replace `LRUCache` internals with `OrderedDict`

Replacing the internals should make the implementation faster
and simpler since the order tracking is done by the `OrderedDict`.

Furthermore, this commit adds type hints to `LRUCache` and
renames the `upd` method to `replace` to make its use more clear.

src/borg/archive.py

@@ -1236,7 +1236,7 @@ class MetadataCollector:
 # (hash_func, chunk_length) -> chunk_hash
 # we play safe and have the hash_func in the mapping key, in case we
 # have different hash_funcs within the same borg run.
-zero_chunk_ids = LRUCache(10, dispose=lambda _: None)
+zero_chunk_ids = LRUCache(10)  # type: ignore[var-annotated]
 
 
 def cached_hash(chunk, id_hash):

src/borg/fuse.py

@@ -115,7 +115,7 @@ class ItemCache:
         # tend to re-read the same chunks over and over.
         # The capacity is kept low because increasing it does not provide any significant advantage,
         # but makes LRUCache's square behaviour noticeable and consumes more memory.
-        self.chunks = LRUCache(capacity=10, dispose=lambda _: None)
+        self.chunks = LRUCache(capacity=10)
 
         # Instrumentation
         # Count of indirect items, i.e. data is cached in the object cache, not directly in this cache
@@ -252,7 +252,7 @@ class FuseBackend:
         # not contained in archives.
         self._items = {}
         # cache up to <FILES> Items
-        self._inode_cache = LRUCache(capacity=FILES, dispose=lambda _: None)
+        self._inode_cache = LRUCache(capacity=FILES)
         # _inode_count is the current count of synthetic inodes, i.e. those in self._items
         self.inode_count = 0
         # Maps inode numbers to the inode number of the parent
@@ -445,8 +445,8 @@ class FuseOperations(llfuse.Operations, FuseBackend):
         self.decrypted_repository = decrypted_repository
         data_cache_capacity = int(os.environ.get("BORG_MOUNT_DATA_CACHE_ENTRIES", os.cpu_count() or 1))
         logger.debug("mount data cache capacity: %d chunks", data_cache_capacity)
-        self.data_cache = LRUCache(capacity=data_cache_capacity, dispose=lambda _: None)
-        self._last_pos = LRUCache(capacity=FILES, dispose=lambda _: None)
+        self.data_cache = LRUCache(capacity=data_cache_capacity)
+        self._last_pos = LRUCache(capacity=FILES)
 
     def sig_info_handler(self, sig_no, stack):
         logger.debug(
@@ -689,7 +689,7 @@ class FuseOperations(llfuse.Operations, FuseBackend):
             size -= n
             if not size:
                 if fh in self._last_pos:
-                    self._last_pos.upd(fh, (chunk_no, chunk_offset))
+                    self._last_pos.replace(fh, (chunk_no, chunk_offset))
                 else:
                     self._last_pos[fh] = (chunk_no, chunk_offset)
                 break

src/borg/helpers/lrucache.py

@@ -1,57 +1,71 @@
+from collections import OrderedDict
+from collections.abc import Callable, ItemsView, Iterator, KeysView, MutableMapping, ValuesView
+from typing import TypeVar
+
 sentinel = object()
+K = TypeVar("K")
+V = TypeVar("V")
 
 
-class LRUCache:
-    def __init__(self, capacity, dispose):
-        self._cache = {}
-        self._lru = []
+class LRUCache(MutableMapping[K, V]):
+    """
+    Mapping which maintains a maximum size by dropping the least recently used value.
+    Items are passed to dispose before being removed and replacing an item without
+    removing it first is forbidden.
+    """
+
+    _cache: OrderedDict[K, V]
+
+    _capacity: int
+
+    _dispose: Callable[[V], None]
+
+    def __init__(self, capacity: int, dispose: Callable[[V], None] = lambda _: None):
+        self._cache = OrderedDict()
         self._capacity = capacity
         self._dispose = dispose
 
-    def __setitem__(self, key, value):
+    def __setitem__(self, key: K, value: V) -> None:
         assert key not in self._cache, (
             "Unexpected attempt to replace a cached item," " without first deleting the old item."
         )
-        self._lru.append(key)
-        while len(self._lru) > self._capacity:
-            del self[self._lru[0]]
+        while len(self._cache) >= self._capacity:
+            self._dispose(self._cache.popitem(last=False)[1])
         self._cache[key] = value
+        self._cache.move_to_end(key)
 
-    def __getitem__(self, key):
-        value = self._cache[key]  # raise KeyError if not found
-        self._lru.remove(key)
-        self._lru.append(key)
-        return value
+    def __getitem__(self, key: K) -> V:
+        self._cache.move_to_end(key)  # raise KeyError if not found
+        return self._cache[key]
 
-    def __delitem__(self, key):
-        value = self._cache.pop(key)  # raise KeyError if not found
-        self._dispose(value)
-        self._lru.remove(key)
+    def __delitem__(self, key: K) -> None:
+        self._dispose(self._cache.pop(key))
 
-    def __contains__(self, key):
+    def __contains__(self, key: object) -> bool:
         return key in self._cache
 
-    def get(self, key, default=None):
-        value = self._cache.get(key, sentinel)
-        if value is sentinel:
-            return default
-        self._lru.remove(key)
-        self._lru.append(key)
-        return value
+    def __len__(self) -> int:
+        return len(self._cache)
 
-    def upd(self, key, value):
-        # special use only: update the value for an existing key without having to dispose it first
+    def replace(self, key: K, value: V) -> None:
+        """Replace an item which is already present, not disposing it in the process"""
         # this method complements __setitem__ which should be used for the normal use case.
         assert key in self._cache, "Unexpected attempt to update a non-existing item."
         self._cache[key] = value
 
-    def clear(self):
+    def clear(self) -> None:
         for value in self._cache.values():
             self._dispose(value)
         self._cache.clear()
 
-    def items(self):
-        return self._cache.items()
+    def __iter__(self) -> Iterator[K]:
+        return iter(self._cache)
 
-    def __len__(self):
-        return len(self._cache)
+    def keys(self) -> KeysView[K]:
+        return self._cache.keys()
+
+    def values(self) -> ValuesView[V]:
+        return self._cache.values()
+
+    def items(self) -> ItemsView[K, V]:
+        return self._cache.items()

src/borg/repository.py

@@ -1536,7 +1536,7 @@ class LoggedIO:
         else:
             # we only have fresh enough stuff here.
             # update the timestamp of the lru cache entry.
-            self.fds.upd(segment, (now, fd))
+            self.fds.replace(segment, (now, fd))
         return fd
 
     def close_segment(self):

src/borg/testsuite/lrucache.py

@@ -7,7 +7,7 @@ from ..helpers.lrucache import LRUCache
 
 class TestLRUCache:
     def test_lrucache(self):
-        c = LRUCache(2, dispose=lambda _: None)
+        c = LRUCache(2)
         assert len(c) == 0
         assert c.items() == set()
         for i, x in enumerate("abc"):