Fix test_passthroughfs() failure due to unexpected mtime/ctime differences (#130)

What's happening here is the following:

- If the FUSE (kernel-level) writeback cache is enabled, the filesystem
  daemon cannot be trusted by the kernel to produce accurate mtime and
  ctime values, because there may be pending writes in the cache that
  have not been flushed to the daemon.
- Therefore, the kernel still calls the getattr() handler, but ignores
  the mtime and ctime values, and instead maintains them internally.
- When a file is closed, the kernel communicates the correct mtime and
  ctime values to the filesystem daemon through an extra setattr() call.
- Therefore, the (correct) mtime value that a userspace program gets
  from the kernel for the FUSE filesystem will not agree with the (also
  correct) mtime value reported by the underlying filesystem as long as
  the file is open.
- Once the file has been closed, the mtime values agree, but the
  filesystem daemon has updated this with an utimens() call which has
  resulted in ctime change, so now the ctime values do not agree.

To fix this, make the writeback cache configurable and only check mtimes
if writeback caching is disabled.

Fixes: #57.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: Nikratio

examples/passthroughfs.py

@@ -72,10 +72,9 @@
 
 
 class Operations(pyfuse3.Operations):
-    enable_writeback_cache = True
-
-    def __init__(self, source: str) -> None:
+    def __init__(self, source: str, enable_writeback_cache: bool = False) -> None:
         super().__init__()
+        self.enable_writeback_cache = enable_writeback_cache
         self._inode_path_map: dict[InodeT, str | set[str]] = {pyfuse3.ROOT_INODE: source}
         self._lookup_cnt: defaultdict[InodeT, int] = defaultdict(lambda: 0)
         self._fd_inode_map: dict[int, InodeT] = dict()
@@ -534,14 +533,20 @@ def parse_args(args: list[str]) -> Namespace:
     parser.add_argument(
         '--debug-fuse', action='store_true', default=False, help='Enable FUSE debugging output'
     )
+    parser.add_argument(
+        '--enable-writeback-cache',
+        action='store_true',
+        default=False,
+        help='Enable writeback cache (default: disabled)',
+    )
 
     return parser.parse_args(args)
 
 
 def main() -> None:
     options = parse_args(sys.argv[1:])
     init_logging(options.debug)
-    operations = Operations(options.source)
+    operations = Operations(options.source, enable_writeback_cache=options.enable_writeback_cache)
 
     log.debug('Mounting...')
     fuse_options = set(pyfuse3.default_options)

test/test_examples.py

@@ -96,7 +96,8 @@ def test_tmpfs(tmpdir):
         umount(mount_process, mnt_dir)
 
 
-def test_passthroughfs(tmpdir):
+@pytest.mark.parametrize('enable_writeback_cache', (True, False))
+def test_passthroughfs(tmpdir, enable_writeback_cache):
     mnt_dir = str(tmpdir.mkdir('mnt'))
     src_dir = str(tmpdir.mkdir('src'))
     cmdline = [
@@ -105,6 +106,8 @@ def test_passthroughfs(tmpdir):
         src_dir,
         mnt_dir,
     ]
+    if enable_writeback_cache:
+        cmdline.append('--enable-writeback-cache')
     mount_process = subprocess.Popen(cmdline, stdin=subprocess.DEVNULL, universal_newlines=True)
     try:
         wait_for_mount(mount_process, mnt_dir)
@@ -125,7 +128,7 @@ def test_passthroughfs(tmpdir):
         tst_truncate_path(mnt_dir)
         tst_truncate_fd(mnt_dir)
         tst_unlink(mnt_dir)
-        tst_passthrough(src_dir, mnt_dir)
+        tst_passthrough(src_dir, mnt_dir, enable_writeback_cache=enable_writeback_cache)
     except:
         cleanup(mount_process, mnt_dir)
         raise
@@ -408,7 +411,7 @@ def tst_rounding(mnt_dir, ns_tol=0):
     checked_unlink(filename, mnt_dir, isdir=True)
 
 
-def tst_passthrough(src_dir, mnt_dir):
+def tst_passthrough(src_dir, mnt_dir, enable_writeback_cache: bool = False):
     # Test propagation from source to mirror
     name = name_generator()
     src_name = os.path.join(src_dir, name)
@@ -419,7 +422,7 @@ def tst_passthrough(src_dir, mnt_dir):
         fh.write('Hello, world')
     assert name in os.listdir(src_dir)
     assert name in os.listdir(mnt_dir)
-    assert_same_stats(src_name, mnt_name)
+    assert_same_stats(src_name, mnt_name, check_times=not enable_writeback_cache)
 
     # Test propagation from mirror to source
     name = name_generator()
@@ -431,7 +434,7 @@ def tst_passthrough(src_dir, mnt_dir):
         fh.write('Hello, world')
     assert name in os.listdir(src_dir)
     assert name in os.listdir(mnt_dir)
-    assert_same_stats(src_name, mnt_name)
+    assert_same_stats(src_name, mnt_name, check_times=not enable_writeback_cache)
 
     # Test propagation inside subdirectory
     name = name_generator()
@@ -446,10 +449,10 @@ def tst_passthrough(src_dir, mnt_dir):
         fh.write('Hello, world')
     assert name in os.listdir(src_dir)
     assert name in os.listdir(mnt_dir)
-    assert_same_stats(src_name, mnt_name)
+    assert_same_stats(src_name, mnt_name, check_times=not enable_writeback_cache)
 
 
-def assert_same_stats(name1, name2):
+def assert_same_stats(name1, name2, check_times: bool = True):
     stat1 = os.stat(name1)
     stat2 = os.stat(name2)
 
@@ -471,4 +474,11 @@ def assert_same_stats(name1, name2):
         if name.endswith('_ns') and os.getenv('CI') == 'true':
             continue
 
+        # When FUSE writeback cache is enabled, the kernel maintains mtime/ctime
+        # internally and only flushes them to the underlying filesystem on close.
+        # Until then, the timestamps reported for the passthrough mount and the
+        # backing directory may legitimately differ, so skip strict time checks.
+        if name.endswith('_ns') and not check_times:
+            continue
+
         assert v1 == v2, 'Attribute {} differs by {} ({} vs {})'.format(name, v1 - v2, v1, v2)