testsuite: sweep stale borg FUSE mounts left by aborted runs

When a test run is aborted (Ctrl-C, a timeout, or a crashed xdist worker),
the fuse_mount context manager's `finally: umount` does not run, leaving a
live FUSE mount under the pytest temp dir. A later run then fails to rm_rf
the old temp tree (Resource busy / Device not configured / Read-only file
system), which surfaces as noisy pytest warnings.

Add pytest_sessionstart/sessionfinish hooks (run once on the xdist
controller) that walk the pytest temp tree for leftover "mountpoint"
directories and unmount them via borg's own cross-platform umount helper,
making runs self-healing across previously aborted sessions.

Discovery uses only the stdlib (os.walk + os.path.ismount, with an
os.listdir fallback for dead/stale mounts) and the mount is named by the
well-known on-disk layout, so there is no OS-specific mount-table parsing or
force-unmount handling.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/borg/conftest.py

@@ -1,5 +1,7 @@
+import getpass
 import os
 import shutil
+import tempfile
 
 import pytest
 
@@ -16,6 +18,62 @@ from borg.testsuite import has_lchflags, has_llfuse, has_pyfuse3, has_mfusepy  #
 from borg.testsuite import are_symlinks_supported, are_hardlinks_supported, is_utime_fully_supported  # noqa: E402
 from borg.testsuite.archiver import BORG_EXES
 from borg.testsuite.platform.platform_test import fakeroot_detected  # noqa: E402
+from borg.helpers import umount  # noqa: E402
+
+
+def _pytest_tmp_root():
+    # Mirror the parent of pytest's basetemp: <temproot>/pytest-of-<user>.
+    # Old (already-removed-from-retention) run dirs live here too, which is where
+    # FUSE mounts orphaned by aborted runs hide.
+    try:
+        user = getpass.getuser()
+    except Exception:
+        user = "unknown"
+    return os.path.join(tempfile.gettempdir(), f"pytest-of-{user}")
+
+
+def _looks_mounted(path):
+    # A live FUSE mount reports as a mount point; a dead/stale one makes the
+    # directory inaccessible (os.path.ismount swallows that and returns False).
+    if os.path.ismount(path):
+        return True
+    try:
+        os.listdir(path)
+    except OSError:
+        return True
+    return False
+
+
+def _sweep_stale_fuse_mounts():
+    # The fuse test helper always mounts at a directory named "mountpoint" under the
+    # pytest temp tree and unmounts it in a finally block.  If a run is aborted, that
+    # cleanup is skipped and the mount lingers, so the next run's temp-dir GC (rm_rf)
+    # trips over it ("Resource busy" etc.).  Walk the temp tree, find such leftover
+    # mountpoints and unmount them with borg's own (cross-platform) umount helper.
+    root = _pytest_tmp_root()
+    for dirpath, dirnames, filenames in os.walk(root, onerror=lambda err: None):
+        if "mountpoint" not in dirnames:
+            continue
+        dirnames.remove("mountpoint")  # never descend into a (possibly stale) mount
+        mountpoint = os.path.join(dirpath, "mountpoint")
+        if _looks_mounted(mountpoint):
+            try:
+                umount(mountpoint)
+            except Exception:  # nosec B110
+                pass  # best-effort: never let cleanup break the test session
+
+
+def pytest_sessionstart(session):
+    # Run once in the main process (not in each xdist worker), before any temp-dir
+    # GC, to clear mounts left behind by a previous, aborted session.
+    if not hasattr(session.config, "workerinput"):
+        _sweep_stale_fuse_mounts()
+
+
+def pytest_sessionfinish(session, exitstatus):
+    # Safety net for anything stranded by this session (e.g. a hard interrupt).
+    if not hasattr(session.config, "workerinput"):
+        _sweep_stale_fuse_mounts()
 
 
 @pytest.fixture(autouse=True)