src/pulsecore/lock-autospawn.c

   1 /***
   2   This file is part of PulseAudio.
   3
   4   Copyright 2008 Lennart Poettering
   5
   6   PulseAudio is free software; you can redistribute it and/or modify
   7   it under the terms of the GNU Lesser General Public License as published
   8   by the Free Software Foundation; either version 2 of the License,
   9   or (at your option) any later version.
  10
  11   PulseAudio is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with PulseAudio; if not, write to the Free Software
  18   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
  19   USA.
  20 ***/
  21
  22 #ifdef HAVE_CONFIG_H
  23 #include <config.h>
  24 #endif
  25
  26 #include <fcntl.h>
  27 #include <errno.h>
  28 #include <string.h>
  29 #include <sys/poll.h>
  30 #include <signal.h>
  31 #include <pthread.h>
  32
  33 #include <pulse/i18n.h>
  34 #include <pulse/xmalloc.h>
  35
  36 #include <pulsecore/mutex.h>
  37 #include <pulsecore/thread.h>
  38 #include <pulsecore/core-util.h>
  39
  40 #include "lock-autospawn.h"
  41
  42 /* So, why do we have this complex code here with threads and pipes
  43  * and stuff? For two reasons: POSIX file locks are per-process, not
  44  * per-file descriptor. That means that two contexts within the same
  45  * process that try to create the autospawn lock might end up assuming
  46  * they both managed to lock the file. And then, POSIX locking
  47  * operations are synchronous. If two contexts run from the same event
  48  * loop it must be made sure that they do not block each other, but
  49  * that the locking operation can happen asynchronously. */
  50
  51 #define AUTOSPAWN_LOCK "autospawn.lock"
  52
  53 static pa_mutex *mutex;
  54
  55 static unsigned n_ref = 0;
  56 static int lock_fd = -1;
  57 static pa_mutex *lock_fd_mutex = NULL;
  58 static pa_bool_t taken = FALSE;
  59 static pa_thread *thread;
  60 static int pipe_fd[2] = { -1, -1 };
  61
  62 static void destroy_mutex(void) PA_GCC_DESTRUCTOR;
  63
  64 static int ref(void) {
  65
  66     if (n_ref > 0) {
  67
  68         pa_assert(pipe_fd[0] >= 0);
  69         pa_assert(pipe_fd[1] >= 0);
  70
  71         n_ref++;
  72
  73         return 0;
  74     }
  75
  76     pa_assert(lock_fd < 0);
  77     pa_assert(!lock_fd_mutex);
  78     pa_assert(!taken);
  79     pa_assert(!thread);
  80     pa_assert(pipe_fd[0] < 0);
  81     pa_assert(pipe_fd[1] < 0);
  82
  83     if (pipe(pipe_fd) < 0)
  84         return -1;
  85
  86     lock_fd_mutex = pa_mutex_new(FALSE, FALSE);
  87
  88     pa_make_fd_cloexec(pipe_fd[0]);
  89     pa_make_fd_cloexec(pipe_fd[1]);
  90
  91     pa_make_fd_nonblock(pipe_fd[1]);
  92     pa_make_fd_nonblock(pipe_fd[0]);
  93
  94     n_ref = 1;
  95     return 0;
  96 }
  97
  98 static void unref(pa_bool_t after_fork) {
  99
 100     pa_assert(n_ref > 0);
 101     pa_assert(pipe_fd[0] >= 0);
 102     pa_assert(pipe_fd[1] >= 0);
 103     pa_assert(lock_fd_mutex);
 104
 105     n_ref--;
 106
 107     if (n_ref > 0)
 108         return;
 109
 110     pa_assert(!taken);
 111
 112     if (thread) {
 113         pa_thread_free(thread);
 114         thread = NULL;
 115     }
 116
 117     pa_mutex_lock(lock_fd_mutex);
 118     if (lock_fd >= 0) {
 119
 120         if (after_fork)
 121             pa_close(lock_fd);
 122         else {
 123             char *lf;
 124
 125             if (!(lf = pa_runtime_path(AUTOSPAWN_LOCK)))
 126                 pa_log_warn(_("Cannot access autospawn lock."));
 127
 128             pa_unlock_lockfile(lf, lock_fd);
 129             pa_xfree(lf);
 130
 131             lock_fd = -1;
 132         }
 133     }
 134     pa_mutex_unlock(lock_fd_mutex);
 135
 136     pa_mutex_free(lock_fd_mutex);
 137     lock_fd_mutex = NULL;
 138
 139     pa_close(pipe_fd[0]);
 140     pa_close(pipe_fd[1]);
 141     pipe_fd[0] = pipe_fd[1] = -1;
 142 }
 143
 144 static void ping(void) {
 145     ssize_t s;
 146
 147     pa_assert(pipe_fd[1] >= 0);
 148
 149     for (;;) {
 150         char x = 'x';
 151
 152         if ((s = write(pipe_fd[1], &x, 1)) == 1)
 153             break;
 154
 155         pa_assert(s < 0);
 156
 157         if (errno == EAGAIN)
 158             break;
 159
 160         pa_assert(errno == EINTR);
 161     }
 162 }
 163
 164 static void wait_for_ping(void) {
 165     ssize_t s;
 166     char x;
 167     struct pollfd pfd;
 168     int k;
 169
 170     pa_assert(pipe_fd[0] >= 0);
 171
 172     memset(&pfd, 0, sizeof(pfd));
 173     pfd.fd = pipe_fd[0];
 174     pfd.events = POLLIN;
 175
 176     if ((k = poll(&pfd, 1, -1)) != 1) {
 177         pa_assert(k < 0);
 178         pa_assert(errno == EINTR);
 179     } else if ((s = read(pipe_fd[0], &x, 1)) != 1) {
 180         pa_assert(s < 0);
 181         pa_assert(errno == EAGAIN);
 182     }
 183 }
 184
 185 static void empty_pipe(void) {
 186     char x[16];
 187     ssize_t s;
 188
 189     pa_assert(pipe_fd[0] >= 0);
 190
 191     if ((s = read(pipe_fd[0], &x, sizeof(x))) < 1) {
 192         pa_assert(s < 0);
 193         pa_assert(errno == EAGAIN);
 194     }
 195 }
 196
 197 static void thread_func(void *u) {
 198     int fd;
 199     char *lf;
 200     sigset_t fullset;
 201
 202     /* No signals in this thread please */
 203     sigfillset(&fullset);
 204     pthread_sigmask(SIG_BLOCK, &fullset, NULL);
 205
 206     if (!(lf = pa_runtime_path(AUTOSPAWN_LOCK))) {
 207         pa_log_warn(_("Cannot access autospawn lock."));
 208         goto finish;
 209     }
 210
 211     if ((fd = pa_lock_lockfile(lf)) < 0)
 212         goto finish;
 213
 214     pa_mutex_lock(lock_fd_mutex);
 215     pa_assert(lock_fd < 0);
 216     lock_fd = fd;
 217     pa_mutex_unlock(lock_fd_mutex);
 218
 219 finish:
 220     pa_xfree(lf);
 221
 222     ping();
 223 }
 224
 225 static int start_thread(void) {
 226
 227     if (!thread)
 228         if (!(thread = pa_thread_new(thread_func, NULL)))
 229             return -1;
 230
 231     return 0;
 232 }
 233
 234 static void create_mutex(void) {
 235     PA_ONCE_BEGIN {
 236         mutex = pa_mutex_new(FALSE, FALSE);
 237     } PA_ONCE_END;
 238 }
 239
 240 static void destroy_mutex(void) {
 241
 242     if (mutex)
 243         pa_mutex_free(mutex);
 244 }
 245
 246
 247 int pa_autospawn_lock_init(void) {
 248     int ret = -1;
 249
 250     create_mutex();
 251     pa_mutex_lock(mutex);
 252
 253     if (ref() < 0)
 254         ret = -1;
 255     else
 256         ret = pipe_fd[0];
 257
 258     pa_mutex_unlock(mutex);
 259
 260     return ret;
 261 }
 262
 263 int pa_autospawn_lock_acquire(pa_bool_t block) {
 264     int ret = -1;
 265
 266     create_mutex();
 267     pa_mutex_lock(mutex);
 268     pa_assert(n_ref >= 1);
 269
 270     pa_mutex_lock(lock_fd_mutex);
 271
 272     for (;;) {
 273
 274         empty_pipe();
 275
 276         if (lock_fd >= 0 && !taken) {
 277             taken = TRUE;
 278             ret = 1;
 279             break;
 280         }
 281
 282         if (lock_fd < 0)
 283             if (start_thread() < 0)
 284                 break;
 285
 286         if (!block) {
 287             ret = 0;
 288             break;
 289         }
 290
 291         pa_mutex_unlock(lock_fd_mutex);
 292         pa_mutex_unlock(mutex);
 293
 294         wait_for_ping();
 295
 296         pa_mutex_lock(mutex);
 297         pa_mutex_lock(lock_fd_mutex);
 298     }
 299
 300     pa_mutex_unlock(lock_fd_mutex);
 301
 302     pa_mutex_unlock(mutex);
 303
 304     return ret;
 305 }
 306
 307 void pa_autospawn_lock_release(void) {
 308
 309     create_mutex();
 310     pa_mutex_lock(mutex);
 311     pa_assert(n_ref >= 1);
 312
 313     pa_assert(taken);
 314     taken = FALSE;
 315
 316     ping();
 317
 318     pa_mutex_unlock(mutex);
 319 }
 320
 321 void pa_autospawn_lock_done(pa_bool_t after_fork) {
 322
 323     create_mutex();
 324     pa_mutex_lock(mutex);
 325     pa_assert(n_ref >= 1);
 326
 327     unref(after_fork);
 328
 329     pa_mutex_unlock(mutex);
 330 }