btrfs-progs: fsck-tests: Add test image for valid half-dropped orphan inode

Btrfs check used to report false alerts on half dropped orphan inodes.
Add test cases to prevent such problem from happening again.

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

tests/fsck-tests/042-half-dropped-inode/test.sh

@@ -0,0 +1,34 @@
+#!/bin/bash
+# To check if btrfs check can handle half dropped inodes.
+# Such inodes are orphan inodes going through file items evicting.
+# During that evicting, btrfs won't bother updating the nbytes of the orphan
+# inode as they will soon be removed completely.
+#
+# Btrfs check should be able to recognize such inodes without giving false
+# alerts
+#
+# The way to reproduce the image:
+# - Create a lot of regular file extents for one inode
+#   Using direct IO with small block size is the easiy method
+# - Modify kernel to commit transaction more aggresively
+#   Two locations are needed:
+#   * btrfs_unlink():
+#     To make the ORPHAN item reach disk asap
+#   * btrfs_evict_inode():
+#     Make the btrfs_end_transaction() call after btrfs_trncate_inode_items()
+#     to commit transaction, so we can catch every file item deletion
+# - Setup dm-log-writes
+#   To catch every transaction commit
+# - Delete the inode
+# - Sync the fs
+# - Replay the log
+
+source "$TEST_TOP/common"
+
+check_prereq btrfs
+
+check_image() {
+	run_check "$TOP/btrfs" check "$1"
+}
+
+check_all_images