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) 1996-2017, PostgreSQL Global Development Group
8 : *
9 : * IDENTIFICATION
10 : * src/backend/postmaster/fork_process.c
11 : */
12 : #include "postgres.h"
13 : #include "postmaster/fork_process.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
31 341 : fork_process(void)
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 : */
47 341 : fflush(stdout);
48 341 : 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 :
61 341 : result = fork();
62 682 : 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 : */
85 341 : oomfilename = getenv("PG_OOM_ADJUST_FILE");
86 :
87 341 : 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 0 : int fd = open(oomfilename, O_WRONLY, 0);
94 :
95 : /* We ignore all errors */
96 0 : if (fd >= 0)
97 : {
98 0 : const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
99 : int rc;
100 :
101 0 : if (oomvalue == NULL) /* supply a useful default */
102 0 : oomvalue = "0";
103 :
104 0 : rc = write(fd, oomvalue, strlen(oomvalue));
105 : (void) rc;
106 0 : 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 :
118 682 : return result;
119 : }
120 :
121 : #endif /* ! WIN32 */
|
