Adding filters should not force initialization of Agent.

src/Elastic.Apm/Agent.cs

@@ -39,7 +39,6 @@ public interface IApmAgent : IApmAgentComponents
 	internal class ApmAgent : IApmAgent, IDisposable
 	{
 		internal readonly CompositeDisposable Disposables = new();
-
 		internal ApmAgent(AgentComponents agentComponents) => Components = agentComponents ?? new AgentComponents();
 
 		internal ICentralConfigurationFetcher CentralConfigurationFetcher => Components.CentralConfigurationFetcher;
@@ -72,16 +71,31 @@ public static class Agent
 			lock (InitializationLock)
 			{
 				var agent = new ApmAgent(Components);
-
 				agent.Logger?.Trace()
 					?.Log("Initialization - Agent instance initialized. Callstack: {callstack}", new StackTrace().ToString());
 
+				if (agent.Components.PayloadSender is not IPayloadSenderWithFilters sender)
+					return agent;
+
+				ErrorFilters.ForEach(f => sender.AddFilter(f));
+				TransactionFilters.ForEach(f => sender.AddFilter(f));
+				SpanFilters.ForEach(f => sender.AddFilter(f));
+				agent.Logger?.Trace()
+					?.Log(@"Initialization - Added filters to agent (errors:{{ErrorFilters}}, transactions:{TransactionFilters} spans:{SpanFilters}",
+						ErrorFilters.Count, TransactionFilters.Count, SpanFilters.Count);
+
 				return agent;
 			}
 		});
 
 		private static readonly object InitializationLock = new object();
 
+		private static readonly List<Func<IError, IError>> ErrorFilters = [];
+
+		private static readonly List<Func<ISpan, ISpan>> SpanFilters = [];
+
+		private static readonly List<Func<ITransaction, ITransaction>> TransactionFilters = [];
+
 		internal static AgentComponents Components { get; private set; }
 
 		public static IConfigurationReader Config => Instance.Configuration;
@@ -114,7 +128,16 @@ public static class Agent
 		/// <code>true</code> if the filter was added successfully, <code>false</code> otherwise. In case the method
 		/// returns <code>false</code> the filter won't be called.
 		/// </returns>
-		public static bool AddFilter(Func<ITransaction, ITransaction> filter) => CheckAndAddFilter(p => p.TransactionFilters.Add(filter));
+		public static bool AddFilter(Func<ITransaction, ITransaction> filter)
+		{
+			if (!IsConfigured)
+			{
+				TransactionFilters.Add(filter);
+				return true;
+			}
+
+			return CheckAndAddFilter(p => p.AddFilter(filter));
+		}
 
 		/// <summary>
 		/// Adds a filter which gets called before each span gets sent to APM Server.
@@ -133,7 +156,16 @@ public static class Agent
 		/// <code>true</code> if the filter was added successfully, <code>false</code> otherwise. In case the method
 		/// returns <code>false</code> the filter won't be called.
 		/// </returns>
-		public static bool AddFilter(Func<ISpan, ISpan> filter) => CheckAndAddFilter(p => p.SpanFilters.Add(filter));
+		public static bool AddFilter(Func<ISpan, ISpan> filter)
+		{
+			if (!IsConfigured)
+			{
+				SpanFilters.Add(filter);
+				return true;
+			}
+
+			return CheckAndAddFilter(p => p.AddFilter(filter));
+		}
 
 		/// <summary>
 		/// Adds a filter which gets called before each error gets sent to APM Server.
@@ -152,15 +184,23 @@ public static class Agent
 		/// <code>true</code> if the filter was added successfully, <code>false</code> otherwise. In case the method
 		/// returns <code>false</code> the filter won't be called.
 		/// </returns>
-		public static bool AddFilter(Func<IError, IError> filter) => CheckAndAddFilter(p => p.ErrorFilters.Add(filter));
+		public static bool AddFilter(Func<IError, IError> filter)
+		{
+			if (!IsConfigured)
+			{
+				ErrorFilters.Add(filter);
+				return true;
+			}
+
+			return CheckAndAddFilter(p => p.AddFilter(filter));
+		}
 
-		private static bool CheckAndAddFilter(Action<PayloadSenderV2> action)
+		private static bool CheckAndAddFilter(Func<IPayloadSenderWithFilters, bool> action)
 		{
-			if (!(Instance.PayloadSender is PayloadSenderV2 payloadSenderV2))
+			if (Instance.PayloadSender is not IPayloadSenderWithFilters sender)
 				return false;
 
-			action(payloadSenderV2);
-			return true;
+			return action(sender);
 		}
 
 		/// <summary>
@@ -199,10 +239,12 @@ public static void Setup(AgentComponents agentComponents)
 					return;
 				}
 
+				Components ??= agentComponents;
+
 				agentComponents?.Logger?.Trace()
 					?.Log("Initialization - Agent.Setup called");
 
-				Components = agentComponents;
+
 				// Force initialization
 				var _ = LazyApmAgent.Value;
 			}

src/Elastic.Apm/Report/IPayloadSender.cs

@@ -2,6 +2,8 @@
 // Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information
 
+using System;
+using System.Collections.Generic;
 using Elastic.Apm.Api;
 
 namespace Elastic.Apm.Report
@@ -16,4 +18,11 @@ public interface IPayloadSender
 
 		void QueueTransaction(ITransaction transaction);
 	}
+
+	public interface IPayloadSenderWithFilters
+	{
+		bool AddFilter(Func<ITransaction, ITransaction> transactionFilter);
+		bool AddFilter(Func<ISpan, ISpan> spanFilter);
+		bool AddFilter(Func<IError, IError> errorFilter);
+	}
 }

src/Elastic.Apm/Report/PayloadSenderV2.cs

@@ -29,12 +29,12 @@ namespace Elastic.Apm.Report
 	/// Responsible for sending the data to APM server. Implements Intake V2.
 	/// Each instance creates its own thread to do the work. Therefore, instances should be reused if possible.
 	/// </summary>
-	internal class PayloadSenderV2 : BackendCommComponentBase, IPayloadSender
+	internal class PayloadSenderV2 : BackendCommComponentBase, IPayloadSender, IPayloadSenderWithFilters
 	{
 		private const string ThisClassName = nameof(PayloadSenderV2);
-		internal readonly List<Func<IError, IError>> ErrorFilters = new List<Func<IError, IError>>();
-		internal readonly List<Func<ISpan, ISpan>> SpanFilters = new List<Func<ISpan, ISpan>>();
-		internal readonly List<Func<ITransaction, ITransaction>> TransactionFilters = new List<Func<ITransaction, ITransaction>>();
+		internal readonly List<Func<IError, IError>> ErrorFilters = new();
+		internal readonly List<Func<ISpan, ISpan>> SpanFilters = new();
+		internal readonly List<Func<ITransaction, ITransaction>> TransactionFilters = new();
 
 		private readonly IApmServerInfo _apmServerInfo;
 
@@ -148,6 +148,7 @@ IApmLogger logger
 
 		private bool _getApmServerVersion;
 		private bool _getCloudMetadata;
+		private bool _allowFilterAdd;
 		private static readonly UTF8Encoding Utf8Encoding;
 		private static readonly MediaTypeHeaderValue MediaTypeHeaderValue;
 
@@ -244,6 +245,8 @@ protected override void WorkLoopIteration()
 			}
 
 			var batch = ReceiveBatch();
+			if (_allowFilterAdd && batch is { Length: > 0 })
+				_allowFilterAdd = false;
 			if (batch != null)
 				ProcessQueueItems(batch);
 		}
@@ -497,5 +500,32 @@ private T TryExecuteFilter<T>(List<Func<T, T>> filters, T item) where T : class
 
 			return item;
 		}
+
+		public bool AddFilter(Func<ITransaction, ITransaction> transactionFilter)
+		{
+			if (!_allowFilterAdd)
+				return false;
+
+			TransactionFilters.Add(transactionFilter);
+			return true;
+		}
+
+		public bool AddFilter(Func<ISpan, ISpan> spanFilter)
+		{
+			if (!_allowFilterAdd)
+				return false;
+
+			SpanFilters.Add(spanFilter);
+			return true;
+		}
+
+		public bool AddFilter(Func<IError, IError> errorFilter)
+		{
+			if (!_allowFilterAdd)
+				return false;
+
+			ErrorFilters.Add(errorFilter);
+			return true;
+		}
 	}
 }

test/Elastic.Apm.Tests.Utilities/MockPayloadSender.cs

@@ -19,7 +19,7 @@
 
 namespace Elastic.Apm.Tests.Utilities
 {
-	internal class MockPayloadSender : IPayloadSender
+	internal class MockPayloadSender : IPayloadSender, IPayloadSenderWithFilters
 	{
 		private static readonly JObject JsonSpanTypesData =
 			JObject.Parse(File.ReadAllText(Path.Combine(SolutionPaths.Root, "test/Elastic.Apm.Tests.Utilities/TestResources/json-specs/span_types.json")));
@@ -211,7 +211,7 @@ public IReadOnlyList<IMetricSet> Metrics
 			get
 			{
 				lock (_metricsLock)
-					return CreateImmutableSnapshot<IMetricSet>(_metrics);
+					return CreateImmutableSnapshot(_metrics);
 			}
 		}
 
@@ -220,7 +220,7 @@ public IReadOnlyList<ISpan> Spans
 			get
 			{
 				lock (_spanLock)
-					return CreateImmutableSnapshot<ISpan>(_spans);
+					return CreateImmutableSnapshot(_spans);
 			}
 		}
 
@@ -229,7 +229,7 @@ public IReadOnlyList<ITransaction> Transactions
 			get
 			{
 				lock (_transactionLock)
-					return CreateImmutableSnapshot<ITransaction>(_transactions);
+					return CreateImmutableSnapshot(_transactions);
 			}
 		}
 
@@ -240,8 +240,8 @@ public void QueueError(IError error)
 		{
 			lock (_errorLock)
 			{
-				error = _errorFilters.Aggregate(error,
-					(current, filter) => filter(current));
+				error = _errorFilters
+					.Aggregate(error, (current, filter) => filter(current));
 				_errors.Add(error);
 				_errorWaitHandle.Set();
 			}
@@ -251,8 +251,8 @@ public virtual void QueueTransaction(ITransaction transaction)
 		{
 			lock (_transactionLock)
 			{
-				transaction = _transactionFilters.Aggregate(transaction,
-					(current, filter) => filter(current));
+				transaction = _transactionFilters
+					.Aggregate(transaction, (current, filter) => filter(current));
 				_transactions.Add(transaction);
 				_transactionWaitHandle.Set();
 			}
@@ -263,7 +263,8 @@ public void QueueSpan(ISpan span)
 			VerifySpan(span);
 			lock (_spanLock)
 			{
-				span = _spanFilters.Aggregate(span, (current, filter) => filter(current));
+				span = _spanFilters
+					.Aggregate(span, (current, filter) => filter(current));
 				_spans.Add(span);
 				_spanWaitHandle.Set();
 			}
@@ -305,6 +306,24 @@ private void VerifySpan(ISpan span)
 			}
 		}
 
+		public bool AddFilter(Func<ITransaction, ITransaction> transactionFilter)
+		{
+			_transactionFilters.Add(transactionFilter);
+			return true;
+		}
+
+		public bool AddFilter(Func<ISpan, ISpan> spanFilter)
+		{
+			_spanFilters.Add(spanFilter);
+			return true;
+		}
+
+		public bool AddFilter(Func<IError, IError> errorFilter)
+		{
+			_errorFilters.Add(errorFilter);
+			return true;
+		}
+
 		public void QueueMetrics(IMetricSet metricSet)
 		{
 			lock (_metricsLock)

test/Elastic.Apm.Tests.Utilities/NoopPayloadSender.cs

@@ -2,12 +2,14 @@
 // Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
 // See the LICENSE file in the project root for more information
 
+using System;
+using System.Collections.Generic;
 using Elastic.Apm.Api;
 using Elastic.Apm.Report;
 
 namespace Elastic.Apm.Tests.Utilities
 {
-	public class NoopPayloadSender : IPayloadSender
+	public class NoopPayloadSender : IPayloadSender, IPayloadSenderWithFilters
 	{
 		public void QueueError(IError error) { }
 
@@ -16,5 +18,12 @@ public void QueueTransaction(ITransaction transaction) { }
 		public void QueueSpan(ISpan span) { }
 
 		public void QueueMetrics(IMetricSet metricSet) { }
+
+		public bool AddFilter(Func<ITransaction, ITransaction> transactionFilter) => true;
+
+		public bool AddFilter(Func<ISpan, ISpan> spanFilter) => true;
+
+		public bool AddFilter(Func<IError, IError> errorFilter) => true;
+
 	}
 }