test: Add test that reproduces a crash observed in the wild

This commit just adds a test for a bug that was very tricky to
reproduce. The test occured on existing backups after recent changes of
the single files handling, which caused these backups to break. Such
crashes are unacceptable! Old backups with valid configurations must
succeed!

This commit just adds the test and marks it as expected failure. Later
commits will refactor the test a bit and then add a fix.

Author: MaxG87

tests/test_backup_backends.py

@@ -1,4 +1,5 @@
 import datetime as dt
+import itertools
 import os
 from collections import Counter
 from pathlib import Path
@@ -230,6 +231,51 @@ def test_do_backup_removes_existing_files_in_exclude_list(
     assert result_content == expected_content
 
 
+@pytest.mark.xfail(reason="Reproduces a bug for which no fix is yet available.")
+@pytest.mark.parametrize(
+    "first_source, second_source",
+    [(FIRST_BACKUP, SECOND_BACKUP)],
+)
+def test_btrfs_backend_gracefully_handles_existing_snapshots_owned_by_root(
+    first_source, second_source, mounted_device
+) -> None:
+    # THIS IS A REGRESSION TEST!
+    #
+    # After the improvements on the handling of single files backups a weird error
+    # ocurred. Sometimes backups would fail due to "PermissionError"s. This happened
+    # when the existing snapshot was owned by root and was missing the single files
+    # target folder. This test reproduces this scenario and ensures that the backup
+    # works correctly, even in this case.
+    empty_config, device = mounted_device
+    if not isinstance(empty_config, cp.BtrFSRsyncConfig):
+        # This test works for BtrfsConfig only. However, encrypted_device on
+        # which mounted_device depends on, is parameterised over all backends.
+        # Since this simplifies many other tests it seemed to be an acceptable
+        # tradeoff to short-circuit the test here.
+        return
+
+    first_config = complement_configuration(empty_config, first_source)
+    first_backend = bb.BackupBackend.from_config(first_config)
+    first_backend.do_backup(device)
+
+    snapshot_root = device / first_config.BackupRepositoryFolder
+    latest_snapshot = sorted(snapshot_root.iterdir())[-1]
+    for cur in itertools.chain(snapshot_root.glob("*"), snapshot_root.glob("*/*")):
+        print(f"Changing ownership of {cur} to root:root")
+        sh.run_cmd(cmd=["sudo", "chown", "root:root", cur])
+    sh.run_cmd(cmd=["sudo", "rm", "-rf", latest_snapshot / first_config.FilesDest])
+
+    second_config = complement_configuration(empty_config, second_source).model_copy(
+        update={"ExcludePatternsFile": EXCLUDE_FILE}
+    )
+    second_backend = bb.BackupBackend.from_config(second_config)
+    second_backend.do_backup(device)
+
+    result_content = get_result_content(second_config, device)
+    expected_content = get_expected_content(second_config, exclude_to_ignore_file=True)
+    assert result_content == expected_content
+
+
 def test_do_backup_for_btrfs_creates_snapshots_with_timestamp_names(
     mounted_device,
 ) -> None: