btrfs-progs: tests: Test backup root retention logic

This tests ensures that the kernel correctly persists backup roots in
case the filesystem has been mounted from a backup root.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
[ cleanup to use common helpers ]
Signed-off-by: David Sterba <dsterba@suse.com>

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

@@ -0,0 +1,45 @@
+#!/bin/bash
+# Test that a corrupted filesystem will correctly handle writing of backup root
+
+source "$TEST_TOP/common"
+
+check_prereq mkfs.btrfs
+check_prereq btrfs
+check_prereq btrfs-corrupt-block
+
+setup_root_helper
+prepare_test_dev
+
+# Create a file and unmount to commit some backup roots
+run_check_mkfs_test_dev
+run_check_mount_test_dev
+run_check $SUDO_HELPER touch "$TEST_MNT/file"
+run_check_umount_test_dev
+
+dump_super() {
+	run_check_stdout $SUDO_HELPER "$TOP/btrfs" inspect-internal dump-super -f "$TEST_DEV"
+}
+
+# Ensure currently active backup slot is the expected one (slot 3)
+backup2_root_ptr=$(dump_super | grep -A1 "backup 2" | grep backup_tree_root | awk '{print $2}')
+
+main_root_ptr=$(dump_super | grep root | head -n1 | awk '{print $2}')
+
+[ "$backup2_root_ptr" -eq "$main_root_ptr" ] || _fail "Backup slot 2 is not in use"
+
+run_check "$TOP/btrfs-corrupt-block" -m $main_root_ptr -f generation "$TEST_DEV"
+
+# Should fail because the root is corrupted
+run_mustfail "Unexpected successful mount" \
+	$SUDO_HELPER mount "$TEST_DEV" "$TEST_MNT"
+
+# Cycle mount with the backup to force rewrite of slot 3
+run_check_mount_test_dev -o usebackuproot
+run_check_umount_test_dev
+
+# Since we've used backup 1 as the usable root, then backup 2 should have been
+# overwritten
+main_root_ptr=$(dump_super | grep root | head -n1 | awk '{print $2}')
+backup2_new_root_ptr=$(dump_super | grep -A1 "backup 2" | grep backup_tree_root | awk '{print $2}')
+
+[ "$backup2_root_ptr" -ne "$backup2_new_root_ptr" ] || _fail "Backup 2 not overwritten"