loop-device: implicitly sync device on detach

Apparently, if IO is still in flight at the moment we invoke LOOP_CLR_FD
it is likely simply dropped (probably because yanking physical storage,
such as a USB stick would drop it too). Let's protect ourselves against
that and always sync explicitly before we invoke it.

Author: poettering

src/shared/loop-util.c

@@ -191,6 +191,10 @@ LoopDevice* loop_device_unref(LoopDevice *d) {
                 return NULL;
 
         if (d->fd >= 0) {
+                /* Implicitly sync the device, since otherwise in-flight blocks might not get written */
+                if (fsync(d->fd) < 0)
+                        log_debug_errno(errno, "Failed to sync loop block device, ignoring: %m");
+
                 if (d->nr >= 0 && !d->relinquished) {
                         if (ioctl(d->fd, LOOP_CLR_FD) < 0)
                                 log_debug_errno(errno, "Failed to clear loop device: %m");
@@ -216,7 +220,7 @@ LoopDevice* loop_device_unref(LoopDevice *d) {
                                         log_warning_errno(errno, "Failed to remove device %s: %m", strna(d->node));
                                         break;
                                 }
-                                usleep(50 * USEC_PER_MSEC);
+                                (void) usleep(50 * USEC_PER_MSEC);
                         }
         }