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![Erik Johnston](/assets/img/avatar_default.png)
During the migration the automated script to update the copyright headers accidentally got rid of some of the existing copyright lines. Reinstate them.
158 lines
5.0 KiB
Python
158 lines
5.0 KiB
Python
#
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# This file is licensed under the Affero General Public License (AGPL) version 3.
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#
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# Copyright 2021 The Matrix.org Foundation C.I.C.
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# Copyright (C) 2023 New Vector, Ltd
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#
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# This program is free software: you can redistribute it and/or modify
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# it under the terms of the GNU Affero General Public License as
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# published by the Free Software Foundation, either version 3 of the
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# License, or (at your option) any later version.
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#
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# See the GNU Affero General Public License for more details:
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# <https://www.gnu.org/licenses/agpl-3.0.html>.
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#
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# Originally licensed under the Apache License, Version 2.0:
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# <http://www.apache.org/licenses/LICENSE-2.0>.
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#
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# [This file includes modifications made by New Vector Limited]
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#
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#
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"""A circular doubly linked list implementation.
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"""
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import threading
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from typing import Generic, Optional, Type, TypeVar
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P = TypeVar("P")
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LN = TypeVar("LN", bound="ListNode")
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class ListNode(Generic[P]):
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"""A node in a circular doubly linked list, with an (optional) reference to
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a cache entry.
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The reference should only be `None` for the root node or if the node has
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been removed from the list.
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"""
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# A lock to protect mutating the list prev/next pointers.
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_LOCK = threading.Lock()
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# We don't use attrs here as in py3.6 you can't have `attr.s(slots=True)`
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# and inherit from `Generic` for some reason
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__slots__ = [
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"cache_entry",
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"prev_node",
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"next_node",
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]
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def __init__(self, cache_entry: Optional[P] = None) -> None:
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self.cache_entry = cache_entry
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self.prev_node: Optional[ListNode[P]] = None
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self.next_node: Optional[ListNode[P]] = None
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@classmethod
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def create_root_node(cls: Type["ListNode[P]"]) -> "ListNode[P]":
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"""Create a new linked list by creating a "root" node, which is a node
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that has prev_node/next_node pointing to itself and no associated cache
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entry.
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"""
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root = cls()
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root.prev_node = root
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root.next_node = root
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return root
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@classmethod
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def insert_after(
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cls: Type[LN],
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cache_entry: P,
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node: "ListNode[P]",
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) -> LN:
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"""Create a new list node that is placed after the given node.
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Args:
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cache_entry: The associated cache entry.
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node: The existing node in the list to insert the new entry after.
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"""
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new_node = cls(cache_entry)
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with cls._LOCK:
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new_node._refs_insert_after(node)
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return new_node
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def remove_from_list(self) -> None:
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"""Remove this node from the list."""
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with self._LOCK:
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self._refs_remove_node_from_list()
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# We drop the reference to the cache entry to break the reference cycle
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# between the list node and cache entry, allowing the two to be dropped
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# immediately rather than at the next GC.
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self.cache_entry = None
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def move_after(self, node: "ListNode[P]") -> None:
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"""Move this node from its current location in the list to after the
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given node.
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"""
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with self._LOCK:
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# We assert that both this node and the target node is still "alive".
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assert self.prev_node
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assert self.next_node
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assert node.prev_node
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assert node.next_node
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assert self is not node
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# Remove self from the list
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self._refs_remove_node_from_list()
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# Insert self back into the list, after target node
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self._refs_insert_after(node)
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def _refs_remove_node_from_list(self) -> None:
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"""Internal method to *just* remove the node from the list, without
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e.g. clearing out the cache entry.
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"""
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if self.prev_node is None or self.next_node is None:
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# We've already been removed from the list.
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return
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prev_node = self.prev_node
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next_node = self.next_node
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prev_node.next_node = next_node
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next_node.prev_node = prev_node
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# We set these to None so that we don't get circular references,
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# allowing us to be dropped without having to go via the GC.
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self.prev_node = None
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self.next_node = None
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def _refs_insert_after(self, node: "ListNode[P]") -> None:
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"""Internal method to insert the node after the given node."""
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# This method should only be called when we're not already in the list.
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assert self.prev_node is None
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assert self.next_node is None
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# We expect the given node to be in the list and thus have valid
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# prev/next refs.
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assert node.next_node
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assert node.prev_node
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prev_node = node
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next_node = node.next_node
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self.prev_node = prev_node
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self.next_node = next_node
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prev_node.next_node = self
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next_node.prev_node = self
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def get_cache_entry(self) -> Optional[P]:
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"""Get the cache entry, returns None if this is the root node (i.e.
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cache_entry is None) or if the entry has been dropped.
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"""
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return self.cache_entry
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