make ttINVOKE much more flexible

src/ripple/app/hook/impl/applyHook.cpp

@@ -204,7 +204,8 @@ namespace hook
             case ttPAYCHAN_CREATE:
             case ttINVOKE:
             {
-                ADD_TSH(*destAcc, canRollback);
+                if (destAcc)
+                    ADD_TSH(*destAcc, canRollback);
                 break;
             }
 

src/ripple/app/tx/impl/Invoke.cpp

@@ -40,9 +40,6 @@ Invoke::preflight(PreflightContext const& ctx)
     auto& tx = ctx.tx;
     auto& j = ctx.j;
 
-    if (!tx.isFieldPresent(sfBlob) && !tx.isFieldPresent(sfHookParameters))
-        return temMALFORMED;
-
     if (tx.getFieldVL(sfBlob).size() > (128*1024))
         return temMALFORMED;
 
@@ -61,8 +58,11 @@ Invoke::preclaim(PreclaimContext const& ctx)
     if (!sle)
         return terNO_ACCOUNT;
 
-    if (!ctx.view.exists(keylet::account(ctx.tx[sfDestination])))
-        return tecNO_TARGET;
+    if (ctx.tx.isFieldPresent(sfDestination))
+    {
+        if (!ctx.view.exists(keylet::account(ctx.tx[sfDestination])))
+            return tecNO_TARGET;
+    }
 
     return tesSUCCESS;
 }

src/ripple/protocol/impl/TxFormats.cpp

@@ -357,7 +357,8 @@ TxFormats::TxFormats()
         {
             {sfBlob, soeOPTIONAL},
             {sfHookParameters, soeOPTIONAL},
-            {sfDestination, soeREQUIRED}
+            {sfDestination, soeOPTIONAL},
+            {sfInvoiceID, soeOPTIONAL},
         },
         commonFields);
 }