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) 1996-2021, 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 <time.h>
      16             : #include <sys/stat.h>
      17             : #include <sys/time.h>
      18             : #include <unistd.h>
      19             : 
      20             : #include "postmaster/fork_process.h"
      21             : 
      22             : #ifndef WIN32
      23             : /*
      24             :  * Wrapper for fork(). Return values are the same as those for fork():
      25             :  * -1 if the fork failed, 0 in the child process, and the PID of the
      26             :  * child in the parent process.
      27             :  */
      28             : pid_t
      29       20712 : fork_process(void)
      30             : {
      31             :     pid_t       result;
      32             :     const char *oomfilename;
      33             : 
      34             : #ifdef LINUX_PROFILE
      35             :     struct itimerval prof_itimer;
      36             : #endif
      37             : 
      38             :     /*
      39             :      * Flush stdio channels just before fork, to avoid double-output problems.
      40             :      * Ideally we'd use fflush(NULL) here, but there are still a few non-ANSI
      41             :      * stdio libraries out there (like SunOS 4.1.x) that coredump if we do.
      42             :      * Presently stdout and stderr are the only stdio output channels used by
      43             :      * the postmaster, so fflush'ing them should be sufficient.
      44             :      */
      45       20712 :     fflush(stdout);
      46       20712 :     fflush(stderr);
      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       20712 :     result = fork();
      60       39320 :     if (result == 0)
      61             :     {
      62             :         /* fork succeeded, in child */
      63             : #ifdef LINUX_PROFILE
      64             :         setitimer(ITIMER_PROF, &prof_itimer, NULL);
      65             : #endif
      66             : 
      67             :         /*
      68             :          * By default, Linux tends to kill the postmaster in out-of-memory
      69             :          * situations, because it blames the postmaster for the sum of child
      70             :          * process sizes *including shared memory*.  (This is unbelievably
      71             :          * stupid, but the kernel hackers seem uninterested in improving it.)
      72             :          * Therefore it's often a good idea to protect the postmaster by
      73             :          * setting its OOM score adjustment negative (which has to be done in
      74             :          * a root-owned startup script).  Since the adjustment is inherited by
      75             :          * child processes, this would ordinarily mean that all the
      76             :          * postmaster's children are equally protected against OOM kill, which
      77             :          * is not such a good idea.  So we provide this code to allow the
      78             :          * children to change their OOM score adjustments again.  Both the
      79             :          * file name to write to and the value to write are controlled by
      80             :          * environment variables, which can be set by the same startup script
      81             :          * that did the original adjustment.
      82             :          */
      83       18614 :         oomfilename = getenv("PG_OOM_ADJUST_FILE");
      84             : 
      85       18614 :         if (oomfilename != NULL)
      86             :         {
      87             :             /*
      88             :              * Use open() not stdio, to ensure we control the open flags. Some
      89             :              * Linux security environments reject anything but O_WRONLY.
      90             :              */
      91           0 :             int         fd = open(oomfilename, O_WRONLY, 0);
      92             : 
      93             :             /* We ignore all errors */
      94           0 :             if (fd >= 0)
      95             :             {
      96           0 :                 const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
      97             :                 int         rc;
      98             : 
      99           0 :                 if (oomvalue == NULL)   /* supply a useful default */
     100           0 :                     oomvalue = "0";
     101             : 
     102           0 :                 rc = write(fd, oomvalue, strlen(oomvalue));
     103             :                 (void) rc;
     104           0 :                 close(fd);
     105             :             }
     106             :         }
     107             : 
     108             :         /* do post-fork initialization for random number generation */
     109       18614 :         pg_strong_random_init();
     110             :     }
     111             : 
     112       39320 :     return result;
     113             : }
     114             : 
     115             : #endif                          /* ! WIN32 */