Allow persistence plugins to commit as a final step. (neo-project#568)

* Allow persistence plugins to commit as a final step.

* Plugins commit before core commits, once all plugins have handled initial work OnPersist.

* Allow PersistencePlugin to determine whether to crash if commit fails.

* Add ShouldThrowExceptionFromCommit method to IPersistencePlugin.

* Throw all commit exceptions that should be thrown in an AggregateException.

neo/Ledger/Blockchain.cs

@@ -610,6 +610,25 @@ private void Persist(Block block)
                 foreach (IPersistencePlugin plugin in Plugin.PersistencePlugins)
                     plugin.OnPersist(snapshot, all_application_executed);
                 snapshot.Commit();
+                List<Exception> commitExceptions = null;
+                foreach (IPersistencePlugin plugin in Plugin.PersistencePlugins)
+                {
+                    try
+                    {
+                        plugin.OnCommit(snapshot);
+                    }
+                    catch (Exception ex)
+                    {
+                        if (plugin.ShouldThrowExceptionFromCommit(ex))
+                        {
+                            if (commitExceptions == null)
+                                commitExceptions = new List<Exception>();
+
+                            commitExceptions.Add(ex);
+                        }
+                    }
+                }
+                if (commitExceptions != null) throw new AggregateException(commitExceptions);
             }
             UpdateCurrentSnapshot();
             OnPersistCompleted(block);

neo/Plugins/IPersistencePlugin.cs

@@ -1,4 +1,5 @@
-using Neo.Persistence;
+using System;
+using Neo.Persistence;
 using System.Collections.Generic;
 using static Neo.Ledger.Blockchain;
 
@@ -7,5 +8,7 @@ namespace Neo.Plugins
     public interface IPersistencePlugin
     {
         void OnPersist(Snapshot snapshot, IReadOnlyList<ApplicationExecuted> applicationExecutedList);
+        void OnCommit(Snapshot snapshot);
+        bool ShouldThrowExceptionFromCommit(Exception ex);
     }
 }