btrfs-progs: tests: in misc/038 test cases

The test case always fails in my VM, with the following error:

  $ sudo TEST=038\* make test-misc
     [TEST]   misc-tests.sh
     [TEST/misc]   038-backup-root-corruption
  Backup 2 not overwritten
  test failed for case 038-backup-root-corruption

After more debugging, it turns out that there is nothing wrong except
the final check:

  [ "$main_root_ptr" -ne "$backup_new_root_ptr" ] || _fail "Backup 2 not overwritten"

The _fail() is only triggered if the previous check returns false, which
is completely the opposite.

Furthermore on the github CI, the kernel commits 2 instead of 1
transaction, resulting the next slot never to match the current
generation/tree root.

The two bugs combined, github CI always passses the test case, while
for my VM which does the expected one transaction, it would always fail.

Fix it by:

- Use a proper "if [] then; fi" block to check the tree root bytenr
- Use the generation diff to calculate the expected backup root slot
- Log the full super block dump for debug usage

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

tests/misc-tests/038-backup-root-corruption/test.sh

@@ -41,6 +41,9 @@ slot_num=$(echo $found | cut -f1 -d:)
 # To follow the dump-super output, where backup slot starts at 0.
 slot_num=$(($slot_num - 1))
 
+_log "Original superblock:"
+_log "$(dump_super)"
+
 # Save the backup slot info into the log
 _log "Backup slot $slot_num will be utilized"
 dump_super | run_check grep -A9 "backup $slot_num:"
@@ -56,9 +59,14 @@ run_check_mount_test_dev -o usebackuproot
 run_check_umount_test_dev
 
 main_root_ptr=$(dump_super | awk '/^root\t/{print $2}')
-
-# The next slot should be overwritten
-slot_num=$(( ($slot_num + 1) % 4 ))
+cur_gen=$(dump_super | grep ^generation | awk '{print $2}')
+# The slot to be used is based on how many transaction are committed.
+slot_num=$(( ($slot_num + $cur_gen - $backup_gen) % 4 ))
 backup_new_root_ptr=$(dump_super | grep -A1 "backup $slot_num" | grep backup_tree_root | awk '{print $2}')
 
-[ "$main_root_ptr" -ne "$backup_new_root_ptr" ] || _fail "Backup 2 not overwritten"
+_log "After the backup usage:"
+_log "$(dump_super)"
+
+if [ "$main_root_ptr" -ne "$backup_new_root_ptr" ]; then
+	_fail "Backup ${slot_num} not overwritten"
+fi