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) 1996-2025, 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       40950 : 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       40950 :     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       40950 :     sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
      66       40950 :     result = fork();
      67       78328 :     if (result == 0)
      68             :     {
      69             :         /* fork succeeded, in child */
      70       37384 :         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       37384 :         oomfilename = getenv("PG_OOM_ADJUST_FILE");
      92             : 
      93       37384 :         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       37384 :         pg_strong_random_init();
     118             :     }
     119             :     else
     120             :     {
     121             :         /* in parent, restore signal mask */
     122       40944 :         sigprocmask(SIG_SETMASK, &save_mask, NULL);
     123             :     }
     124             : 
     125       78328 :     return result;
     126             : }
     127             : 
     128             : #endif                          /* ! WIN32 */