Ensure cached filesystems' protocol is preserved

Since `protocol` is a class attribute, it was not caught in
`__getattribute__`'s call to `__dict__`, leading to the target protocol
being used instead.

Author: mraspaud

fsspec/implementations/cached.py

@@ -483,6 +483,8 @@ def __getattribute__(self, item):
             return getattr(type(self), item)
         if item == "__class__":
             return type(self)
+        if item == "protocol":
+            return type(self).protocol
         d = object.__getattribute__(self, "__dict__")
         fs = d.get("fs", None)  # fs is not immediately defined
         if item in d:

fsspec/implementations/tests/test_cached.py

@@ -1337,3 +1337,8 @@ def test_filecache_write(tmpdir, m):
 
     assert m.cat(fn) == data.encode()
     assert fs.cat(fn) == data.encode()
+
+
+def test_cache_protocol_is_preserved():
+    fs = fsspec.filesystem("filecache", target_protocol="file")
+    assert fs.protocol == "filecache"