lib/fatal-signal.c

   1 /*
   2  * Copyright (c) 2008, 2009 Nicira Networks.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16 #include <config.h>
  17 #include "fatal-signal.h"
  18 #include <assert.h>
  19 #include <errno.h>
  20 #include <signal.h>
  21 #include <stdbool.h>
  22 #include <stdio.h>
  23 #include <stdlib.h>
  24 #include <string.h>
  25 #include <unistd.h>
  26 #include "shash.h"
  27 #include "util.h"
  28
  29 #define THIS_MODULE VLM_fatal_signal
  30 #include "vlog.h"
  31
  32 /* Signals to catch. */
  33 static const int fatal_signals[] = { SIGTERM, SIGINT, SIGHUP, SIGALRM };
  34
  35 /* Signals to catch as a sigset_t. */
  36 static sigset_t fatal_signal_set;
  37
  38 /* Hooks to call upon catching a signal */
  39 struct hook {
  40     void (*func)(void *aux);
  41     void *aux;
  42     bool run_at_exit;
  43 };
  44 #define MAX_HOOKS 32
  45 static struct hook hooks[MAX_HOOKS];
  46 static size_t n_hooks;
  47
  48 /* Number of nesting signal blockers. */
  49 static int block_level = 0;
  50
  51 /* Signal mask saved by outermost signal blocker. */
  52 static sigset_t saved_signal_mask;
  53
  54 /* Disabled by fatal_signal_fork()? */
  55 static bool disabled;
  56
  57 static void call_sigprocmask(int how, sigset_t* new_set, sigset_t* old_set);
  58 static void atexit_handler(void);
  59 static void call_hooks(int sig_nr);
  60
  61 /* Registers 'hook' to be called when a process termination signal is raised.
  62  * If 'run_at_exit' is true, 'hook' is also called during normal process
  63  * termination, e.g. when exit() is called or when main() returns.
  64  *
  65  * 'func' will be invoked from an asynchronous signal handler, so it must be
  66  * written appropriately.  For example, it must not call most C library
  67  * functions, including malloc() or free(). */
  68 void
  69 fatal_signal_add_hook(void (*func)(void *aux), void *aux, bool run_at_exit)
  70 {
  71     fatal_signal_block();
  72     assert(n_hooks < MAX_HOOKS);
  73     hooks[n_hooks].func = func;
  74     hooks[n_hooks].aux = aux;
  75     hooks[n_hooks].run_at_exit = run_at_exit;
  76     n_hooks++;
  77     fatal_signal_unblock();
  78 }
  79
  80 /* Blocks program termination signals until fatal_signal_unblock() is called.
  81  * May be called multiple times with nesting; if so, fatal_signal_unblock()
  82  * must be called the same number of times to unblock signals.
  83  *
  84  * This is needed while adjusting a data structure that will be accessed by a
  85  * fatal signal hook, so that the hook is not invoked while the data structure
  86  * is in an inconsistent state. */
  87 void
  88 fatal_signal_block(void)
  89 {
  90     static bool inited = false;
  91     if (!inited) {
  92         size_t i;
  93
  94         inited = true;
  95         sigemptyset(&fatal_signal_set);
  96         for (i = 0; i < ARRAY_SIZE(fatal_signals); i++) {
  97             int sig_nr = fatal_signals[i];
  98             struct sigaction old_sa;
  99
 100             sigaddset(&fatal_signal_set, sig_nr);
 101             if (sigaction(sig_nr, NULL, &old_sa)) {
 102                 ovs_fatal(errno, "sigaction");
 103             }
 104             if (old_sa.sa_handler == SIG_DFL
 105                 && signal(sig_nr, fatal_signal_handler) == SIG_ERR) {
 106                 ovs_fatal(errno, "signal");
 107             }
 108         }
 109         atexit(atexit_handler);
 110     }
 111
 112     if (++block_level == 1) {
 113         call_sigprocmask(SIG_BLOCK, &fatal_signal_set, &saved_signal_mask);
 114     }
 115 }
 116
 117 /* Unblocks program termination signals blocked by fatal_signal_block() is
 118  * called.  If multiple calls to fatal_signal_block() are nested,
 119  * fatal_signal_unblock() must be called the same number of times to unblock
 120  * signals. */
 121 void
 122 fatal_signal_unblock(void)
 123 {
 124     assert(block_level > 0);
 125     if (--block_level == 0) {
 126         call_sigprocmask(SIG_SETMASK, &saved_signal_mask, NULL);
 127     }
 128 }
 129
 130 /* Handles fatal signal number 'sig_nr'.
 131  *
 132  * Ordinarily this is the actual signal handler.  When other code needs to
 133  * handle one of our signals, however, it can register for that signal and, if
 134  * and when necessary, call this function to do fatal signal processing for it
 135  * and terminate the process.  Currently only timeval.c does this, for SIGALRM.
 136  * (It is not important whether the other code sets up its signal handler
 137  * before or after this file, because this file will only set up a signal
 138  * handler in the case where the signal has its default handling.)  */
 139 void
 140 fatal_signal_handler(int sig_nr)
 141 {
 142     call_hooks(sig_nr);
 143
 144     /* Re-raise the signal with the default handling so that the program
 145      * termination status reflects that we were killed by this signal */
 146     signal(sig_nr, SIG_DFL);
 147     raise(sig_nr);
 148 }
 149
 150 static void
 151 atexit_handler(void)
 152 {
 153     if (!disabled) {
 154         call_hooks(0);
 155     }
 156 }
 157
 158 static void
 159 call_hooks(int sig_nr)
 160 {
 161     static volatile sig_atomic_t recurse = 0;
 162     if (!recurse) {
 163         size_t i;
 164
 165         recurse = 1;
 166
 167         for (i = 0; i < n_hooks; i++) {
 168             struct hook *h = &hooks[i];
 169             if (sig_nr || h->run_at_exit) {
 170                 h->func(h->aux);
 171             }
 172         }
 173     }
 174 }
 175 \f
 176 static struct shash files = SHASH_INITIALIZER(&files);
 177
 178 static void unlink_files(void *aux);
 179 static void do_unlink_files(void);
 180
 181 /* Registers 'file' to be unlinked when the program terminates via exit() or a
 182  * fatal signal. */
 183 void
 184 fatal_signal_add_file_to_unlink(const char *file)
 185 {
 186     static bool added_hook = false;
 187     if (!added_hook) {
 188         added_hook = true;
 189         fatal_signal_add_hook(unlink_files, NULL, true);
 190     }
 191
 192     fatal_signal_block();
 193     if (!shash_find(&files, file)) {
 194         shash_add(&files, file, NULL);
 195     }
 196     fatal_signal_unblock();
 197 }
 198
 199 /* Unregisters 'file' from being unlinked when the program terminates via
 200  * exit() or a fatal signal. */
 201 void
 202 fatal_signal_remove_file_to_unlink(const char *file)
 203 {
 204     struct shash_node *node;
 205
 206     fatal_signal_block();
 207     node = shash_find(&files, file);
 208     if (node) {
 209         shash_delete(&files, node);
 210     }
 211     fatal_signal_unblock();
 212 }
 213
 214 /* Like fatal_signal_remove_file_to_unlink(), but also unlinks 'file'.
 215  * Returns 0 if successful, otherwise a positive errno value. */
 216 int
 217 fatal_signal_unlink_file_now(const char *file)
 218 {
 219     int error = unlink(file) ? errno : 0;
 220     if (error) {
 221         VLOG_WARN("could not unlink \"%s\" (%s)", file, strerror(error));
 222     }
 223
 224     fatal_signal_remove_file_to_unlink(file);
 225
 226     return error;
 227 }
 228
 229 static void
 230 unlink_files(void *aux UNUSED)
 231 {
 232     do_unlink_files();
 233 }
 234
 235 /* This is a fatal_signal_add_hook() callback (via unlink_files()).  It will be
 236  * invoked from an asynchronous signal handler, so it cannot call most C
 237  * library functions (unlink() is an explicit exception, see
 238  * http://www.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html).
 239  * That includes free(), so it doesn't try to free the 'files' data
 240  * structure. */
 241 static void
 242 do_unlink_files(void)
 243 {
 244     struct shash_node *node;
 245
 246     SHASH_FOR_EACH (node, &files) {
 247         unlink(node->name);
 248     }
 249 }
 250 \f
 251 /* Disables the fatal signal hook mechanism.  Following a fork, one of the
 252  * resulting processes can call this function to allow it to terminate without
 253  * triggering fatal signal processing or removing files.  Fatal signal
 254  * processing is still enabled in the other process. */
 255 void
 256 fatal_signal_fork(void)
 257 {
 258     size_t i;
 259
 260     disabled = true;
 261
 262     for (i = 0; i < ARRAY_SIZE(fatal_signals); i++) {
 263         int sig_nr = fatal_signals[i];
 264         if (signal(sig_nr, SIG_DFL) == SIG_IGN) {
 265             signal(sig_nr, SIG_IGN);
 266         }
 267     }
 268 }
 269 \f
 270 static void
 271 call_sigprocmask(int how, sigset_t* new_set, sigset_t* old_set)
 272 {
 273     int error = sigprocmask(how, new_set, old_set);
 274     if (error) {
 275         fprintf(stderr, "sigprocmask: %s\n", strerror(errno));
 276     }
 277 }