src/basic/async.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <errno.h>
   4 #include <pthread.h>
   5 #include <stddef.h>
   6 #include <unistd.h>
   7
   8 #include "async.h"
   9 #include "fd-util.h"
  10 #include "log.h"
  11 #include "macro.h"
  12 #include "process-util.h"
  13 #include "signal-util.h"
  14 #include "util.h"
  15
  16 int asynchronous_job(void* (*func)(void *p), void *arg) {
  17         sigset_t ss, saved_ss;
  18         pthread_attr_t a;
  19         pthread_t t;
  20         int r, k;
  21
  22         /* It kinda sucks that we have to resort to threads to implement an asynchronous close(), but well, such is
  23          * life. */
  24
  25         r = pthread_attr_init(&a);
  26         if (r > 0)
  27                 return -r;
  28
  29         r = pthread_attr_setdetachstate(&a, PTHREAD_CREATE_DETACHED);
  30         if (r > 0) {
  31                 r = -r;
  32                 goto finish;
  33         }
  34
  35         assert_se(sigfillset(&ss) >= 0);
  36
  37         /* Block all signals before forking off the thread, so that the new thread is started with all signals
  38          * blocked. This way the existence of the new thread won't affect signal handling in other threads. */
  39
  40         r = pthread_sigmask(SIG_BLOCK, &ss, &saved_ss);
  41         if (r > 0) {
  42                 r = -r;
  43                 goto finish;
  44         }
  45
  46         r = pthread_create(&t, &a, func, arg);
  47
  48         k = pthread_sigmask(SIG_SETMASK, &saved_ss, NULL);
  49
  50         if (r > 0)
  51                 r = -r;
  52         else if (k > 0)
  53                 r = -k;
  54         else
  55                 r = 0;
  56
  57 finish:
  58         pthread_attr_destroy(&a);
  59         return r;
  60 }
  61
  62 int asynchronous_sync(pid_t *ret_pid) {
  63         int r;
  64
  65         /* This forks off an invocation of fork() as a child process, in order to initiate synchronization to
  66          * disk. Note that we implement this as helper process rather than thread as we don't want the sync() to hang our
  67          * original process ever, and a thread would do that as the process can't exit with threads hanging in blocking
  68          * syscalls. */
  69
  70         r = safe_fork("(sd-sync)", FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS, ret_pid);
  71         if (r < 0)
  72                 return r;
  73         if (r == 0) {
  74                 /* Child process */
  75                 (void) sync();
  76                 _exit(EXIT_SUCCESS);
  77         }
  78
  79         return 0;
  80 }
  81
  82 static void *close_thread(void *p) {
  83         (void) pthread_setname_np(pthread_self(), "close");
  84
  85         assert_se(close_nointr(PTR_TO_FD(p)) != -EBADF);
  86         return NULL;
  87 }
  88
  89 int asynchronous_close(int fd) {
  90         int r;
  91
  92         /* This is supposed to behave similar to safe_close(), but
  93          * actually invoke close() asynchronously, so that it will
  94          * never block. Ideally the kernel would have an API for this,
  95          * but it doesn't, so we work around it, and hide this as a
  96          * far away as we can. */
  97
  98         if (fd >= 0) {
  99                 PROTECT_ERRNO;
 100
 101                 r = asynchronous_job(close_thread, FD_TO_PTR(fd));
 102                 if (r < 0)
 103                          assert_se(close_nointr(fd) != -EBADF);
 104         }
 105
 106         return -1;
 107 }