fix: preserve existing files when checking FSMap write access

fsspec/mapping.py

@@ -1,6 +1,7 @@
 import array
 import logging
 import posixpath
+import uuid
 import warnings
 from collections.abc import MutableMapping
 from functools import cached_property
@@ -60,8 +61,9 @@ def __init__(self, root, fs, check=False, create=False, missing_exceptions=None)
                     f"Path {root} does not exist. Create "
                     f" with the ``create=True`` keyword"
                 )
-            self.fs.touch(root + "/a")
-            self.fs.rm(root + "/a")
+            check_path = f"{root}/{uuid.uuid4().hex}"
+            self.fs.touch(check_path)
+            self.fs.rm(check_path)
 
     @cached_property
     def dirfs(self):

fsspec/tests/test_mapping.py

@@ -28,6 +28,19 @@ def test_mapping_prefix(tmpdir):
     assert m == m2 == m3
 
 
+@pytest.mark.parametrize("protocol", ["file", "memory"])
+@pytest.mark.parametrize("key", ["a", "a/nested"])
+def test_check_preserves_contents(tmp_path, protocol, key):
+    url = f"{protocol}://{tmp_path.as_posix()}/mapping"
+    mapper = fsspec.get_mapper(url, create=True)
+    contents = {key: b"existing data", "other": b"more data"}
+    mapper.update(contents)
+
+    checked = fsspec.get_mapper(url, check=True)
+
+    assert dict(checked) == contents
+
+
 def test_getitems_errors(tmpdir):
     tmpdir = str(tmpdir)
     os.makedirs(os.path.join(tmpdir, "afolder"))