
     120              :     {
     121              :         /* in parent, restore signal mask */
     122        26768 :         sigprocmask(SIG_SETMASK, &save_mask, NULL);
     123              :     }
     124              : 
     125        50473 :     return result;
     126              : }
     127              : 
     128              : #endif                          /* ! WIN32 */