rpcrt4: Implement RPC_FC_CALLBACK_HANDLE handle type for NdrClientCall2().

dlls/rpcrt4/ndr_stubless.c

@@ -344,7 +344,11 @@ static PFORMAT_STRING client_get_handle(
         *phBinding = *pStubMsg->StubDesc->IMPLICIT_HANDLE_INFO.pPrimitiveHandle;
         break;
     case RPC_FC_CALLBACK_HANDLE: /* implicit callback */
-        FIXME("RPC_FC_CALLBACK_HANDLE\n");
+        TRACE("RPC_FC_CALLBACK_HANDLE\n");
+        /* server calls callback procedures only in response to remote call, and most recent
+           binding handle is used. Calling back to a client can potentially result in another
+           callback with different current handle. */
+        *phBinding = I_RpcGetCurrentCallHandle();
         break;
     case RPC_FC_AUTO_HANDLE: /* implicit auto handle */
         /* strictly speaking, it isn't necessary to set hBinding here

dlls/rpcrt4/ndr_stubless.h

@@ -35,7 +35,8 @@ typedef struct _NDR_PROC_HEADER
      * RPC_FC_BIND_PRIMITIVE = 32 - Implicit handle using handle_t created by
      *   calling application
      * RPC_FC_AUTO_HANDLE = 33 - Automatic handle
-     * RPC_FC_CALLBACK_HANDLE = 34 - undocumented
+     * RPC_FC_CALLBACK_HANDLE = 34 - Implicit handle used for a callback: current handle
+     *   from last remote call
      */
     unsigned char handle_type;