Implemented the missing details for the concurrent setup, it now compiles

Duplicati/Library/Main/Database/LocalBackupDatabase.cs

@@ -799,13 +799,12 @@ namespace Duplicati.Library.Main.Database
             }            
         }
 
-        public object SafeDeleteRemoteVolume(string name, System.Data.IDbTransaction transaction)
+        public void SafeDeleteRemoteVolume(string name, System.Data.IDbTransaction transaction)
         {
             var volumeid = GetRemoteVolumeID(name, transaction);
 
-            using(var cmd = m_connection.CreateCommand())
+            using(var cmd = m_connection.CreateCommand(transaction))
             {
-                cmd.Transaction = transaction;
                 var c = cmd.ExecuteNonQuery(@"SELECT COUNT(*) FROM ""Block"" WHERE ""VolumeID"" = ? ", volumeid);
                 if (c != 0)
                     throw new Exception(string.Format("Failed to safe-delete volume {0}, blocks: {1}", name, c));
@@ -813,5 +812,18 @@ namespace Duplicati.Library.Main.Database
                 RemoveRemoteVolume(name, transaction);
             }            
         }
+
+        public IEnumerable<string> GetBlocklistHashes(string name, System.Data.IDbTransaction transaction)
+        {
+            var volumeid = GetRemoteVolumeID(name, transaction);
+            using(var cmd = m_connection.CreateCommand(transaction))
+            {
+                // Grab the strings and return as array to avoid concurrent access to the IEnumerable
+                return cmd.ExecuteReaderEnumerable(
+                    @"SELECT DISTINCT ""Block"".""Hash"" FROM ""Block"" WHERE ""Block"".""VolumeID"" = ? AND ""Block"".""Hash"" IN (SELECT ""Hash"" FROM ""BlocklistHash"")", volumeid)
+                    .Select(x => x.ConvertValueToString(0))
+                    .ToArray();
+            }            
+        }
     }
 }

Duplicati/Library/Main/Duplicati.Library.Main.csproj

@@ -122,7 +122,6 @@
     <Compile Include="Operation\Backup\FilePreFilterProcess.cs" />
     <Compile Include="Operation\Backup\DataBlockProcessor.cs" />
     <Compile Include="Operation\Common\BackendOperation.cs" />
-    <Compile Include="Operation\BackendProcess.cs" />
     <Compile Include="Operation\Backup\SpillCollectorProcess.cs" />
     <Compile Include="Operation\Backup\BackupDatabase.cs" />
     <Compile Include="Operation\Common\DatabaseCommon.cs" />

Duplicati/Library/Main/Operation/BackendProcess.cs

@@ -1,47 +0,0 @@
-//  Copyright (C) 2015, The Duplicati Team
-//  http://www.duplicati.com, info@duplicati.com
-//
-//  This library is free software; you can redistribute it and/or modify
-//  it under the terms of the GNU Lesser General Public License as
-//  published by the Free Software Foundation; either version 2.1 of the
-//  License, or (at your option) any later version.
-//
-//  This library is distributed in the hope that it will be useful, but
-//  WITHOUT ANY WARRANTY; without even the implied warranty of
-//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
-//  Lesser General Public License for more details.
-//
-//  You should have received a copy of the GNU Lesser General Public
-//  License along with this library; if not, write to the Free Software
-//  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
-using System;
-using CoCoL;
-using System.Threading.Tasks;
-using Duplicati.Library.Main.Operation.Common;
-
-namespace Duplicati.Library.Main.Operation
-{
-    internal static class BackendProcess
-    {
-        public static Task Run()
-        {
-            return AutomationExtensions.RunTask(
-                new
-                {
-                    LogChannel = ChannelMarker.ForWrite<LogMessage>("LogChannel"),
-                    Input = ChannelMarker.ForRead<IBackendOperation>("BackendRequests"),
-                },
-
-                async self => 
-                {
-                    while(true)
-                    {
-                        var request = await self.Input.ReadAsync();
-
-                    }
-                }
-            );
-        }
-    }
-}
-

Duplicati/Library/Main/Operation/Backup/BackupDatabase.cs

@@ -117,7 +117,7 @@ namespace Duplicati.Library.Main.Operation.Backup
 
         public Task SafeDeleteRemoteVolumeAsync(string remotename)
         {
-            return RunOnMain(() => Task.FromResult(m_database.SafeDeleteRemoteVolume(remotename, m_transaction)));
+            return RunOnMain(() => m_database.SafeDeleteRemoteVolume(remotename, m_transaction));
         }
 
         public Task<IEnumerable<string>> GetBlocklistHashesAsync(string remotename)

Duplicati/Library/Main/Operation/Backup/BackupStatsCollector.cs

@@ -25,7 +25,7 @@ namespace Duplicati.Library.Main.Operation.Backup
         private BackupResults m_res;
 
         public BackupStatsCollector(BackupResults res)
-            : base(res)
+            : base(res.BackendWriter)
         {
             m_res = res;
         }

Duplicati/Library/Main/Operation/Common/BackendHandler.cs

@@ -267,14 +267,14 @@ namespace Duplicati.Library.Main.Operation.Common
             });
         }
 
-        public Task<Library.Utility.TempFile> GetFileForTestingAsync(string remotename, long size, string remotehash)
+        /*public Task<Library.Utility.TempFile> GetFileForTestingAsync(string remotename, long size, string remotehash)
         {
             var fe = new FileEntryItem(BackendActionType.Get, remotename, null);
             return RunRetryOnMain(fe, async() =>
             {
 
             });
-        }
+        }*/
 
         private async Task ResetBackendAsync(Exception ex)
         {