lib/util/tfork.c

   1 /*
   2    fork on steroids to avoid SIGCHLD and waitpid
   3
   4    Copyright (C) Stefan Metzmacher 2010
   5    Copyright (C) Ralph Boehme 2017
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  19 */
  20
  21 #include "replace.h"
  22 #include "system/wait.h"
  23 #include "system/filesys.h"
  24 #include "system/network.h"
  25 #include "lib/util/samba_util.h"
  26 #include "lib/util/sys_rw.h"
  27 #include "lib/util/tfork.h"
  28 #include "lib/util/debug.h"
  29
  30 #ifdef HAVE_PTHREAD
  31 #include <pthread.h>
  32 #endif
  33
  34 #ifdef NDEBUG
  35 #undef NDEBUG
  36 #endif
  37 #include <assert.h>
  38
  39 /*
  40  * This is how the process hierarchy looks like:
  41  *
  42  *   +----------+
  43  *   |  caller  |
  44  *   +----------+
  45  *         |
  46  *       fork
  47  *         |
  48  *         v
  49  *   +----------+
  50  *   |  waiter  |
  51  *   +----------+
  52  *         |
  53  *       fork
  54  *         |
  55  *         v
  56  *   +----------+
  57  *   |  worker  |
  58  *   +----------+
  59  */
  60
  61 /*
  62  * The resulting (private) state per tfork_create() call, returned as a opaque
  63  * handle to the caller.
  64  */
  65 struct tfork {
  66         /*
  67          * This is returned to the caller with tfork_event_fd()
  68          */
  69         int event_fd;
  70
  71         /*
  72          * This is used in the caller by tfork_status() to read the worker exit
  73          * status and to tell the waiter to exit by closing the fd.
  74          */
  75         int status_fd;
  76
  77         pid_t waiter_pid;
  78         pid_t worker_pid;
  79 };
  80
  81 /*
  82  * Internal per-thread state maintained while inside tfork.
  83  */
  84 struct tfork_state {
  85         pid_t waiter_pid;
  86         int waiter_errno;
  87
  88         pid_t worker_pid;
  89 };
  90
  91 /*
  92  * A global state that synchronizes access to handling SIGCHLD and waiting for
  93  * childs.
  94  */
  95 struct tfork_signal_state {
  96         bool available;
  97
  98 #ifdef HAVE_PTHREAD
  99         pthread_cond_t cond;
 100         pthread_mutex_t mutex;
 101 #endif
 102
 103         /*
 104          * pid of the waiter child. This points at waiter_pid in either struct
 105          * tfork or struct tfork_state, depending on who called
 106          * tfork_install_sigchld_handler().
 107          *
 108          * When tfork_install_sigchld_handler() is called the waiter_pid is
 109          * still -1 and only set later after fork(), that's why this is must be
 110          * a pointer. The signal handler checks this.
 111          */
 112         pid_t *pid;
 113
 114         struct sigaction oldact;
 115         sigset_t oldset;
 116 };
 117
 118 static struct tfork_signal_state signal_state;
 119
 120 #ifdef HAVE_PTHREAD
 121 static pthread_once_t tfork_global_is_initialized = PTHREAD_ONCE_INIT;
 122 static pthread_key_t tfork_global_key;
 123 #else
 124 static struct tfork_state *global_state;
 125 #endif
 126
 127 static void tfork_sigchld_handler(int signum, siginfo_t *si, void *p);
 128
 129 #ifdef HAVE_PTHREAD
 130 static void tfork_global_destructor(void *state)
 131 {
 132         anonymous_shared_free(state);
 133 }
 134 #endif
 135
 136 static int tfork_acquire_sighandling(void)
 137 {
 138         int ret = 0;
 139
 140 #ifdef HAVE_PTHREAD
 141         ret = pthread_mutex_lock(&signal_state.mutex);
 142         if (ret != 0) {
 143                 return ret;
 144         }
 145
 146         while (!signal_state.available) {
 147                 ret = pthread_cond_wait(&signal_state.cond,
 148                                         &signal_state.mutex);
 149                 if (ret != 0) {
 150                         return ret;
 151                 }
 152         }
 153
 154         signal_state.available = false;
 155
 156         ret = pthread_mutex_unlock(&signal_state.mutex);
 157         if (ret != 0) {
 158                 return ret;
 159         }
 160 #endif
 161
 162         return ret;
 163 }
 164
 165 static int tfork_release_sighandling(void)
 166 {
 167         int ret = 0;
 168
 169 #ifdef HAVE_PTHREAD
 170         ret = pthread_mutex_lock(&signal_state.mutex);
 171         if (ret != 0) {
 172                 return ret;
 173         }
 174
 175         signal_state.available = true;
 176
 177         ret = pthread_cond_signal(&signal_state.cond);
 178         if (ret != 0) {
 179                 pthread_mutex_unlock(&signal_state.mutex);
 180                 return ret;
 181         }
 182
 183         ret = pthread_mutex_unlock(&signal_state.mutex);
 184         if (ret != 0) {
 185                 return ret;
 186         }
 187 #endif
 188
 189         return ret;
 190 }
 191
 192 #ifdef HAVE_PTHREAD
 193 static void tfork_atfork_prepare(void)
 194 {
 195         int ret;
 196
 197         ret = pthread_mutex_lock(&signal_state.mutex);
 198         assert(ret == 0);
 199 }
 200
 201 static void tfork_atfork_parent(void)
 202 {
 203         int ret;
 204
 205         ret = pthread_mutex_unlock(&signal_state.mutex);
 206         assert(ret == 0);
 207 }
 208 #endif
 209
 210 static void tfork_atfork_child(void)
 211 {
 212         int ret;
 213
 214 #ifdef HAVE_PTHREAD
 215         ret = pthread_mutex_unlock(&signal_state.mutex);
 216         assert(ret == 0);
 217
 218         ret = pthread_key_delete(tfork_global_key);
 219         assert(ret == 0);
 220
 221         ret = pthread_key_create(&tfork_global_key, tfork_global_destructor);
 222         assert(ret == 0);
 223
 224         /*
 225          * There's no way to destroy a condition variable if there are waiters,
 226          * pthread_cond_destroy() will return EBUSY. Just zero out memory and
 227          * then initialize again. This is not backed by POSIX but should be ok.
 228          */
 229         ZERO_STRUCT(signal_state.cond);
 230         ret = pthread_cond_init(&signal_state.cond, NULL);
 231         assert(ret == 0);
 232 #endif
 233
 234         if (signal_state.pid != NULL) {
 235
 236                 ret = sigaction(SIGCHLD, &signal_state.oldact, NULL);
 237                 assert(ret == 0);
 238
 239 #ifdef HAVE_PTHREAD
 240                 ret = pthread_sigmask(SIG_SETMASK, &signal_state.oldset, NULL);
 241 #else
 242                 ret = sigprocmask(SIG_SETMASK, &signal_state.oldset, NULL);
 243                 assert(ret == 0);
 244 #endif
 245
 246                 signal_state.pid = NULL;
 247         }
 248
 249         signal_state.available = true;
 250 }
 251
 252 static void tfork_global_initialize(void)
 253 {
 254 #ifdef HAVE_PTHREAD
 255         int ret;
 256
 257         pthread_atfork(tfork_atfork_prepare,
 258                        tfork_atfork_parent,
 259                        tfork_atfork_child);
 260
 261         ret = pthread_key_create(&tfork_global_key, tfork_global_destructor);
 262         assert(ret == 0);
 263
 264         ret = pthread_mutex_init(&signal_state.mutex, NULL);
 265         assert(ret == 0);
 266
 267         ret = pthread_cond_init(&signal_state.cond, NULL);
 268         assert(ret == 0);
 269 #endif
 270
 271         signal_state.available = true;
 272 }
 273
 274 static struct tfork_state *tfork_global_get(void)
 275 {
 276         struct tfork_state *state = NULL;
 277 #ifdef HAVE_PTHREAD
 278         int ret;
 279 #endif
 280
 281 #ifdef HAVE_PTHREAD
 282         state = (struct tfork_state *)pthread_getspecific(tfork_global_key);
 283 #else
 284         state = global_state;
 285 #endif
 286         if (state != NULL) {
 287                 return state;
 288         }
 289
 290         state = (struct tfork_state *)anonymous_shared_allocate(
 291                 sizeof(struct tfork_state));
 292         if (state == NULL) {
 293                 return NULL;
 294         }
 295
 296 #ifdef HAVE_PTHREAD
 297         ret = pthread_setspecific(tfork_global_key, state);
 298         if (ret != 0) {
 299                 anonymous_shared_free(state);
 300                 return NULL;
 301         }
 302 #endif
 303         return state;
 304 }
 305
 306 static void tfork_global_free(void)
 307 {
 308         struct tfork_state *state = NULL;
 309 #ifdef HAVE_PTHREAD
 310         int ret;
 311 #endif
 312
 313 #ifdef HAVE_PTHREAD
 314         state = (struct tfork_state *)pthread_getspecific(tfork_global_key);
 315 #else
 316         state = global_state;
 317 #endif
 318         if (state == NULL) {
 319                 return;
 320         }
 321
 322 #ifdef HAVE_PTHREAD
 323         ret = pthread_setspecific(tfork_global_key, NULL);
 324         if (ret != 0) {
 325                 return;
 326         }
 327 #endif
 328         anonymous_shared_free(state);
 329 }
 330
 331 /**
 332  * Only one thread at a time is allowed to handle SIGCHLD signals
 333  **/
 334 static int tfork_install_sigchld_handler(pid_t *pid)
 335 {
 336         int ret;
 337         struct sigaction act;
 338         sigset_t set;
 339
 340         ret = tfork_acquire_sighandling();
 341         if (ret != 0) {
 342                 return -1;
 343         }
 344
 345         assert(signal_state.pid == NULL);
 346         signal_state.pid = pid;
 347
 348         act = (struct sigaction) {
 349                 .sa_sigaction = tfork_sigchld_handler,
 350                 .sa_flags = SA_SIGINFO,
 351         };
 352
 353         ret = sigaction(SIGCHLD, &act, &signal_state.oldact);
 354         if (ret != 0) {
 355                 return -1;
 356         }
 357
 358         sigemptyset(&set);
 359         sigaddset(&set, SIGCHLD);
 360 #ifdef HAVE_PTHREAD
 361         ret = pthread_sigmask(SIG_UNBLOCK, &set, &signal_state.oldset);
 362 #else
 363         ret = sigprocmask(SIG_UNBLOCK, &set, &signal_state.oldset);
 364 #endif
 365         if (ret != 0) {
 366                 return -1;
 367         }
 368
 369         return 0;
 370 }
 371
 372 static int tfork_uninstall_sigchld_handler(void)
 373 {
 374         int ret;
 375
 376         signal_state.pid = NULL;
 377
 378         ret = sigaction(SIGCHLD, &signal_state.oldact, NULL);
 379         if (ret != 0) {
 380                 return -1;
 381         }
 382
 383 #ifdef HAVE_PTHREAD
 384         ret = pthread_sigmask(SIG_SETMASK, &signal_state.oldset, NULL);
 385 #else
 386         ret = sigprocmask(SIG_SETMASK, &signal_state.oldset, NULL);
 387 #endif
 388         if (ret != 0) {
 389                 return -1;
 390         }
 391
 392         ret = tfork_release_sighandling();
 393         if (ret != 0) {
 394                 return -1;
 395         }
 396
 397         return 0;
 398 }
 399
 400 static void tfork_sigchld_handler(int signum, siginfo_t *si, void *p)
 401 {
 402         if ((signal_state.pid != NULL) &&
 403             (*signal_state.pid != -1) &&
 404             (si->si_pid == *signal_state.pid))
 405         {
 406                 return;
 407         }
 408
 409         /*
 410          * Not our child, forward to old handler
 411          */
 412         if (signal_state.oldact.sa_flags & SA_SIGINFO) {
 413                 signal_state.oldact.sa_sigaction(signum, si, p);
 414                 return;
 415         }
 416
 417         if (signal_state.oldact.sa_handler == SIG_IGN) {
 418                 return;
 419         }
 420         if (signal_state.oldact.sa_handler == SIG_DFL) {
 421                 return;
 422         }
 423         signal_state.oldact.sa_handler(signum);
 424 }
 425
 426 static pid_t tfork_start_waiter_and_worker(struct tfork_state *state,
 427                                            int *_event_fd,
 428                                            int *_status_fd)
 429 {
 430         int p[2];
 431         int status_sp_caller_fd = -1;
 432         int status_sp_waiter_fd = -1;
 433         int event_pipe_caller_fd = -1;
 434         int event_pipe_waiter_fd = -1;
 435         int ready_pipe_caller_fd = -1;
 436         int ready_pipe_worker_fd = -1;
 437         ssize_t nwritten;
 438         ssize_t nread;
 439         pid_t pid;
 440         int status;
 441         int fd;
 442         char c;
 443         int ret;
 444
 445         *_event_fd = -1;
 446         *_status_fd = -1;
 447
 448         if (state == NULL) {
 449                 return -1;
 450         }
 451
 452         ret = socketpair(AF_UNIX, SOCK_STREAM, 0, p);
 453         if (ret != 0) {
 454                 return -1;
 455         }
 456         set_close_on_exec(p[0]);
 457         set_close_on_exec(p[1]);
 458         status_sp_caller_fd = p[0];
 459         status_sp_waiter_fd = p[1];
 460
 461         ret = pipe(p);
 462         if (ret != 0) {
 463                 close(status_sp_caller_fd);
 464                 close(status_sp_waiter_fd);
 465                 return -1;
 466         }
 467         set_close_on_exec(p[0]);
 468         set_close_on_exec(p[1]);
 469         event_pipe_caller_fd = p[0];
 470         event_pipe_waiter_fd = p[1];
 471
 472
 473         ret = pipe(p);
 474         if (ret != 0) {
 475                 close(status_sp_caller_fd);
 476                 close(status_sp_waiter_fd);
 477                 close(event_pipe_caller_fd);
 478                 close(event_pipe_waiter_fd);
 479                 return -1;
 480         }
 481         set_close_on_exec(p[0]);
 482         set_close_on_exec(p[1]);
 483         ready_pipe_worker_fd = p[0];
 484         ready_pipe_caller_fd = p[1];
 485
 486         pid = fork();
 487         if (pid == -1) {
 488                 close(status_sp_caller_fd);
 489                 close(status_sp_waiter_fd);
 490                 close(event_pipe_caller_fd);
 491                 close(event_pipe_waiter_fd);
 492                 close(ready_pipe_caller_fd);
 493                 close(ready_pipe_worker_fd);
 494                 return -1;
 495         }
 496         if (pid != 0) {
 497                 /* The caller */
 498
 499                 state->waiter_pid = pid;
 500
 501                 close(status_sp_waiter_fd);
 502                 close(event_pipe_waiter_fd);
 503                 close(ready_pipe_worker_fd);
 504
 505                 set_blocking(event_pipe_caller_fd, false);
 506
 507                 /*
 508                  * wait for the waiter to get ready.
 509                  */
 510                 nread = sys_read(status_sp_caller_fd, &c, sizeof(char));
 511                 if (nread != sizeof(char)) {
 512                         return -1;
 513                 }
 514
 515                 /*
 516                  * Notify the worker to start.
 517                  */
 518                 nwritten = sys_write(ready_pipe_caller_fd,
 519                                      &(char){0}, sizeof(char));
 520                 if (nwritten != sizeof(char)) {
 521                         close(ready_pipe_caller_fd);
 522                         return -1;
 523                 }
 524                 close(ready_pipe_caller_fd);
 525
 526                 *_event_fd = event_pipe_caller_fd;
 527                 *_status_fd = status_sp_caller_fd;
 528
 529                 return pid;
 530         }
 531
 532 #ifndef HAVE_PTHREAD
 533         /* cleanup sigchld_handler */
 534         tfork_atfork_child();
 535 #endif
 536
 537         /*
 538          * The "waiter" child.
 539          */
 540         setproctitle("tfork waiter process");
 541         CatchSignal(SIGCHLD, SIG_DFL);
 542
 543         close(status_sp_caller_fd);
 544         close(event_pipe_caller_fd);
 545         close(ready_pipe_caller_fd);
 546
 547         pid = fork();
 548         if (pid == -1) {
 549                 state->waiter_errno = errno;
 550                 _exit(0);
 551         }
 552         if (pid == 0) {
 553                 /*
 554                  * The worker child.
 555                  */
 556
 557                 close(status_sp_waiter_fd);
 558                 close(event_pipe_waiter_fd);
 559
 560                 /*
 561                  * Wait for the caller to give us a go!
 562                  */
 563                 nread = sys_read(ready_pipe_worker_fd, &c, sizeof(char));
 564                 if (nread != sizeof(char)) {
 565                         _exit(1);
 566                 }
 567                 close(ready_pipe_worker_fd);
 568
 569                 return 0;
 570         }
 571         state->worker_pid = pid;
 572
 573         close(ready_pipe_worker_fd);
 574
 575         /*
 576          * We're going to stay around until child2 exits, so lets close all fds
 577          * other then the pipe fd we may have inherited from the caller.
 578          *
 579          * Dup event_sp_waiter_fd and status_sp_waiter_fd onto fds 0 and 1 so we
 580          * can then call closefrom(2).
 581          */
 582         if (event_pipe_waiter_fd > 0) {
 583                 int dup_fd = 0;
 584
 585                 if (status_sp_waiter_fd == 0) {
 586                         dup_fd = 1;
 587                 }
 588
 589                 do {
 590                         fd = dup2(event_pipe_waiter_fd, dup_fd);
 591                 } while ((fd == -1) && (errno == EINTR));
 592                 if (fd == -1) {
 593                         state->waiter_errno = errno;
 594                         kill(state->worker_pid, SIGKILL);
 595                         state->worker_pid = -1;
 596                         _exit(1);
 597                 }
 598                 event_pipe_waiter_fd = fd;
 599         }
 600
 601         if (status_sp_waiter_fd > 1) {
 602                 do {
 603                         fd = dup2(status_sp_waiter_fd, 1);
 604                 } while ((fd == -1) && (errno == EINTR));
 605                 if (fd == -1) {
 606                         state->waiter_errno = errno;
 607                         kill(state->worker_pid, SIGKILL);
 608                         state->worker_pid = -1;
 609                         _exit(1);
 610                 }
 611                 status_sp_waiter_fd = fd;
 612         }
 613
 614         closefrom(2);
 615
 616         /* Tell the caller we're ready */
 617         nwritten = sys_write(status_sp_waiter_fd, &(char){0}, sizeof(char));
 618         if (nwritten != sizeof(char)) {
 619                 _exit(1);
 620         }
 621
 622         tfork_global_free();
 623         state = NULL;
 624
 625         do {
 626                 ret = waitpid(pid, &status, 0);
 627         } while ((ret == -1) && (errno == EINTR));
 628         if (ret == -1) {
 629                 status = errno;
 630                 kill(pid, SIGKILL);
 631         }
 632
 633         /*
 634          * This writes the worker child exit status via our internal socketpair
 635          * so the tfork_status() implementation can read it from its end.
 636          */
 637         nwritten = sys_write(status_sp_waiter_fd, &status, sizeof(status));
 638         if (nwritten == -1) {
 639                 if (errno != EPIPE && errno != ECONNRESET) {
 640                         _exit(errno);
 641                 }
 642                 /*
 643                  * The caller exitted and didn't call tfork_status().
 644                  */
 645                 _exit(0);
 646         }
 647         if (nwritten != sizeof(status)) {
 648                 _exit(1);
 649         }
 650
 651         /*
 652          * This write to the event_fd returned by tfork_event_fd() and notifies
 653          * the caller that the worker child is done and he may now call
 654          * tfork_status().
 655          */
 656         nwritten = sys_write(event_pipe_waiter_fd, &(char){0}, sizeof(char));
 657         if (nwritten != sizeof(char)) {
 658                 _exit(1);
 659         }
 660
 661         /*
 662          * Wait for our parent (the process that called tfork_create()) to
 663          * close() the socketpair fd in tfork_status().
 664          *
 665          * Again, the caller might have exitted without calling tfork_status().
 666          */
 667         nread = sys_read(status_sp_waiter_fd, &c, 1);
 668         if (nread == -1) {
 669                 if (errno == EPIPE || errno == ECONNRESET) {
 670                         _exit(0);
 671                 }
 672                 _exit(errno);
 673         }
 674         if (nread != 1) {
 675                 _exit(255);
 676         }
 677
 678         _exit(0);
 679 }
 680
 681 static int tfork_create_reap_waiter(pid_t waiter_pid)
 682 {
 683         pid_t pid;
 684         int waiter_status;
 685
 686         if (waiter_pid == -1) {
 687                 return 0;
 688         }
 689
 690         kill(waiter_pid, SIGKILL);
 691
 692         do {
 693                 pid = waitpid(waiter_pid, &waiter_status, 0);
 694         } while ((pid == -1) && (errno == EINTR));
 695         assert(pid == waiter_pid);
 696
 697         return 0;
 698 }
 699
 700 struct tfork *tfork_create(void)
 701 {
 702         struct tfork_state *state = NULL;
 703         struct tfork *t = NULL;
 704         pid_t pid;
 705         int saved_errno;
 706         int ret = 0;
 707
 708 #ifdef HAVE_PTHREAD
 709         ret = pthread_once(&tfork_global_is_initialized,
 710                            tfork_global_initialize);
 711         if (ret != 0) {
 712                 return NULL;
 713         }
 714 #else
 715         tfork_global_initialize();
 716 #endif
 717
 718         state = tfork_global_get();
 719         if (state == NULL) {
 720                 return NULL;
 721         }
 722         *state = (struct tfork_state) {
 723                 .waiter_pid = -1,
 724                 .waiter_errno = ECANCELED,
 725                 .worker_pid = -1,
 726         };
 727
 728         t = malloc(sizeof(struct tfork));
 729         if (t == NULL) {
 730                 ret = -1;
 731                 goto cleanup;
 732         }
 733
 734         *t = (struct tfork) {
 735                 .event_fd = -1,
 736                 .status_fd = -1,
 737                 .waiter_pid = -1,
 738                 .worker_pid = -1,
 739         };
 740
 741         ret = tfork_install_sigchld_handler(&state->waiter_pid);
 742         if (ret != 0) {
 743                 goto cleanup;
 744         }
 745
 746         pid = tfork_start_waiter_and_worker(state,
 747                                             &t->event_fd,
 748                                             &t->status_fd);
 749         if (pid == -1) {
 750                 ret = -1;
 751                 goto cleanup;
 752         }
 753         if (pid == 0) {
 754                 /* In the worker */
 755                 tfork_global_free();
 756                 t->worker_pid = 0;
 757                 return t;
 758         }
 759
 760         t->waiter_pid = pid;
 761         t->worker_pid = state->worker_pid;
 762
 763 cleanup:
 764         if (ret == -1) {
 765                 saved_errno = errno;
 766
 767                 if (t != NULL) {
 768                         if (t->status_fd != -1) {
 769                                 close(t->status_fd);
 770                         }
 771                         if (t->event_fd != -1) {
 772                                 close(t->event_fd);
 773                         }
 774
 775                         ret = tfork_create_reap_waiter(state->waiter_pid);
 776                         assert(ret == 0);
 777
 778                         free(t);
 779                         t = NULL;
 780                 }
 781         }
 782
 783         ret = tfork_uninstall_sigchld_handler();
 784         assert(ret == 0);
 785
 786         tfork_global_free();
 787
 788         if (ret == -1) {
 789                 errno = saved_errno;
 790         }
 791         return t;
 792 }
 793
 794 pid_t tfork_child_pid(const struct tfork *t)
 795 {
 796         return t->worker_pid;
 797 }
 798
 799 int tfork_event_fd(struct tfork *t)
 800 {
 801         int fd = t->event_fd;
 802
 803         assert(t->event_fd != -1);
 804         t->event_fd = -1;
 805
 806         return fd;
 807 }
 808
 809 int tfork_status(struct tfork **_t, bool wait)
 810 {
 811         struct tfork *t = *_t;
 812         int status;
 813         ssize_t nread;
 814         int waiter_status;
 815         pid_t pid;
 816         int ret;
 817
 818         if (t == NULL) {
 819                 return -1;
 820         }
 821
 822         if (wait) {
 823                 set_blocking(t->status_fd, true);
 824
 825                 nread = sys_read(t->status_fd, &status, sizeof(int));
 826         } else {
 827                 set_blocking(t->status_fd, false);
 828
 829                 nread = read(t->status_fd, &status, sizeof(int));
 830                 if ((nread == -1) &&
 831                     ((errno == EAGAIN) || (errno == EWOULDBLOCK) || errno == EINTR)) {
 832                         errno = EAGAIN;
 833                         return -1;
 834                 }
 835         }
 836         if (nread != sizeof(int)) {
 837                 return -1;
 838         }
 839
 840         ret = tfork_install_sigchld_handler(&t->waiter_pid);
 841         if (ret != 0) {
 842                 return -1;
 843         }
 844
 845         /*
 846          * This triggers process exit in the waiter.
 847          * We write to the fd as well as closing it, as any tforked sibling
 848          * processes will also have the writable end of this socket open.
 849          *
 850          */
 851         {
 852                 size_t nwritten;
 853                 nwritten = sys_write(t->status_fd, &(char){0}, sizeof(char));
 854                 if (nwritten != sizeof(char)) {
 855                         close(t->status_fd);
 856                         return -1;
 857                 }
 858         }
 859         close(t->status_fd);
 860
 861         do {
 862                 pid = waitpid(t->waiter_pid, &waiter_status, 0);
 863         } while ((pid == -1) && (errno == EINTR));
 864         assert(pid == t->waiter_pid);
 865
 866         if (t->event_fd != -1) {
 867                 close(t->event_fd);
 868                 t->event_fd = -1;
 869         }
 870
 871         free(t);
 872         t = NULL;
 873         *_t = NULL;
 874
 875         ret = tfork_uninstall_sigchld_handler();
 876         assert(ret == 0);
 877
 878         return status;
 879 }
 880
 881 int tfork_destroy(struct tfork **_t)
 882 {
 883         struct tfork *t = *_t;
 884         int ret;
 885
 886         if (t == NULL) {
 887                 errno = EINVAL;
 888                 return -1;
 889         }
 890
 891         kill(t->worker_pid, SIGKILL);
 892
 893         ret = tfork_status(_t, true);
 894         if (ret == -1) {
 895                 return -1;
 896         }
 897
 898         return 0;
 899 }