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a37e1b9 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 | """Shared concurrency helpers for plugin authors.
The most common plugin footgun is the lazy process-wide singleton:
_client = None
def get_client():
global _client
if _client is not None:
return _client
_client = ExpensiveClient(...) # <-- TOCTOU: two threads both run this
return _client
When two threads call ``get_client()`` before the singleton is set, both pass
the ``is not None`` guard, both run the expensive initialization, and the
second write clobbers the first — leaking whatever resource the first client
opened (connections, file handles, background threads).
Multi-threaded agent sessions share one process (delegated tool calls,
background workers, the self-improvement fork), so this race is reachable in
practice. Rather than make every plugin author remember to hand-roll
double-checked locking, this module gives them two thread-safe primitives:
* :func:`lazy_singleton` — decorator for the zero-arg accessor case.
* :class:`SingletonSlot` — manual slot for accessors that build different
instances depending on a config/key argument.
Both are import-light (stdlib ``threading`` only) so any plugin can import
them without dragging in heavyweight host modules.
"""
from __future__ import annotations
import functools
import threading
from typing import Callable, Generic, Optional, TypeVar
__all__ = ["lazy_singleton", "SingletonSlot"]
T = TypeVar("T")
def lazy_singleton(factory: Callable[[], T]) -> Callable[[], T]:
"""Wrap a zero-argument factory into a thread-safe lazy singleton accessor.
The wrapped callable returns the same instance on every call; the factory
runs exactly once even under concurrent first calls, using double-checked
locking. A ``.reset()`` attribute is attached for tests/teardown.
Example::
@lazy_singleton
def get_client():
return ExpensiveClient(load_config())
client = get_client() # built once, safe across threads
get_client.reset() # drop the instance (next call rebuilds)
Note: if the factory raises, no instance is cached and the next call
retries (the lock is released either way).
"""
lock = threading.Lock()
box: list = [] # one-element [instance]; empty == not yet built
@functools.wraps(factory)
def accessor() -> T:
if box:
return box[0]
with lock:
if box: # re-check inside the lock
return box[0]
instance = factory()
box.append(instance)
return instance
def reset() -> None:
with lock:
box.clear()
accessor.reset = reset # type: ignore[attr-defined]
return accessor
class SingletonSlot(Generic[T]):
"""Thread-safe lazy slot for accessors that take a build argument.
Use this when the cached instance depends on a config/key passed to the
accessor (so a bare zero-arg :func:`lazy_singleton` doesn't fit). The slot
caches the first successfully-built instance and ignores the argument on
subsequent calls — matching the established "first config wins" singleton
semantics most plugins already rely on.
Example::
_slot: SingletonSlot[Honcho] = SingletonSlot()
def get_honcho_client(config=None):
return _slot.get(lambda: Honcho(**resolve(config)))
def reset_honcho_client():
_slot.reset()
The factory runs at most once even under concurrent first calls. If the
factory raises, nothing is cached and the next call retries.
"""
__slots__ = ("_lock", "_value", "_set")
def __init__(self) -> None:
self._lock = threading.Lock()
self._value: Optional[T] = None
self._set = False
def get(self, factory: Callable[[], T]) -> T:
# Fast path: already built, no lock needed (a set bool + ref read is
# atomic under CPython's GIL).
if self._set:
return self._value # type: ignore[return-value]
with self._lock:
if self._set: # re-check inside the lock
return self._value # type: ignore[return-value]
value = factory()
self._value = value
self._set = True
return value
def peek(self) -> Optional[T]:
"""Return the cached instance without building it (None if unset)."""
return self._value if self._set else None
def reset(self) -> None:
"""Drop the cached instance so the next ``get()`` rebuilds it."""
with self._lock:
self._value = None
self._set = False
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