Add discard as an option for custom recoverability policies

src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.netframework.approved.txt

@@ -256,6 +256,10 @@ namespace NServiceBus
         public static void CustomDiagnosticsWriter(this NServiceBus.EndpointConfiguration config, System.Func<string, System.Threading.Tasks.Task> customDiagnosticsWriter) { }
         public static void SetDiagnosticsPath(this NServiceBus.EndpointConfiguration config, string path) { }
     }
+    public sealed class Discard : NServiceBus.RecoverabilityAction
+    {
+        public string Reason { get; }
+    }
     public class DistributionPolicy : NServiceBus.IDistributionPolicy
     {
         public DistributionPolicy() { }
@@ -750,6 +754,8 @@ namespace NServiceBus
     {
         protected internal RecoverabilityAction() { }
         public static NServiceBus.DelayedRetry DelayedRetry(System.TimeSpan timeSpan) { }
+        public static NServiceBus.Discard Discard() { }
+        public static NServiceBus.Discard Discard(string reason) { }
         public static NServiceBus.ImmediateRetry ImmediateRetry() { }
         public static NServiceBus.MoveToError MoveToError(string errorQueue) { }
     }

src/NServiceBus.Core.Tests/ApprovalFiles/APIApprovals.ApproveNServiceBus.netstandard.approved.txt

@@ -256,6 +256,10 @@ namespace NServiceBus
         public static void CustomDiagnosticsWriter(this NServiceBus.EndpointConfiguration config, System.Func<string, System.Threading.Tasks.Task> customDiagnosticsWriter) { }
         public static void SetDiagnosticsPath(this NServiceBus.EndpointConfiguration config, string path) { }
     }
+    public sealed class Discard : NServiceBus.RecoverabilityAction
+    {
+        public string Reason { get; }
+    }
     public class DistributionPolicy : NServiceBus.IDistributionPolicy
     {
         public DistributionPolicy() { }
@@ -750,6 +754,8 @@ namespace NServiceBus
     {
         protected internal RecoverabilityAction() { }
         public static NServiceBus.DelayedRetry DelayedRetry(System.TimeSpan timeSpan) { }
+        public static NServiceBus.Discard Discard() { }
+        public static NServiceBus.Discard Discard(string reason) { }
         public static NServiceBus.ImmediateRetry ImmediateRetry() { }
         public static NServiceBus.MoveToError MoveToError(string errorQueue) { }
     }

src/NServiceBus.Core.Tests/Recoverability/RecoverabilityExecutorTests.cs

@@ -130,7 +130,21 @@ public async Task When_unsupported_action_returned_should_move_to_errors()
             Assert.AreEqual(1, eventAggregator.NotificationsRaised.Count);
             Assert.AreEqual("message-id", failure.Message.MessageId);
         }
+
+        [Test]
+        public async Task When_discard_action_returned_should_discard_message()
+        {
+            var recoverabilityExecutor = CreateExecutor(
+                RetryPolicy.Discard());
+            var errorContext = CreateErrorContext(messageId: "message-id");
+
+            var result = await recoverabilityExecutor.Invoke(errorContext);
+
+            Assert.AreEqual(ErrorHandleResult.Handled, result);
+            Assert.AreEqual(0, eventAggregator.NotificationsRaised.Count);
+        }
 
+        [Test]
         public async Task When_moving_to_custom_error_queue_custom_error_queue_address_should_be_set_on_notification()
         {
             var customErrorQueueAddress = "custom-error-queue";
@@ -217,6 +231,14 @@ public static Func<RecoverabilityConfig, ErrorContext, RecoverabilityAction> Uns
                     new UnsupportedAction()
                 }).Invoke;
             }
+
+            public static Func<RecoverabilityConfig, ErrorContext, RecoverabilityAction> Discard()
+            {
+                return new RetryPolicy(new[]
+                {
+                    new Discard(), 
+                }).Invoke;
+            }
 
             Queue<RecoverabilityAction> actions;
         }

src/NServiceBus.Core/Recoverability/Discard.cs

@@ -0,0 +1,18 @@
+namespace NServiceBus
+{
+    /// <summary>
+    /// Indicates recoverability is required to discard/ignore the current message.
+    /// </summary>
+    public sealed class Discard : RecoverabilityAction
+    {
+        internal Discard(string reason = null)
+        {
+            Reason = reason;
+        }
+
+        /// <summary>
+        /// The reason why a message was discarded.
+        /// </summary>
+        public string Reason { get; }
+    }
+}

src/NServiceBus.Core/Recoverability/RecoverabilityAction.cs

@@ -45,7 +45,24 @@ public static MoveToError MoveToError(string errorQueue)
             Guard.AgainstNullAndEmpty(nameof(errorQueue), errorQueue);
             return new MoveToError(errorQueue);
         }
+
+        /// <summary>
+        /// Creates a discard recoverability action.
+        /// </summary>
+        /// <returns>Discard action.</returns>
+        public static Discard Discard() => Discard(null);
+
+        /// <summary>
+        /// Creates a discard recoverability action.
+        /// </summary>
+        /// <param name="reason">The reason why the message was discarded.</param>
+        /// <returns>Discard action.</returns>
+        public static Discard Discard(string reason)
+        {
+            return string.IsNullOrEmpty(reason) ? CachedDiscard : new Discard(reason);
+        }
 
         static ImmediateRetry CachedImmediateRetry = new ImmediateRetry();
+        static Discard CachedDiscard = new Discard();
     }
 }

src/NServiceBus.Core/Recoverability/RecoverabilityExecutor.cs

@@ -24,6 +24,12 @@ public Task<ErrorHandleResult> Invoke(ErrorContext errorContext)
         {
             var recoveryAction = recoverabilityPolicy(configuration, errorContext);
 
+            if (recoveryAction is Discard discard)
+            {
+                Logger.Info($"Discarding message with id '{errorContext.Message.MessageId}'.{(string.IsNullOrEmpty(discard.Reason) ? string.Empty : $" Reason: {discard.Reason}")}", errorContext.Exception);
+                return HandledTask;
+            }
+
             // When we can't do immediate retries and policy did not honor MaxNumberOfRetries for ImmediateRetries
             if (recoveryAction is ImmediateRetry && !immediateRetriesAvailable)
             {
@@ -119,7 +125,8 @@ await eventAggregator.Raise(
         bool immediateRetriesAvailable;
         bool delayedRetriesAvailable;
 
+        static Task<ErrorHandleResult> HandledTask = Task.FromResult(ErrorHandleResult.Handled);
         static ILog Logger = LogManager.GetLogger<RecoverabilityExecutor>();
         RecoverabilityConfig configuration;
     }
-}
+}