fix: bug???

src/Data.Async.RwLock.purs

@@ -50,8 +50,8 @@ data RwLock a = RwLock
 makeReadGuard :: forall a. AVar ReaderId -> AVar (Set ReaderId) -> AVar a -> a -> Aff (ReadGuard a)
 makeReadGuard nextReaderIdCell readersCell stateCell state = do
   nextReaderId <- AVar.take nextReaderIdCell
-  ids <- AVar.take readersCell
   AVar.put (nextReaderId + ReaderId 1) nextReaderIdCell
+  ids <- AVar.take readersCell
   AVar.put (Set.insert nextReaderId ids) readersCell
   AVar.put state stateCell
   pure $ ReadGuard nextReaderId state
@@ -59,8 +59,8 @@ makeReadGuard nextReaderIdCell readersCell stateCell state = do
 mayWrite :: AVar (Array (Effect Unit)) -> Aff Unit
 mayWrite writerQCell = do
   writerQueue <- AVar.take writerQCell
-  liftEffect $ for_ (Array.head writerQueue) identity
   AVar.put (fromMaybe [] $ Array.tail writerQueue) writerQCell
+  liftEffect $ fromMaybe (pure unit) $ Array.head writerQueue
 
 instance AsyncState RwLock WriteGuard ReadGuard where
   boxed a = liftAff do
@@ -74,13 +74,14 @@ instance AsyncStateWritable RwLock WriteGuard where
   write _ (WriteGuard stateCell) s = do
     void $ liftAff $ liftMaybe (error "WriteGuard used after `unlock` invoked!") =<< AVar.tryTake stateCell
     liftAff $ void $ whileJust $ void <$> AVar.tryTake stateCell
-    liftAff $ AVar.put s stateCell
+    wasPut <- liftAff $ AVar.tryPut s stateCell
+    if not wasPut then
+      liftAff $ throwError $ error "WriteGuard has multiple pending writes - why?"
+    else
+      pure unit
 
 instance AsyncStateReadable RwLock WriteGuard where
-  read _ (WriteGuard stateCell) =
-    liftAff
-      $ liftMaybe (error "WriteGuard used after `unlock` invoked!")
-          =<< AVar.tryRead stateCell
+  read _ (WriteGuard stateCell) = liftAff $ liftMaybe (error "WriteGuard used after `unlock` invoked!") =<< AVar.tryRead stateCell
 
 instance AsyncStateReadable RwLock ReadGuard where
   read (RwLock { readers: readersCell }) (ReadGuard id a) = liftAff do
@@ -103,6 +104,7 @@ instance AsyncStateLock RwLock ReadGuard where
 instance AsyncStateLock RwLock WriteGuard where
   unlock (RwLock { writerQueue: writerQCell, state: rwLockStateCell }) (WriteGuard localStateCell) = liftAff do
     state <- AVar.take localStateCell
+    void $ AVar.tryTake rwLockStateCell
     AVar.put state rwLockStateCell
     mayWrite writerQCell
   tryLock (RwLock { readers: readersCell, state: stateCell }) = liftAff do
@@ -133,4 +135,4 @@ instance AsyncStateLock RwLock WriteGuard where
     else do
       AVar.put readers readersCell
       blockOnWritable
-      lock rw
+      onWritable