Fix Backend service crash when RabbitMQ queues are predeclared

Signed-off-by: Fred Heinecke <fred.heinecke@yahoo.com>

back/src/Kyoo.RabbitMq/RabbitProducer.cs

@@ -30,6 +30,7 @@ public class RabbitProducer
 	{
 		_channel = rabbitConnection.CreateModel();
 
+		if (!doesExchangeExist(rabbitConnection, "events.resource"))
 			_channel.ExchangeDeclare("events.resource", ExchangeType.Topic);
 		_ListenResourceEvents<Collection>("events.resource");
 		_ListenResourceEvents<Movie>("events.resource");
@@ -39,6 +40,7 @@ public class RabbitProducer
 		_ListenResourceEvents<Studio>("events.resource");
 		_ListenResourceEvents<User>("events.resource");
 
+		if (!doesExchangeExist(rabbitConnection, "events.watched"))
 			_channel.ExchangeDeclare("events.watched", ExchangeType.Topic);
 		IWatchStatusRepository.OnMovieStatusChangedHandler += _PublishWatchStatus<Movie>("movie");
 		IWatchStatusRepository.OnShowStatusChangedHandler += _PublishWatchStatus<Show>("show");
@@ -47,6 +49,30 @@ public class RabbitProducer
 		);
 	}
 
+	/// <summary>
+	/// Checks if the exchange exists. Needed to avoid crashing when re-declaring an existing
+	/// queue with different parameters.
+	/// </summary>
+	/// <param name="rabbitConnection">The RabbitMQ connection.</param>
+	/// <param name="exchangeName">The name of the channel.</param>
+	/// <returns>True if the queue exists, false otherwise.</returns>
+	private bool doesExchangeExist(IConnection rabbitConnection, string exchangeName)
+	{
+		// If the queue does not exist when QueueDeclarePassive is called,
+		// an exception will be thrown. According to the docs, when this
+		// happens, the entire channel should be thrown away.
+		using var channel = rabbitConnection.CreateModel();
+		try
+		{
+			channel.ExchangeDeclarePassive(exchangeName);
+			return true;
+		}
+		catch (Exception)
+		{
+			return false;
+		}
+	}
+
 	private void _ListenResourceEvents<T>(string exchange)
 		where T : IResource, IQuery
 	{

back/src/Kyoo.RabbitMq/ScannerProducer.cs

@@ -31,10 +31,36 @@ public class ScannerProducer : IScanner
 	public ScannerProducer(IConnection rabbitConnection)
 	{
 		_channel = rabbitConnection.CreateModel();
+		if (!doesQueueExist(rabbitConnection, "scanner"))
 			_channel.QueueDeclare("scanner", exclusive: false, autoDelete: false);
+		if (!doesQueueExist(rabbitConnection, "scanner.rescan"))
 			_channel.QueueDeclare("scanner.rescan", exclusive: false, autoDelete: false);
 	}
 
+	/// <summary>
+	/// Checks if the queue exists. Needed to avoid crashing when re-declaring an existing
+	/// queue with different parameters.
+	/// </summary>
+	/// <param name="rabbitConnection">The RabbitMQ connection.</param>
+	/// <param name="queueName">The name of the channel.</param>
+	/// <returns>True if the queue exists, false otherwise.</returns>
+	private bool doesQueueExist(IConnection rabbitConnection, string queueName)
+	{
+		// If the queue does not exist when QueueDeclarePassive is called,
+		// an exception will be thrown. According to the docs, when this
+		// happens, the entire channel should be thrown away.
+		using var channel = rabbitConnection.CreateModel();
+		try
+		{
+			channel.QueueDeclarePassive(queueName);
+			return true;
+		}
+		catch (Exception)
+		{
+			return false;
+		}
+	}
+
 	private Task _Publish<T>(T message, string queue = "scanner")
 	{
 		var body = Encoding.UTF8.GetBytes(JsonSerializer.Serialize(message, Utility.JsonOptions));