Branch data 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) 1996-2026, 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 "miscadmin.h"
23 : : #include "postmaster/fork_process.h"
24 : :
25 : : #ifndef WIN32
26 : : /*
27 : : * Wrapper for fork(). Return values are the same as those for fork():
28 : : * -1 if the fork failed, 0 in the child process, and the PID of the
29 : : * child in the parent process. Signals are blocked while forking, so
30 : : * the child must unblock.
31 : : */
32 : : pid_t
33 : 812 : fork_process(void)
34 : : {
35 : 812 : pid_t result;
36 : 812 : const char *oomfilename;
37 : 812 : sigset_t save_mask;
38 : :
39 : : #ifdef LINUX_PROFILE
40 : : struct itimerval prof_itimer;
41 : : #endif
42 : :
43 : : /*
44 : : * Flush stdio channels just before fork, to avoid double-output problems.
45 : : */
46 : 812 : fflush(NULL);
47 : :
48 : : #ifdef LINUX_PROFILE
49 : :
50 : : /*
51 : : * Linux's fork() resets the profiling timer in the child process. If we
52 : : * want to profile child processes then we need to save and restore the
53 : : * timer setting. This is a waste of time if not profiling, however, so
54 : : * only do it if commanded by specific -DLINUX_PROFILE switch.
55 : : */
56 : : getitimer(ITIMER_PROF, &prof_itimer);
57 : : #endif
58 : :
59 : : /*
60 : : * We start postmaster children with signals blocked. This allows them to
61 : : * install their own handlers before unblocking, to avoid races where they
62 : : * might run the postmaster's handler and miss an important control
63 : : * signal. With more analysis this could potentially be relaxed.
64 : : */
65 : 812 : sigprocmask(SIG_SETMASK, &BlockSig, &save_mask);
66 : 812 : result = fork();
67 [ + + ]: 812 : if (result == 0)
68 : : {
69 : : /* fork succeeded, in child */
70 : 804 : MyProcPid = getpid();
71 : : #ifdef LINUX_PROFILE
72 : : setitimer(ITIMER_PROF, &prof_itimer, NULL);
73 : : #endif
74 : :
75 : : /*
76 : : * By default, Linux tends to kill the postmaster in out-of-memory
77 : : * situations, because it blames the postmaster for the sum of child
78 : : * process sizes *including shared memory*. (This is unbelievably
79 : : * stupid, but the kernel hackers seem uninterested in improving it.)
80 : : * Therefore it's often a good idea to protect the postmaster by
81 : : * setting its OOM score adjustment negative (which has to be done in
82 : : * a root-owned startup script). Since the adjustment is inherited by
83 : : * child processes, this would ordinarily mean that all the
84 : : * postmaster's children are equally protected against OOM kill, which
85 : : * is not such a good idea. So we provide this code to allow the
86 : : * children to change their OOM score adjustments again. Both the
87 : : * file name to write to and the value to write are controlled by
88 : : * environment variables, which can be set by the same startup script
89 : : * that did the original adjustment.
90 : : */
91 : 804 : oomfilename = getenv("PG_OOM_ADJUST_FILE");
92 : :
93 : 804 : if (oomfilename != NULL)
94 : : {
95 : : /*
96 : : * Use open() not stdio, to ensure we control the open flags. Some
97 : : * Linux security environments reject anything but O_WRONLY.
98 : : */
99 : 0 : int fd = open(oomfilename, O_WRONLY, 0);
100 : :
101 : : /* We ignore all errors */
102 : 0 : if (fd >= 0)
103 : : {
104 : 0 : const char *oomvalue = getenv("PG_OOM_ADJUST_VALUE");
105 : 0 : int rc;
106 : :
107 : 0 : if (oomvalue == NULL) /* supply a useful default */
108 : 0 : oomvalue = "0";
109 : :
110 : 0 : rc = write(fd, oomvalue, strlen(oomvalue));
111 : 0 : (void) rc;
112 : 0 : close(fd);
113 : 0 : }
114 : 0 : }
115 : :
116 : : /* do post-fork initialization for random number generation */
117 : 804 : pg_strong_random_init();
118 : 804 : }
119 : : else
120 : : {
121 : : /* in parent, restore signal mask */
122 : 812 : sigprocmask(SIG_SETMASK, &save_mask, NULL);
123 : : }
124 : :
125 : 3232 : return result;
126 : 1616 : }
127 : :
128 : : #endif /* ! WIN32 */
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