Decorators: replace asyncio.Lock with a custom object

Concerns were raised over possible race conditions due `asyncio.Lock`
internally awaiting coroutines. Does a mere `await` suspend the current
coroutine, or does it have to actually await something asynchronous,
like a future?

Avoid answering that question by doing away with the awaits, which
aren't necessary but are there as a consequence of using `asyncio.Lock`.
Instead, add a custom `LockGuard` object to replace the previous locks.

3 files changed, 28 insertions, 4 deletions

diff --git a/bot/decorators.py b/bot/decorators.py
index 0e84cf37e..3418dfd11 100644
--- a/bot/decorators.py
+++ b/bot/decorators.py
@@ -13,7 +13,7 @@ from discord.ext.commands import Cog, Context, check
 
 from bot.constants import Channels, ERROR_REPLIES, RedirectOutput
 from bot.errors import LockedResourceError
-from bot.utils import function
+from bot.utils import LockGuard, function
 from bot.utils.checks import in_whitelist_check, with_role_check, without_role_check
 
 log = logging.getLogger(__name__)
@@ -144,11 +144,11 @@ def mutually_exclusive(
 
             # Get the lock for the ID. Create a lock if one doesn't exist yet.
             locks = __lock_dicts[namespace]
-            lock = locks.setdefault(id_, asyncio.Lock())
+            lock = locks.setdefault(id_, LockGuard())
 
             if not lock.locked():
                 log.debug(f"{name}: resource {namespace!r}:{id_!r} is free; acquiring it...")
-                async with lock:
+                with lock:
                     return await func(*args, **kwargs)
             else:
                 log.info(f"{name}: aborted because resource {namespace!r}:{id_!r} is locked")
diff --git a/bot/utils/__init__.py b/bot/utils/__init__.py
index 5a6e1811b..0dd9605e8 100644
--- a/bot/utils/__init__.py
+++ b/bot/utils/__init__.py
@@ -2,9 +2,10 @@ from abc import ABCMeta
 
 from discord.ext.commands import CogMeta
 
+from bot.utils.lock import LockGuard
 from bot.utils.redis_cache import RedisCache
 
-__all__ = ['RedisCache', 'CogABCMeta']
+__all__ = ["CogABCMeta", "LockGuard", "RedisCache"]
 
 
 class CogABCMeta(CogMeta, ABCMeta):
diff --git a/bot/utils/lock.py b/bot/utils/lock.py
new file mode 100644
index 000000000..8f1b738aa
--- /dev/null
+++ b/bot/utils/lock.py
@@ -0,0 +1,23 @@
+class LockGuard:
+    """
+    A context manager which acquires and releases a lock (mutex).
+
+    Raise RuntimeError if trying to acquire a locked lock.
+    """
+
+    def __init__(self):
+        self._locked = False
+
+    def locked(self) -> bool:
+        """Return True if currently locked or False if unlocked."""
+        return self._locked
+
+    def __enter__(self):
+        if self._locked:
+            raise RuntimeError("Cannot acquire a locked lock.")
+
+        self._locked = True
+
+    def __exit__(self, _exc_type, _exc_value, _traceback):  # noqa: ANN001
+        self._locked = False
+        return False  # Indicate any raised exception shouldn't be suppressed.