Age Owner TLA 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
6604 neilc 32 GIC 14038 : fork_process(void)
33 : {
34 : pid_t result;
3217 tgl 35 ECB : 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 : */
223 tgl 45 GNC 14038 : 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 signal.
62 : * With more analysis this could potentially be relaxed.
63 : */
87 tmunro 64 14038 : sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
6604 neilc 65 GIC 14038 : result = fork();
6303 bruce 66 26766 : if (result == 0)
67 : {
68 : /* fork succeeded, in child */
69 : #ifdef LINUX_PROFILE
6604 neilc 70 ECB : 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 : */
3217 tgl 89 GIC 12728 : oomfilename = getenv("PG_OOM_ADJUST_FILE");
90 :
91 12728 : if (oomfilename != NULL)
92 : {
93 : /*
94 : * Use open() not stdio, to ensure we control the open flags. Some
4790 bruce 95 ECB : * Linux security environments reject anything but O_WRONLY.
96 : */
3217 tgl 97 LBC 0 : int fd = open(oomfilename, O_WRONLY, 0);
98 :
99 : /* We ignore all errors */
4836 tgl 100 UIC 0 : if (fd >= 0)
101 : {
3217 102 0 : const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
3969 peter_e 103 EUB : int rc;
104 :
3217 tgl 105 UIC 0 : if (oomvalue == NULL) /* supply a useful default */
3217 tgl 106 UBC 0 : oomvalue = "0";
107 :
108 0 : rc = write(fd, oomvalue, strlen(oomvalue));
109 : (void) rc;
4836 tgl 110 UIC 0 : close(fd);
4836 tgl 111 EUB : }
112 : }
113 :
884 magnus 114 : /* do post-fork initialization for random number generation */
884 magnus 115 GIC 12728 : pg_strong_random_init();
6604 neilc 116 EUB : }
117 : else
118 : {
119 : /* in parent, restore signal mask */
87 tmunro 120 GNC 14038 : sigprocmask(SIG_SETMASK, &save_mask, NULL);
121 : }
122 :
6604 neilc 123 GIC 26766 : return result;
124 : }
125 :
2118 tgl 126 ECB : #endif /* ! WIN32 */
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