[PATCH 2/2] btrfs-progs: find-root: Fix a bug that will cause wrong result

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

[BUG]
btrfs-find-root may not output desire result, as due to
search_extent_cache() may return result which doesn't cover the desired
range, generation cache can be screwed up if higher generation tree root
is found before lower generation tree root.

For example:
=======
./btrfs-find-root  /dev/sda6  -a
Superblock thinks the generation is 8
Superblock thinks the level is 0
adding bytenr: 4194304, gen: 8	<<< Debug output
adding bytenr: 24715264, gen: 7 <<< gen is 7 at read_tree_block time
Well block 4194304(gen: 8 level: 0) seems good, and it matches
superblock
Well block 24715264(gen: 8 level: 0) seems good, and it matches
superblock  <<< But its gen is wrong at result output time
=======

[Fix]
Add a new judgment to make sure the search_extent_cache() is returning
desired result.

Reported-by: Marc Merlin <marc@merlins.org>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
---
 find-root.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/find-root.c b/find-root.c
index 1af37b5..0eca0ab 100644
--- a/find-root.c
+++ b/find-root.c
@@ -46,9 +46,14 @@ static int add_eb_to_result(struct extent_buffer *eb,
 	    generation < filter->generation)
 		return ret;
 
-	/* Get the generation cache or create one */
+	/*
+	 * Get the generation cache or create one
+	 *
+	 * NOTE: search_cache_extent() may return cache doesn't cover
+	 * the range. So need extra judgement to make sure it's a match.
+	 */
 	cache = search_cache_extent(result, generation);
-	if (!cache) {
+	if (!cache || cache->start != generation) {
 		gen_cache = malloc(sizeof(*gen_cache));
 		BUG_ON(!gen_cache);
 		cache = &gen_cache->cache;
-- 
2.5.0