Write cache atomically using a tempfile in the same directory (#1111)

* Write cache atomically using a tempfile in the same directory

* Docstring

* update flake8 precommit

Co-authored-by: Martin Durant <[email protected]>

Author: jwodder

.pre-commit-config.yaml

@@ -16,8 +16,8 @@ repos:
     rev: 22.3.0
     hooks:
       - id: black
-  - repo: https://gitlab.com/pycqa/flake8
-    rev: 3.8.4
+  - repo: https://github.com/PyCQA/flake8
+    rev: 5.0.4
     hooks:
       - id: flake8
   - repo: https://github.com/asottile/seed-isort-config

fsspec/implementations/cached.py

@@ -1,11 +1,12 @@
+import contextlib
 import hashlib
 import inspect
 import logging
 import os
 import pickle
 import tempfile
 import time
-from shutil import move, rmtree
+from shutil import rmtree
 
 from fsspec import AbstractFileSystem, filesystem
 from fsspec.callbacks import _DEFAULT_CALLBACK
@@ -184,11 +185,9 @@ def save_cache(self):
         for c in cache.values():
             if isinstance(c["blocks"], set):
                 c["blocks"] = list(c["blocks"])
-        fd2, fn2 = tempfile.mkstemp()
-        with open(fd2, "wb") as f:
-            pickle.dump(cache, f)
         self._mkcache()
-        move(fn2, fn)
+        with atomic_write(fn) as f:
+            pickle.dump(cache, f)
         self.cached_files[-1] = cached_files
         self.last_cache = time.time()
 
@@ -264,7 +263,7 @@ def clear_expired_cache(self, expiry_time=None):
 
         if self.cached_files[-1]:
             cache_path = os.path.join(self.storage[-1], "cache")
-            with open(cache_path, "wb") as fc:
+            with atomic_write(cache_path) as fc:
                 pickle.dump(self.cached_files[-1], fc)
         else:
             rmtree(self.storage[-1])
@@ -834,3 +833,24 @@ def hash_name(path, same_name):
     else:
         hash = hashlib.sha256(path.encode()).hexdigest()
     return hash
+
+
+@contextlib.contextmanager
+def atomic_write(path, mode="wb"):
+    """
+    A context manager that opens a temporary file next to `path` and, on exit,
+    replaces `path` with the temporary file, thereby updating `path`
+    atomically.
+    """
+    fd, fn = tempfile.mkstemp(
+        dir=os.path.dirname(path), prefix=os.path.basename(path) + "-"
+    )
+    try:
+        with open(fd, mode) as fp:
+            yield fp
+    except BaseException:
+        with contextlib.suppress(FileNotFoundError):
+            os.unlink(fn)
+        raise
+    else:
+        os.replace(fn, path)