Add support for evicting cache entries based on last access time. (#10205)

1 files changed, 150 insertions, 0 deletions

diff --git a/synapse/util/linked_list.py b/synapse/util/linked_list.py
new file mode 100644
index 0000000000..a456b136f0
--- /dev/null
+++ b/synapse/util/linked_list.py
@@ -0,0 +1,150 @@
+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""A circular doubly linked list implementation.
+"""
+
+import threading
+from typing import Generic, Optional, Type, TypeVar
+
+P = TypeVar("P")
+LN = TypeVar("LN", bound="ListNode")
+
+
+class ListNode(Generic[P]):
+    """A node in a circular doubly linked list, with an (optional) reference to
+    a cache entry.
+
+    The reference should only be `None` for the root node or if the node has
+    been removed from the list.
+    """
+
+    # A lock to protect mutating the list prev/next pointers.
+    _LOCK = threading.Lock()
+
+    # We don't use attrs here as in py3.6 you can't have `attr.s(slots=True)`
+    # and inherit from `Generic` for some reason
+    __slots__ = [
+        "cache_entry",
+        "prev_node",
+        "next_node",
+    ]
+
+    def __init__(self, cache_entry: Optional[P] = None) -> None:
+        self.cache_entry = cache_entry
+        self.prev_node: Optional[ListNode[P]] = None
+        self.next_node: Optional[ListNode[P]] = None
+
+    @classmethod
+    def create_root_node(cls: Type["ListNode[P]"]) -> "ListNode[P]":
+        """Create a new linked list by creating a "root" node, which is a node
+        that has prev_node/next_node pointing to itself and no associated cache
+        entry.
+        """
+        root = cls()
+        root.prev_node = root
+        root.next_node = root
+        return root
+
+    @classmethod
+    def insert_after(
+        cls: Type[LN],
+        cache_entry: P,
+        node: "ListNode[P]",
+    ) -> LN:
+        """Create a new list node that is placed after the given node.
+
+        Args:
+            cache_entry: The associated cache entry.
+            node: The existing node in the list to insert the new entry after.
+        """
+        new_node = cls(cache_entry)
+        with cls._LOCK:
+            new_node._refs_insert_after(node)
+        return new_node
+
+    def remove_from_list(self):
+        """Remove this node from the list."""
+        with self._LOCK:
+            self._refs_remove_node_from_list()
+
+        # We drop the reference to the cache entry to break the reference cycle
+        # between the list node and cache entry, allowing the two to be dropped
+        # immediately rather than at the next GC.
+        self.cache_entry = None
+
+    def move_after(self, node: "ListNode"):
+        """Move this node from its current location in the list to after the
+        given node.
+        """
+        with self._LOCK:
+            # We assert that both this node and the target node is still "alive".
+            assert self.prev_node
+            assert self.next_node
+            assert node.prev_node
+            assert node.next_node
+
+            assert self is not node
+
+            # Remove self from the list
+            self._refs_remove_node_from_list()
+
+            # Insert self back into the list, after target node
+            self._refs_insert_after(node)
+
+    def _refs_remove_node_from_list(self):
+        """Internal method to *just* remove the node from the list, without
+        e.g. clearing out the cache entry.
+        """
+        if self.prev_node is None or self.next_node is None:
+            # We've already been removed from the list.
+            return
+
+        prev_node = self.prev_node
+        next_node = self.next_node
+
+        prev_node.next_node = next_node
+        next_node.prev_node = prev_node
+
+        # We set these to None so that we don't get circular references,
+        # allowing us to be dropped without having to go via the GC.
+        self.prev_node = None
+        self.next_node = None
+
+    def _refs_insert_after(self, node: "ListNode"):
+        """Internal method to insert the node after the given node."""
+
+        # This method should only be called when we're not already in the list.
+        assert self.prev_node is None
+        assert self.next_node is None
+
+        # We expect the given node to be in the list and thus have valid
+        # prev/next refs.
+        assert node.next_node
+        assert node.prev_node
+
+        prev_node = node
+        next_node = node.next_node
+
+        self.prev_node = prev_node
+        self.next_node = next_node
+
+        prev_node.next_node = self
+        next_node.prev_node = self
+
+    def get_cache_entry(self) -> Optional[P]:
+        """Get the cache entry, returns None if this is the root node (i.e.
+        cache_entry is None) or if the entry has been dropped.
+        """
+        return self.cache_entry