Handle synchronization

src/pdns-dhcp/Kea/KeaLeaseWatcher.cs

@@ -92,20 +92,22 @@ public abstract class KeaDhcpLeaseWatcher : IHostedService
 			var loopToken = loopCts.Token;
 			_eventChannel = Channel.CreateUnbounded<FileSystemEventArgs>();
 			_fsw.EnableRaisingEvents = true;
-			using SemaphoreSlim waitTrap = new(0, 1);
-			using SemaphoreSlim changeEvent = new(0, 1);
-			Task reader = FileReader(waitTrap, changeEvent, loopToken);
+			using AutoResetEvent resetEvent = new(false);
+			Task reader = Task.CompletedTask;
 			try
 			{
-				await foreach (var @event in _eventChannel.Reader.ReadAllAsync(loopToken))
+				for (
+					EventArgs readArgs = EventArgs.Empty;
+					!loopToken.IsCancellationRequested;
+					readArgs = await _eventChannel.Reader.ReadAsync(loopToken))
 				{
 					// Guard for Deleted and renamed away events,
 					// both have to stop this reader immediately.
 					// Just wait for the file being created/renamed to _leaseFile.
 					// Described in [The LFC Process](https://kea.readthedocs.io/en/latest/arm/lfc.html#kea-lfc)
-					switch (@event)
+					switch (readArgs)
 					{
-						case { ChangeType: WatcherChangeTypes.Deleted }:
+						case FileSystemEventArgs { ChangeType: WatcherChangeTypes.Deleted }:
 						case RenamedEventArgs renamed when renamed.OldName == _leaseFile:
 							loopCts.Cancel();
 							continue;
@@ -113,12 +115,15 @@ public abstract class KeaDhcpLeaseWatcher : IHostedService
 
 					if (reader is not { IsCompleted: false })
 					{
-						reader = FileReader(waitTrap, changeEvent, loopToken);
+						// In any case that the reader failed (for whatever reason)
+						// restart now.
+						// Incoming event could be Changed/Created/Renamed
+						// This doesn't care, as we already lost the file handle.
+						reader = FileReader(resetEvent, stoppingToken);
 					}
-
-					if (waitTrap.Wait(0, CancellationToken.None))
+					else
 					{
-						changeEvent.Release();
+						resetEvent.Set();
 					}
 				}
 			}
@@ -138,7 +143,7 @@ public abstract class KeaDhcpLeaseWatcher : IHostedService
 		}
 	}
 
-	private async Task FileReader(SemaphoreSlim waitTrap, SemaphoreSlim changeEvent, CancellationToken stoppingToken)
+	private async Task FileReader(AutoResetEvent waitHandle, CancellationToken stoppingToken)
 	{
 		PipeWriter writer = _pipe.Writer;
 		SepReader? reader = null;
@@ -174,9 +179,7 @@ public abstract class KeaDhcpLeaseWatcher : IHostedService
 				}
 				else
 				{
-					var acquireLock = changeEvent.WaitAsync(stoppingToken);
-					waitTrap.Release();
-					await acquireLock.ConfigureAwait(continueOnCapturedContext: false);
+					await waitHandle.WaitOneAsync(stoppingToken).ConfigureAwait(continueOnCapturedContext: false);
 				}
 			}
 		}

src/pdns-dhcp/System/Threading/Tasks/AsyncWaitHandle.cs

@@ -0,0 +1,68 @@
+namespace System.Threading.Tasks;
+
+public static class AsyncWaitHandle
+{
+	public static Task WaitOneAsync(this WaitHandle waitHandle, CancellationToken cancellationToken = default)
+	{
+		WaitOneAsyncState state = new(waitHandle, cancellationToken);
+		return state.WaitOneAsync();
+	}
+
+	private class WaitOneAsyncState : IDisposable
+	{
+		private readonly CancellationTokenRegistration _cancellationTokenRegistration;
+		private readonly RegisteredWaitHandle _registeredWaitHandle;
+		private readonly TaskCompletionSource _tcs;
+		private bool _disposed;
+
+		public WaitOneAsyncState(WaitHandle waitHandle, CancellationToken cancellationToken)
+		{
+			_tcs = new();
+			_cancellationTokenRegistration = cancellationToken.Register((state, token) => ((WaitOneAsyncState)state!).Canceled(token), this);
+			_registeredWaitHandle = ThreadPool.RegisterWaitForSingleObject(waitHandle, (state, timeout) => ((WaitOneAsyncState)state!).Signaled(timeout), this, Timeout.Infinite, true);
+		}
+
+		public void Dispose()
+		{
+			if (_disposed)
+			{
+				return;
+			}
+			
+			_registeredWaitHandle.Unregister(default);
+			_cancellationTokenRegistration.Dispose();
+
+			_disposed = true;
+		}
+
+		public Task WaitOneAsync()
+		{
+			const TaskContinuationOptions options = TaskContinuationOptions.AttachedToParent | TaskContinuationOptions.ExecuteSynchronously;
+			return _tcs.Task.ContinueWith((upstream, state) => ((WaitOneAsyncState)state!).Continuation(upstream), this, options).Unwrap();
+		}
+
+		private void Canceled(CancellationToken token)
+		{
+			_tcs.SetCanceled(token);
+			this.Dispose();
+		}
+
+		private Task Continuation(Task task)
+		{
+			this.Dispose();
+			return task;
+		}
+
+		private void Signaled(bool timeout)
+		{
+			if (timeout)
+			{
+				Canceled(CancellationToken.None);
+			}
+			else
+			{
+				_tcs.SetResult();
+			}
+		}
+	}
+}