Currently btrfs-progs will happily enumerate any device which has a
btrfs filesystem on it irrespective of its type. For the majority of
use cases that's fine and there haven't been any problems with that.
However, there was a recent report that in multipath scenario when
running "btrfs fi show" after a path flap (path going down and then
coming back up) instead of the multipath device being show the device
which represents the flapped path is shown. So a multipath filesystem
might look like:

Label: none  uuid: d3c1261f-18be-4015-9fef-6b35759dfdba
        Total devices 1 FS bytes used 192.00KiB
        devid    1 size 10.00GiB used 536.00MiB path /dev/mapper/3600140501cc1f49e5364f0093869c763

/dev/mapper/xxx is actually backed by an arbitrary number of paths,
which in turn are presented to the system as ordinary scsi devices i.e
/dev/sdX. If a path flaps and a user re-runs 'btrfs fi show' the output
would look like:

Label: none  uuid: d3c1261f-18be-4015-9fef-6b35759dfdba
        Total devices 1 FS bytes used 192.00KiB
        devid    1 size 10.00GiB used 536.00MiB path /dev/sdd

This only occurs on unmounted filesystems as those are enumerated by
btrfs-progs, for mounted filesystem the kernel properly deals only with
the actual multipath device.

Turns out the output of this command is consumed by libraries and the
presence of a path device rather than the actual multipath causes
issues.

Fix this by checking for the presence of DM_MULTIPATH_DEVICE_PATH
udev attribute as multipath path devices are tagged with this attribute
by the multipath udev scripts.

Signed-off-by: Nikolay Borisov <redacted>
---

David,

I haven't renamed is_path_device to is_multipath_device as per our chat offline,
in my mind multipath is the actual /dev/mapper/XXXXX device which precisely the
device we want scanned and individual paths are what I call path devices (don't
know if this is official nomenclature) and is what we want to ignore.

 common/device-scan.c | 47 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/common/device-scan.c b/common/device-scan.c
index b5bfe844104b..a1fd9f38b9df 100644
--- a/common/device-scan.c
+++ b/common/device-scan.c

@@ -14,6 +14,10 @@
  * Boston, MA 021110-1307, USA.
  */

+#ifdef STATIC_BUILD
+#undef HAVE_LIBUDEV
+#endif
+
 #include "kerncompat.h"
 #include <sys/ioctl.h>
 #include <stdlib.h>

@@ -25,6 +29,10 @@
 #include <dirent.h>
 #include <blkid/blkid.h>
 #include <uuid/uuid.h>
+#ifdef HAVE_LIBUDEV
+#include <sys/stat.h>
+#include <libudev.h>
+#endif
 #include "kernel-lib/overflow.h"
 #include "common/path-utils.h"
 #include "common/device-scan.h"

@@ -364,6 +372,42 @@ void free_seen_fsid(struct seen_fsid *seen_fsid_hash[])
 	}
 }

+#ifdef HAVE_LIBUDEV
+static bool is_path_device(char *device_path)
+{
+	struct udev *udev = NULL;
+	struct udev_device *dev = NULL;
+	struct stat dev_stat;
+	const char *val;
+	bool ret = false;
+
+	if (stat(device_path, &dev_stat) < 0)
+		return false;
+
+	udev = udev_new();
+	if (!udev)
+		goto out;
+
+	dev = udev_device_new_from_devnum(udev, 'b', dev_stat.st_rdev);
+	if (!dev)
+		goto out;
+
+	val = udev_device_get_property_value(dev, "DM_MULTIPATH_DEVICE_PATH");
+	if (val && atoi(val) > 0)
+		ret = true;
+out:
+	udev_device_unref(dev);
+	udev_unref(udev);
+
+	return ret;
+}
+#else
+static bool is_path_device(char *device_path)
+{
+	return false;
+}
+#endif
+
 int btrfs_scan_devices(int verbose)
 {
 	int fd = -1;

@@ -394,6 +438,9 @@ int btrfs_scan_devices(int verbose)
 		/* if we are here its definitely a btrfs disk*/
 		strncpy_null(path, blkid_dev_devname(dev));

+		if (is_path_device(path))
+			continue;
+
 		fd = open(path, O_RDONLY);
 		if (fd < 0) {
 			error("cannot open %s: %m", path);
--

2.17.1