[PATCH] btrfs-progs: Force 'btrfs dev scan' not using old libblkid cache

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

'btrfs scan' uses libblkid to scan devices by default, and libblkid uses
cache to reduce the probe.

But if operations below is done in less than 2 seconds(BLKID_PROBE_MIN),
'btrfs scan' will still use the uncorrect cache and scan on the deleted device.
0. /dev/sda[1-4] mounted on /mnt using single data/metadata
1. btrfs dev scan
2. btrfs dev del /dev/sda3 /mnt
3. btrfs dev scan

Since the cache made by step 1 is still validated, step 3 will use the
cache and consider /dev/sda3 as a btrfs filesystem and try to scan it.
But the superblock(at least the first one) is wiped and failed to scan,
a error message, which can be avoided and is unneeded, is output.

This patch will force scan_for_btrfs() not to use cache to avoid the
problem.

Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Singed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
 utils.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/utils.c b/utils.c
index 0bfb9d9..1cd1d28 100644
--- a/utils.c
+++ b/utils.c
@@ -2057,7 +2057,8 @@ int btrfs_scan_lblkid(int update_kernel)
 	blkid_cache cache = NULL;
 	char path[PATH_MAX];
 
-	if (blkid_get_cache(&cache, 0) < 0) {
+	/* No to use libblkid cache to avoid old data */
+	if (blkid_get_cache(&cache, "/dev/null") < 0) {
 		printf("ERROR: lblkid cache get failed\n");
 		return 1;
 	}
-- 
1.9.1