Tag cache directories with CACHEDIR.TAG

diskcache/core.py

@@ -39,6 +39,14 @@ def __repr__(self):
 ENOVAL = Constant('ENOVAL')
 UNKNOWN = Constant('UNKNOWN')
 
+CACHEDIR_TAG = 'CACHEDIR.TAG'
+CACHEDIR_TAG_CONTENTS = (
+    b'Signature: 8a477f597d28d172789f06886806bc55\n'
+    b'# This file is a cache directory tag automatically created by diskcache.\n'
+    b'# For information about cache directory tags, see:\n'
+    b'#\thttps://bford.info/cachedir/\n'
+)
+
 MODE_NONE = 0
 MODE_RAW = 1
 MODE_BINARY = 2
@@ -453,6 +461,14 @@ def __init__(self, directory=None, timeout=60, disk=Disk, **settings):
                         ' and could not be created' % self._directory,
                     ) from None
 
+        tag_path = op.join(directory, CACHEDIR_TAG)
+        if not op.exists(tag_path):
+            try:
+                with open(tag_path, 'xb') as writer:
+                    writer.write(CACHEDIR_TAG_CONTENTS)
+            except OSError:
+                pass
+
         sql = self._sql_retry
 
         # Setup Settings table.
@@ -1967,7 +1983,7 @@ def check(self, fix=False, retry=False):
                     error = set(paths) - filenames
 
                     for full_path in error:
-                        if DBNAME in full_path:
+                        if DBNAME in full_path or CACHEDIR_TAG in full_path:
                             continue
 
                         message = 'unknown file: %s' % full_path

tests/test_core.py

@@ -565,6 +565,17 @@ def test_least_frequently_used(cache):
     assert len(cache.check()) == 0
 
 
+def test_cachedir_tag(cache):
+    tag_path = op.join(cache.directory, dc.core.CACHEDIR_TAG)
+    assert op.exists(tag_path)
+    with open(tag_path, 'rb') as reader:
+        assert reader.read().startswith(b'Signature: 8a477f597d28d172789f06886806bc55\n')
+
+    # The tag file should not be reported (or removed) as an unknown file.
+    assert len(cache.check(fix=True)) == 0
+    assert op.exists(tag_path)
+
+
 def test_check(cache):
     blob = b'a' * 2**20
     keys = (0, 1, 1234, 56.78, 'hello', b'world', None)