127 lines
3.6 KiB
C
127 lines
3.6 KiB
C
/*
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* fork_process.c
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* A simple wrapper on top of fork(). This does not handle the
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* EXEC_BACKEND case; it might be extended to do so, but it would be
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* considerably more complex.
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*
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* Copyright (c) 1996-2023, PostgreSQL Global Development Group
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*
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* IDENTIFICATION
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* src/backend/postmaster/fork_process.c
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*/
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#include "postgres.h"
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#include <fcntl.h>
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#include <signal.h>
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#include <time.h>
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#include <sys/stat.h>
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#include <sys/time.h>
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#include <unistd.h>
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#include "libpq/pqsignal.h"
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#include "postmaster/fork_process.h"
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#ifndef WIN32
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/*
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* Wrapper for fork(). Return values are the same as those for fork():
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* -1 if the fork failed, 0 in the child process, and the PID of the
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* child in the parent process. Signals are blocked while forking, so
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* the child must unblock.
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*/
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pid_t
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fork_process(void)
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{
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pid_t result;
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const char *oomfilename;
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sigset_t save_mask;
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#ifdef LINUX_PROFILE
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struct itimerval prof_itimer;
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#endif
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/*
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* Flush stdio channels just before fork, to avoid double-output problems.
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*/
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fflush(NULL);
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#ifdef LINUX_PROFILE
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/*
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* Linux's fork() resets the profiling timer in the child process. If we
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* want to profile child processes then we need to save and restore the
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* timer setting. This is a waste of time if not profiling, however, so
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* only do it if commanded by specific -DLINUX_PROFILE switch.
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*/
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getitimer(ITIMER_PROF, &prof_itimer);
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#endif
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/*
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* We start postmaster children with signals blocked. This allows them to
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* install their own handlers before unblocking, to avoid races where they
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* might run the postmaster's handler and miss an important control
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* signal. With more analysis this could potentially be relaxed.
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*/
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sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
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result = fork();
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if (result == 0)
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{
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/* fork succeeded, in child */
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#ifdef LINUX_PROFILE
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setitimer(ITIMER_PROF, &prof_itimer, NULL);
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#endif
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/*
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* By default, Linux tends to kill the postmaster in out-of-memory
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* situations, because it blames the postmaster for the sum of child
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* process sizes *including shared memory*. (This is unbelievably
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* stupid, but the kernel hackers seem uninterested in improving it.)
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* Therefore it's often a good idea to protect the postmaster by
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* setting its OOM score adjustment negative (which has to be done in
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* a root-owned startup script). Since the adjustment is inherited by
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* child processes, this would ordinarily mean that all the
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* postmaster's children are equally protected against OOM kill, which
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* is not such a good idea. So we provide this code to allow the
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* children to change their OOM score adjustments again. Both the
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* file name to write to and the value to write are controlled by
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* environment variables, which can be set by the same startup script
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* that did the original adjustment.
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*/
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oomfilename = getenv("PG_OOM_ADJUST_FILE");
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if (oomfilename != NULL)
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{
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/*
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* Use open() not stdio, to ensure we control the open flags. Some
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* Linux security environments reject anything but O_WRONLY.
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*/
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int fd = open(oomfilename, O_WRONLY, 0);
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/* We ignore all errors */
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if (fd >= 0)
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{
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const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
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int rc;
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if (oomvalue == NULL) /* supply a useful default */
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oomvalue = "0";
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rc = write(fd, oomvalue, strlen(oomvalue));
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(void) rc;
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close(fd);
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}
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}
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/* do post-fork initialization for random number generation */
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pg_strong_random_init();
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}
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else
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{
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/* in parent, restore signal mask */
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sigprocmask(SIG_SETMASK, &save_mask, NULL);
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}
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return result;
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}
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#endif /* ! WIN32 */
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