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fork_process.c
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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) 1996-2017, PostgreSQL Global Development Group
8  *
9  * IDENTIFICATION
10  * src/backend/postmaster/fork_process.c
11  */
12 #include "postgres.h"
14 
15 #include <fcntl.h>
16 #include <time.h>
17 #include <sys/stat.h>
18 #include <sys/time.h>
19 #include <unistd.h>
20 #ifdef USE_OPENSSL
21 #include <openssl/rand.h>
22 #endif
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.
29  */
30 pid_t
32 {
33  pid_t result;
34  const char *oomfilename;
35 
36 #ifdef LINUX_PROFILE
37  struct itimerval prof_itimer;
38 #endif
39 
40  /*
41  * Flush stdio channels just before fork, to avoid double-output problems.
42  * Ideally we'd use fflush(NULL) here, but there are still a few non-ANSI
43  * stdio libraries out there (like SunOS 4.1.x) that coredump if we do.
44  * Presently stdout and stderr are the only stdio output channels used by
45  * the postmaster, so fflush'ing them should be sufficient.
46  */
47  fflush(stdout);
48  fflush(stderr);
49 
50 #ifdef LINUX_PROFILE
51 
52  /*
53  * Linux's fork() resets the profiling timer in the child process. If we
54  * want to profile child processes then we need to save and restore the
55  * timer setting. This is a waste of time if not profiling, however, so
56  * only do it if commanded by specific -DLINUX_PROFILE switch.
57  */
58  getitimer(ITIMER_PROF, &prof_itimer);
59 #endif
60 
61  result = fork();
62  if (result == 0)
63  {
64  /* fork succeeded, in child */
65 #ifdef LINUX_PROFILE
66  setitimer(ITIMER_PROF, &prof_itimer, NULL);
67 #endif
68 
69  /*
70  * By default, Linux tends to kill the postmaster in out-of-memory
71  * situations, because it blames the postmaster for the sum of child
72  * process sizes *including shared memory*. (This is unbelievably
73  * stupid, but the kernel hackers seem uninterested in improving it.)
74  * Therefore it's often a good idea to protect the postmaster by
75  * setting its OOM score adjustment negative (which has to be done in
76  * a root-owned startup script). Since the adjustment is inherited by
77  * child processes, this would ordinarily mean that all the
78  * postmaster's children are equally protected against OOM kill, which
79  * is not such a good idea. So we provide this code to allow the
80  * children to change their OOM score adjustments again. Both the
81  * file name to write to and the value to write are controlled by
82  * environment variables, which can be set by the same startup script
83  * that did the original adjustment.
84  */
85  oomfilename = getenv("PG_OOM_ADJUST_FILE");
86 
87  if (oomfilename != NULL)
88  {
89  /*
90  * Use open() not stdio, to ensure we control the open flags. Some
91  * Linux security environments reject anything but O_WRONLY.
92  */
93  int fd = open(oomfilename, O_WRONLY, 0);
94 
95  /* We ignore all errors */
96  if (fd >= 0)
97  {
98  const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
99  int rc;
100 
101  if (oomvalue == NULL) /* supply a useful default */
102  oomvalue = "0";
103 
104  rc = write(fd, oomvalue, strlen(oomvalue));
105  (void) rc;
106  close(fd);
107  }
108  }
109 
110  /*
111  * Make sure processes do not share OpenSSL randomness state.
112  */
113 #ifdef USE_OPENSSL
114  RAND_cleanup();
115 #endif
116  }
117 
118  return result;
119 }
120 
121 #endif /* ! WIN32 */
#define write(a, b, c)
Definition: win32.h:19
pid_t fork_process(void)
Definition: fork_process.c:31
return result
Definition: formatting.c:1618
static int fd(const char *x, int i)
Definition: preproc-init.c:105
int setitimer(int which, const struct itimerval *value, struct itimerval *ovalue)
Definition: timer.c:86
#define NULL
Definition: c.h:229
#define close(a)
Definition: win32.h:17