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, 7 insertions, 0 deletions

diff --git a/src/gpt-auto-generator/gpt-auto-generator.c b/src/gpt-auto-generator/gpt-auto-generator.c
index 08bb9ac724..4731687b7f 100644
--- a/src/gpt-auto-generator/gpt-auto-generator.c
+++ b/src/gpt-auto-generator/gpt-auto-generator.c
@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <stdlib.h>
+#include <sys/file.h>
 #include <unistd.h>
 
 #include "sd-device.h"
@@ -696,6 +697,12 @@ static int enumerate_partitions(dev_t devnum) {
         if (r <= 0)
                 return r;
 
+        /* Let's take a LOCK_SH lock on the block device, in case udevd is already running. If we don't take
+         * the lock, udevd might end up issuing BLKRRPART in the middle, and we don't want that, since that
+         * might remove all partitions while we are operating on them. */
+        if (flock(fd, LOCK_SH) < 0)
+                return log_error_errno(errno, "Failed to lock root block device: %m");
+
         r = dissect_image(
                         fd,
                         NULL, NULL,