New: Cancel tasks in command queue

src/NzbDrone.Core/Messaging/Commands/CommandQueue.cs

@@ -71,6 +71,24 @@ namespace NzbDrone.Core.Messaging.Commands
             }
         }
 
+        public bool RemoveIfQueued(int id)
+        {
+            var rval = false;
+
+            lock (_mutex)
+            {
+                var command = _items.FirstOrDefault(q => q.Id == id);
+
+                if (command?.Status == CommandStatus.Queued)
+                {
+                    _items.Remove(command);
+                    rval = true;
+                }
+            }
+
+            return rval;
+        }
+
         public List<CommandModel> QueuedOrStarted()
         {
             return All().Where(q => q.Status == CommandStatus.Queued || q.Status == CommandStatus.Started)

src/NzbDrone.Core/Messaging/Commands/CommandQueueManager.cs

@@ -1,12 +1,14 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Net;
 using System.Threading;
 using NLog;
 using NzbDrone.Common;
 using NzbDrone.Common.EnsureThat;
 using NzbDrone.Common.Serializer;
 using NzbDrone.Core.Lifecycle;
+using NzbDrone.Core.Exceptions;
 using NzbDrone.Core.Messaging.Events;
 
 namespace NzbDrone.Core.Messaging.Commands
@@ -25,6 +27,7 @@ namespace NzbDrone.Core.Messaging.Commands
         void Complete(CommandModel command, string message);
         void Fail(CommandModel command, string message, Exception e);
         void Requeue();
+        void Cancel(int id);
         void CleanCommands();
     }
 
@@ -193,6 +196,14 @@ namespace NzbDrone.Core.Messaging.Commands
             }
         }
 
+        public void Cancel(int id)
+        {
+            if (!_commandQueue.RemoveIfQueued(id))
+            {
+                throw new NzbDroneClientException(HttpStatusCode.Conflict, "Unable to cancel task");
+            }
+        }
+
         public void CleanCommands()
         {
             _logger.Trace("Cleaning up old commands");