add lock to ReAwaitable

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: proboscis

returns/primitives/reawaitable.py

@@ -2,6 +2,8 @@
 from functools import wraps
 from typing import NewType, ParamSpec, TypeVar, cast, final
 
+import anyio
+
 _ValueType = TypeVar('_ValueType')
 _AwaitableT = TypeVar('_AwaitableT', bound=Awaitable)
 _Ps = ParamSpec('_Ps')
@@ -48,10 +50,11 @@ class ReAwaitable:
 
     """
 
-    __slots__ = ('_cache', '_coro')
+    __slots__ = ('_cache', '_coro', '_lock')
 
     def __init__(self, coro: Awaitable[_ValueType]) -> None:
         """We need just an awaitable to work with."""
+        self._lock = anyio.Lock()
         self._coro = coro
         self._cache: _ValueType | _Sentinel = _sentinel
 
@@ -101,9 +104,10 @@ def __repr__(self) -> str:
 
     async def _awaitable(self) -> _ValueType:
         """Caches the once awaited value forever."""
-        if self._cache is _sentinel:
-            self._cache = await self._coro
-        return self._cache  # type: ignore
+        async with self._lock:
+            if self._cache is _sentinel:
+                self._cache = await self._coro
+            return self._cache  # type: ignore
 
 
 def reawaitable(

tests/test_primitives/test_reawaitable/test_reawaitable_concurrency.py

@@ -0,0 +1,20 @@
+import anyio
+import pytest
+
+from returns.primitives.reawaitable import ReAwaitable
+
+
+async def sample_coro():
+    await anyio.sleep(0.1)
+    return "done"
+
+@pytest.mark.anyio
+async def test_concurrent_awaitable():
+    reawaitable = ReAwaitable(sample_coro())
+
+    async def await_reawaitable():
+        return await reawaitable
+    async with anyio.create_task_group() as tg:
+        task1 = tg.start_soon(await_reawaitable)
+        task2 = tg.start_soon(await_reawaitable)
+