tree-wide: take BSD lock on loopback devices we dissect/mount/operate on

So here's something we should always keep in mind:

systemd-udevd actually does *two* things with BSD file locks on block
devices:

1. While it probes a device it takes a LOCK_SH lock. Thus everyone else
   taking a LOCK_EX lock will temporarily block udev from probing
   devices, which is good when making changes to it.

2. Whenever a device is closed after write (detected via inotify), udevd
   will issue BLKRRPART (requesting the kernel to reread the partition
   table). It does this while holding a LOCK_EX lock on the block
   device. Thus anyone else taking LOCK_SH or LOCK_EX will temporarily
   block udevd from issuing that ioctl. And that's quite relevant, since
   the kernel will temporarily flush out all partitions while re-reading
   the partition table and then create them anew. Thus it is smart to
   take LOCK_SH when dissecting a block device to ensure that no
   BLKRRPART is issued in the background, until we mounted the devices.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/sysext/sysext.c b/src/sysext/sysext.c
index 6d4df0afd2..ccc0bd2687 100644
--- a/src/sysext/sysext.c
+++ b/src/sysext/sysext.c
@@ -533,6 +533,10 @@ static int merge_subprocess(Hashmap *images, const char *workspace) {
                         if (r < 0)
                                 return log_error_errno(r, "Failed to set up loopback device for %s: %m", img->path);
 
+                        r = loop_device_flock(d, LOCK_SH);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to lock loopback device: %m");
+
                         r = dissect_image_and_warn(
                                         d->fd,
                                         img->path,