src/basic/async.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2 /***
   3   This file is part of systemd.
   4
   5   Copyright 2013 Lennart Poettering
   6
   7   systemd is free software; you can redistribute it and/or modify it
   8   under the terms of the GNU Lesser General Public License as published by
   9   the Free Software Foundation; either version 2.1 of the License, or
  10   (at your option) any later version.
  11
  12   systemd is distributed in the hope that it will be useful, but
  13   WITHOUT ANY WARRANTY; without even the implied warranty of
  14   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  15   Lesser General Public License for more details.
  16
  17   You should have received a copy of the GNU Lesser General Public License
  18   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  19 ***/
  20
  21 #include <errno.h>
  22 #include <pthread.h>
  23 #include <stddef.h>
  24 #include <unistd.h>
  25
  26 #include "async.h"
  27 #include "fd-util.h"
  28 #include "log.h"
  29 #include "macro.h"
  30 #include "process-util.h"
  31 #include "signal-util.h"
  32 #include "util.h"
  33
  34 int asynchronous_job(void* (*func)(void *p), void *arg) {
  35         sigset_t ss, saved_ss;
  36         pthread_attr_t a;
  37         pthread_t t;
  38         int r, k;
  39
  40         /* It kinda sucks that we have to resort to threads to implement an asynchronous close(), but well, such is
  41          * life. */
  42
  43         r = pthread_attr_init(&a);
  44         if (r > 0)
  45                 return -r;
  46
  47         r = pthread_attr_setdetachstate(&a, PTHREAD_CREATE_DETACHED);
  48         if (r > 0) {
  49                 r = -r;
  50                 goto finish;
  51         }
  52
  53         if (sigfillset(&ss) < 0) {
  54                 r = -errno;
  55                 goto finish;
  56         }
  57
  58         /* Block all signals before forking off the thread, so that the new thread is started with all signals
  59          * blocked. This way the existence of the new thread won't affect signal handling in other threads. */
  60
  61         r = pthread_sigmask(SIG_BLOCK, &ss, &saved_ss);
  62         if (r > 0) {
  63                 r = -r;
  64                 goto finish;
  65         }
  66
  67         r = pthread_create(&t, &a, func, arg);
  68
  69         k = pthread_sigmask(SIG_SETMASK, &saved_ss, NULL);
  70
  71         if (r > 0)
  72                 r = -r;
  73         else if (k > 0)
  74                 r = -k;
  75         else
  76                 r = 0;
  77
  78 finish:
  79         pthread_attr_destroy(&a);
  80         return r;
  81 }
  82
  83 int asynchronous_sync(pid_t *ret_pid) {
  84         int r;
  85
  86         /* This forks off an invocation of fork() as a child process, in order to initiate synchronization to
  87          * disk. Note that we implement this as helper process rather than thread as we don't want the sync() to hang our
  88          * original process ever, and a thread would do that as the process can't exit with threads hanging in blocking
  89          * syscalls. */
  90
  91         r = safe_fork("(sd-sync)", FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS, ret_pid);
  92         if (r < 0)
  93                 return r;
  94         if (r == 0) {
  95                 /* Child process */
  96                 (void) sync();
  97                 _exit(EXIT_SUCCESS);
  98         }
  99
 100         return 0;
 101 }
 102
 103 static void *close_thread(void *p) {
 104         (void) pthread_setname_np(pthread_self(), "close");
 105
 106         assert_se(close_nointr(PTR_TO_FD(p)) != -EBADF);
 107         return NULL;
 108 }
 109
 110 int asynchronous_close(int fd) {
 111         int r;
 112
 113         /* This is supposed to behave similar to safe_close(), but
 114          * actually invoke close() asynchronously, so that it will
 115          * never block. Ideally the kernel would have an API for this,
 116          * but it doesn't, so we work around it, and hide this as a
 117          * far away as we can. */
 118
 119         if (fd >= 0) {
 120                 PROTECT_ERRNO;
 121
 122                 r = asynchronous_job(close_thread, FD_TO_PTR(fd));
 123                 if (r < 0)
 124                          assert_se(close_nointr(fd) != -EBADF);
 125         }
 126
 127         return -1;
 128 }