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1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * pqsignal.c
4 : : * reliable BSD-style signal(2) routine stolen from RWW who stole it
5 : : * from Stevens...
6 : : *
7 : : * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
8 : : * Portions Copyright (c) 1994, Regents of the University of California
9 : : *
10 : : *
11 : : * IDENTIFICATION
12 : : * src/port/pqsignal.c
13 : : *
14 : : * This is the signal() implementation from "Advanced Programming in the UNIX
15 : : * Environment", with minor changes. It was originally a replacement needed
16 : : * for old SVR4 systems whose signal() behaved as if sa_flags = SA_RESETHAND |
17 : : * SA_NODEFER, also known as "unreliable" signals due to races when the
18 : : * handler was reset.
19 : : *
20 : : * By now, all known modern Unix systems have a "reliable" signal() call.
21 : : * We still don't want to use it though, because it remains
22 : : * implementation-defined by both C99 and POSIX whether the handler is reset
23 : : * or signals are blocked when the handler runs, and default restart behavior
24 : : * is also unspecified. Therefore we take POSIX's advice and call sigaction()
25 : : * so we can provide explicit sa_flags, but wrap it in this more convenient
26 : : * traditional interface style. It also provides a place to set any extra
27 : : * flags we want everywhere, such as SA_NOCLDSTOP.
28 : : *
29 : : * Windows, of course, is resolutely in a class by itself. In the backend,
30 : : * this relies on pqsigaction() in src/backend/port/win32/signal.c, which
31 : : * provides limited emulation of reliable signals.
32 : : *
33 : : * Frontend programs can use this version of pqsignal() to forward to the
34 : : * native Windows signal() call if they wish, but beware that Windows signals
35 : : * behave quite differently. Only the 6 signals required by C are supported.
36 : : * SIGINT handlers run in another thread instead of interrupting an existing
37 : : * thread, and the others don't interrupt system calls either, so SA_RESTART
38 : : * is moot. All except SIGFPE have SA_RESETHAND semantics, meaning the
39 : : * handler is reset to SIG_DFL each time it runs. The set of things you are
40 : : * allowed to do in a handler is also much more restricted than on Unix,
41 : : * according to the documentation.
42 : : *
43 : : * ------------------------------------------------------------------------
44 : : */
45 : :
46 : : #include "c.h"
47 : :
48 : : #include <signal.h>
49 : : #ifndef FRONTEND
50 : : #include <unistd.h>
51 : : #endif
52 : :
53 : : #ifndef FRONTEND
54 : : #include "libpq/pqsignal.h"
55 : : #include "miscadmin.h"
56 : : #endif
57 : :
58 : : #ifdef PG_SIGNAL_COUNT /* Windows */
59 : : #define PG_NSIG (PG_SIGNAL_COUNT)
60 : : #elif defined(NSIG)
61 : : #define PG_NSIG (NSIG)
62 : : #else
63 : : #define PG_NSIG (64) /* XXX: wild guess */
64 : : #endif
65 : :
66 : : /* Check a couple of common signals to make sure PG_NSIG is accurate. */
67 : : StaticAssertDecl(SIGUSR2 < PG_NSIG, "SIGUSR2 >= PG_NSIG");
68 : : StaticAssertDecl(SIGHUP < PG_NSIG, "SIGHUP >= PG_NSIG");
69 : : StaticAssertDecl(SIGTERM < PG_NSIG, "SIGTERM >= PG_NSIG");
70 : : StaticAssertDecl(SIGALRM < PG_NSIG, "SIGALRM >= PG_NSIG");
71 : :
72 : : static volatile pqsigfunc pqsignal_handlers[PG_NSIG];
73 : :
74 : : /*
75 : : * Except when called with SIG_IGN or SIG_DFL, pqsignal() sets up this function
76 : : * as the handler for all signals. This wrapper handler function checks that
77 : : * it is called within a process that knew to maintain MyProcPid, and not a
78 : : * child process forked by system(3), etc. This check ensures that such child
79 : : * processes do not modify shared memory, which is often detrimental. If the
80 : : * check succeeds, the function originally provided to pqsignal() is called.
81 : : * Otherwise, the default signal handler is installed and then called.
82 : : *
83 : : * This wrapper also handles restoring the value of errno.
84 : : */
85 : : static void
86 : 2633 : wrapper_handler(SIGNAL_ARGS)
87 : : {
88 : 2633 : int save_errno = errno;
89 : :
90 [ + - ]: 2633 : Assert(postgres_signal_arg > 0);
91 [ + - ]: 2633 : Assert(postgres_signal_arg < PG_NSIG);
92 : :
93 : : #ifndef FRONTEND
94 : :
95 : : /*
96 : : * We expect processes to set MyProcPid before calling pqsignal() or
97 : : * before accepting signals.
98 : : */
99 [ + - ]: 2633 : Assert(MyProcPid);
100 [ + + + - ]: 2633 : Assert(MyProcPid != PostmasterPid || !IsUnderPostmaster);
101 : :
102 [ - + ]: 2633 : if (unlikely(MyProcPid != (int) getpid()))
103 : : {
104 : 0 : pqsignal(postgres_signal_arg, SIG_DFL);
105 : 0 : raise(postgres_signal_arg);
106 : 0 : return;
107 : : }
108 : : #endif
109 : :
110 : 2633 : (*pqsignal_handlers[postgres_signal_arg]) (postgres_signal_arg);
111 : :
112 : 2633 : errno = save_errno;
113 [ - + ]: 2633 : }
114 : :
115 : : /*
116 : : * Set up a signal handler, with SA_RESTART, for signal "signo"
117 : : *
118 : : * Note: the actual name of this function is either pqsignal_fe when
119 : : * compiled with -DFRONTEND, or pqsignal_be when compiled without that.
120 : : * This is to avoid a name collision with libpq's legacy-pqsignal.c.
121 : : */
122 : : void
123 : 11410 : pqsignal(int signo, pqsigfunc func)
124 : : {
125 : : #if !(defined(WIN32) && defined(FRONTEND))
126 : 11410 : struct sigaction act;
127 : : #endif
128 : :
129 [ + - ]: 11410 : Assert(signo > 0);
130 [ + - ]: 11410 : Assert(signo < PG_NSIG);
131 : :
132 [ + + + + ]: 11410 : if (func != SIG_IGN && func != SIG_DFL)
133 : : {
134 : 7407 : pqsignal_handlers[signo] = func; /* assumed atomic */
135 : 7407 : func = wrapper_handler;
136 : 7407 : }
137 : :
138 : : #if !(defined(WIN32) && defined(FRONTEND))
139 : 11410 : act.sa_handler = func;
140 : 11410 : sigemptyset(&act.sa_mask);
141 : 11410 : act.sa_flags = SA_RESTART;
142 : : #ifdef SA_NOCLDSTOP
143 [ + + ]: 11410 : if (signo == SIGCHLD)
144 : 1076 : act.sa_flags |= SA_NOCLDSTOP;
145 : : #endif
146 [ + - ]: 11410 : if (sigaction(signo, &act, NULL) < 0)
147 : 0 : Assert(false); /* probably indicates coding error */
148 : : #else
149 : : /* Forward to Windows native signal system. */
150 : : if (signal(signo, func) == SIG_ERR)
151 : : Assert(false); /* probably indicates coding error */
152 : : #endif
153 : 11410 : }
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