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) 1996-2023, 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 "postmaster/fork_process.h"
      23             : 
      24             : #ifndef WIN32
      25             : /*
      26             :  * Wrapper for fork(). Return values are the same as those for fork():
      27             :  * -1 if the fork failed, 0 in the child process, and the PID of the
      28             :  * child in the parent process.  Signals are blocked while forking, so
      29             :  * the child must unblock.
      30             :  */
      31             : pid_t
      32       30812 : fork_process(void)
      33             : {
      34             :     pid_t       result;
      35             :     const char *oomfilename;
      36             :     sigset_t    save_mask;
      37             : 
      38             : #ifdef LINUX_PROFILE
      39             :     struct itimerval prof_itimer;
      40             : #endif
      41             : 
      42             :     /*
      43             :      * Flush stdio channels just before fork, to avoid double-output problems.
      44             :      */
      45       30812 :     fflush(NULL);
      46             : 
      47             : #ifdef LINUX_PROFILE
      48             : 
      49             :     /*
      50             :      * Linux's fork() resets the profiling timer in the child process. If we
      51             :      * want to profile child processes then we need to save and restore the
      52             :      * timer setting.  This is a waste of time if not profiling, however, so
      53             :      * only do it if commanded by specific -DLINUX_PROFILE switch.
      54             :      */
      55             :     getitimer(ITIMER_PROF, &prof_itimer);
      56             : #endif
      57             : 
      58             :     /*
      59             :      * We start postmaster children with signals blocked.  This allows them to
      60             :      * install their own handlers before unblocking, to avoid races where they
      61             :      * might run the postmaster's handler and miss an important control
      62             :      * signal. With more analysis this could potentially be relaxed.
      63             :      */
      64       30812 :     sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
      65       30812 :     result = fork();
      66       58714 :     if (result == 0)
      67             :     {
      68             :         /* fork succeeded, in child */
      69             : #ifdef LINUX_PROFILE
      70             :         setitimer(ITIMER_PROF, &prof_itimer, NULL);
      71             : #endif
      72             : 
      73             :         /*
      74             :          * By default, Linux tends to kill the postmaster in out-of-memory
      75             :          * situations, because it blames the postmaster for the sum of child
      76             :          * process sizes *including shared memory*.  (This is unbelievably
      77             :          * stupid, but the kernel hackers seem uninterested in improving it.)
      78             :          * Therefore it's often a good idea to protect the postmaster by
      79             :          * setting its OOM score adjustment negative (which has to be done in
      80             :          * a root-owned startup script).  Since the adjustment is inherited by
      81             :          * child processes, this would ordinarily mean that all the
      82             :          * postmaster's children are equally protected against OOM kill, which
      83             :          * is not such a good idea.  So we provide this code to allow the
      84             :          * children to change their OOM score adjustments again.  Both the
      85             :          * file name to write to and the value to write are controlled by
      86             :          * environment variables, which can be set by the same startup script
      87             :          * that did the original adjustment.
      88             :          */
      89       27908 :         oomfilename = getenv("PG_OOM_ADJUST_FILE");
      90             : 
      91       27908 :         if (oomfilename != NULL)
      92             :         {
      93             :             /*
      94             :              * Use open() not stdio, to ensure we control the open flags. Some
      95             :              * Linux security environments reject anything but O_WRONLY.
      96             :              */
      97           0 :             int         fd = open(oomfilename, O_WRONLY, 0);
      98             : 
      99             :             /* We ignore all errors */
     100           0 :             if (fd >= 0)
     101             :             {
     102           0 :                 const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
     103             :                 int         rc;
     104             : 
     105           0 :                 if (oomvalue == NULL)   /* supply a useful default */
     106           0 :                     oomvalue = "0";
     107             : 
     108           0 :                 rc = write(fd, oomvalue, strlen(oomvalue));
     109             :                 (void) rc;
     110           0 :                 close(fd);
     111             :             }
     112             :         }
     113             : 
     114             :         /* do post-fork initialization for random number generation */
     115       27908 :         pg_strong_random_init();
     116             :     }
     117             :     else
     118             :     {
     119             :         /* in parent, restore signal mask */
     120       30806 :         sigprocmask(SIG_SETMASK, &save_mask, NULL);
     121             :     }
     122             : 
     123       58714 :     return result;
     124             : }
     125             : 
     126             : #endif                          /* ! WIN32 */