Implement pull request mechanism in RsGxsNetService

This could be used to request the online peers to pull updates from us
  ASAP, as an exaple when a group is created a pull request can be
  emitted too so the online peers pull the groups from us ASAP instead
  of waiting for the usual 60 seconds. A mechanism like this is
  especially useful on mobile phones where the internet connection is
  usually turned on only in a few moments (as an example while the user
  is interacting with the app).
Cleanup a few old corners in the code keeping retro-compatibility and
  make the code more welcoming to new developers.
Put a bunch of dead code under #ifdef.

libretroshare/src/gxs/rsgxsnetservice.cc

@@ -35,7 +35,7 @@
 //    |
 //    +----------- sharePublishKeys()
 //    |
-//    +----------- syncWithPeers()
+//    +----------- pullFromPeers()
 //    |              |
 //    |              +--if AutoSync--- send global UpdateTS of each peer to itself => the peer knows the last
 //    |              |                 time current peer has received an updated from himself
@@ -127,14 +127,14 @@
 //                                                        (Set at server side to be mGrpServerUpdateItem->grpUpdateTS)
 //
 //                                                        Only updated in processCompletedIncomingTransaction() from Grp list transaction.
-//                                                        Used in syncWithPeers() sending in RsNxsSyncGrp once to all peers: peer will send data if
+//                                                        Used in pullFromPeers() sending in RsNxsSyncGrp once to all peers: peer will send data if
 //                                                        has something new. All time comparisons are in the friends' clock time.
 //
 //     mClientMsgUpdateMap: map< RsPeerId, map<grpId,TimeStamp > >
 //
 //                                                        Last msg list modification time sent by that peer Id
 //                                                        Updated in processCompletedIncomingTransaction() from Grp list trans.
-//                                                        Used in syncWithPeers() sending in RsNxsSyncGrp once to all peers.
+//                                                        Used in pullFromPeers() sending in RsNxsSyncGrp once to all peers.
 //                                                        Set at server to be mServerMsgUpdateMap[grpId]->msgUpdateTS
 //
 //     mGrpServerUpdateItem:  TimeStamp                   Last group local modification timestamp over all groups
@@ -150,7 +150,7 @@
 //
 //    tick()                                                                                                    tick()
 //      |                                                                                                         |
-//      +---- SyncWithPeers                                                                                       +-- recvNxsItemQueue()
+//      +---- pullFromPeers                                                                                       +-- recvNxsItemQueue()
 //                 |                                                                                                   |
 //                 +---------------- Send global UpdateTS of each peer to itself => the peer knows        +--------->  +------ handleRecvSyncGroup( RsNxsSyncGrp*)
 //                 |                 the last msg sent (stored in mClientGrpUpdateMap[peer_id]),          |            |            - parse all subscribed groups. For each, send a RsNxsSyncGrpItem with publish TS
@@ -457,7 +457,7 @@ int RsGxsNetService::tick()
 
     if((elapsed) < now)
     {
-        syncWithPeers();
+		pullFromPeers();
         syncGrpStatistics();
 		checkDistantSyncState();
 
@@ -570,39 +570,39 @@ RsGxsGroupId RsGxsNetService::hashGrpId(const RsGxsGroupId& gid,const RsPeerId&
     return RsGxsGroupId( RsDirUtil::sha1sum(tmpmem,SIZE).toByteArray() );
 }
 
-void RsGxsNetService::syncWithPeers()
+void RsGxsNetService::pullFromPeers(std::set<RsPeerId> peers)
 {
 #ifdef NXS_NET_DEBUG_0
-    GXSNETDEBUG___ << "RsGxsNetService::syncWithPeers() this=" << (void*)this << ". serviceInfo=" << mServiceInfo << std::endl;
+	RS_DBG("this=", (void*)this, ". serviceInfo=", mServiceInfo);
 #endif
 
-    static RsNxsSerialiser ser(mServType) ;	// this is used to estimate bandwidth.
+	/* If specific peers are passed as paramether ask only to them */
-
+	if(peers.empty())
-    RS_STACK_MUTEX(mNxsMutex) ;
-
-    std::set<RsPeerId> peers;
-    mNetMgr->getOnlineList(mServiceInfo.mServiceType, peers);
-
-	if(mAllowDistSync && mGxsNetTunnel != NULL)
 	{
-		// Grab all online virtual peers of distant tunnels for the current service.
+		mNetMgr->getOnlineList(mServiceInfo.mServiceType, peers);
 
-		std::list<RsGxsNetTunnelVirtualPeerId> vpids ;
+		if(mAllowDistSync && mGxsNetTunnel != nullptr)
-		mGxsNetTunnel->getVirtualPeers(vpids);
+		{
+			/* Grab all online virtual peers of distant tunnels for the current
+			 * service. */
 
-		for(auto it(vpids.begin());it!=vpids.end();++it)
+			std::list<RsGxsNetTunnelVirtualPeerId> vpids ;
-			peers.insert(RsPeerId(*it)) ;
+			mGxsNetTunnel->getVirtualPeers(vpids);
+
+			for(auto it(vpids.begin());it!=vpids.end();++it)
+				peers.insert(RsPeerId(*it)) ;
+		}
 	}
 
-    if (peers.empty()) {
+	// Still empty? Then nothing to do
-        // nothing to do
+	if (peers.empty()) return;
-        return;
+
-    }
+
+	RS_STACK_MUTEX(mNxsMutex);
 
 	// for now just grps
 	for(auto sit = peers.begin(); sit != peers.end(); ++sit)
 	{
-
 		const RsPeerId peerId = *sit;
 
 		ClientGrpMap::const_iterator cit = mClientGrpUpdateMap.find(peerId);
@@ -624,8 +624,7 @@ void RsGxsNetService::syncWithPeers()
 		generic_sendItem(grp);
 	}
 
-    if(!mAllowMsgSync)
+	if(!mAllowMsgSync) return;
-        return ;
 
 #ifndef GXS_DISABLE_SYNC_MSGS
 
@@ -746,7 +745,7 @@ void RsGxsNetService::syncWithPeers()
 #endif
 }
 
-void RsGxsNetService::generic_sendItem(RsNxsItem *si)
+void RsGxsNetService::generic_sendItem(rs_owner_ptr<RsItem> si)
 {
 	// check if the item is to be sent to a distant peer or not
 
@@ -1718,13 +1717,25 @@ RsItem *RsGxsNetService::generic_recvItem()
 
 void RsGxsNetService::recvNxsItemQueue()
 {
-    RsItem *item ;
+	RsItem* item;
 
-    while(NULL != (item=generic_recvItem()))
+	while(nullptr != (item=generic_recvItem()))
-    {
+	{
 #ifdef NXS_NET_DEBUG_1
-        GXSNETDEBUG_P_(item->PeerId()) << "Received RsGxsNetService Item:" << (void*)item << " type=" << std::hex << item->PacketId() << std::dec << std::endl ;
+		RS_DBG( "Received RsGxsNetService Item: ", (void*)item, " type=",
+		        item->PacketId() );
 #endif
+		/* Handle pull request and other new items here to not mess with all the
+		 * old nested code and items hell */
+		switch(static_cast<RsNxsSubtype>(item->PacketSubType()))
+		{
+		case RsNxsSubtype::PULL_REQUEST:
+			std::unique_ptr<RsNxsPullRequestItem> pullItem(
+			            static_cast<RsNxsPullRequestItem*>(item) );
+			handlePullRequest(std::move(pullItem));
+			continue;
+		}
+
         // RsNxsItem needs dynamic_cast, since they have derived siblings.
         //
         RsNxsItem *ni = dynamic_cast<RsNxsItem*>(item) ;
@@ -5075,6 +5086,46 @@ void RsGxsNetService::handleRecvPublishKeys(RsNxsGroupPublishKeyItem *item)
 	}
 }
 
+std::error_condition RsGxsNetService::requestPull(std::set<RsPeerId> peers)
+{
+	/* If specific peers are passed as paramether ask only to them */
+	if(peers.empty())
+	{
+		mNetMgr->getOnlineList(mServiceInfo.mServiceType, peers);
+
+		if(mAllowDistSync && mGxsNetTunnel != nullptr)
+		{
+			/* Grab all online virtual peers of distant tunnels for the current
+			 * service. */
+
+			std::list<RsGxsNetTunnelVirtualPeerId> vpids ;
+			mGxsNetTunnel->getVirtualPeers(vpids);
+
+			for(auto it(vpids.begin());it!=vpids.end();++it)
+				peers.insert(RsPeerId(*it)) ;
+		}
+	}
+
+	// Still empty? Reports there are no available peers
+	if (peers.empty()) return std::errc::network_down;
+
+	for(auto& peerId: std::as_const(peers))
+	{
+		auto item = new RsNxsPullRequestItem(
+		            static_cast<RsServiceType>(mServType) );
+		item->PeerId(peerId);
+		generic_sendItem(item);
+	}
+
+	return std::error_condition();
+}
+
+void RsGxsNetService::handlePullRequest(
+        std::unique_ptr<RsNxsPullRequestItem> item )
+{
+	pullFromPeers(std::set<RsPeerId>{item->PeerId()});
+}
+
 bool RsGxsNetService::getGroupServerUpdateTS(const RsGxsGroupId& gid,rstime_t& group_server_update_TS, rstime_t& msg_server_update_TS)
 {
     RS_STACK_MUTEX(mNxsMutex) ;

libretroshare/src/gxs/rsgxsnetservice.h

@@ -423,6 +423,8 @@ private:
      */
     void handleRecvPublishKeys(RsNxsGroupPublishKeyItem*) ;
 
+	void handlePullRequest(std::unique_ptr<RsNxsPullRequestItem> item);
+
     /** E: item handlers **/
 
 
@@ -459,7 +461,23 @@ private:
     void locked_pushMsgRespFromList(std::list<RsNxsItem*>& itemL, const RsPeerId& sslId, const RsGxsGroupId &grp_id, const uint32_t& transN);
     
 	void checkDistantSyncState();
-    void syncWithPeers();
+
+	/**
+	 * @brief Pull new stuff from peers
+	 * @param peers peers to pull from, if empty all available peers are pulled
+	 */
+	void pullFromPeers(std::set<RsPeerId> peers = std::set<RsPeerId>());
+
+	/**
+	 * @brief request online peers to pull updates from our node ASAP
+	 * @param peers peers to which request pull from, if empty all available
+	 * peers are requested to pull
+	 * @return success or error details
+	 * TODO: should this be exposed via RsNetworkExchangeService?
+	 */
+	std::error_condition requestPull(
+	        std::set<RsPeerId> peers = std::set<RsPeerId>() );
+
     void syncGrpStatistics();
     void addGroupItemToList(NxsTransaction*& tr,
     		const RsGxsGroupId& grpId, uint32_t& transN,
@@ -559,7 +577,7 @@ private:
     void cleanRejectedMessages();
     void processObserverNotifications();
 
-	void generic_sendItem(RsNxsItem *si);
+	void generic_sendItem(rs_owner_ptr<RsItem> si);
 	RsItem *generic_recvItem();
 
 private:

libretroshare/src/libretroshare.pro

@@ -507,7 +507,8 @@ HEADERS +=	util/folderiterator.h \
     util/cxx11retrocompat.h \
     util/cxx14retrocompat.h \
     util/cxx17retrocompat.h \
-            util/rsurl.h
+    util/cxx23retrocompat.h \
+    util/rsurl.h
 
 SOURCES +=	ft/ftchunkmap.cc \
 			ft/ftcontroller.cc \

libretroshare/src/rsitems/itempriorities.h

@@ -3,7 +3,9 @@
  *                                                                             *
  * libretroshare: retroshare core library                                      *
  *                                                                             *
- * Copyright 2011-2011 by Cyril Soler <csoler@users.sourceforge.net>           *
+ * Copyright (C) 2011-2018 Cyril Soler <csoler@users.sourceforge.net>          *
+ * Copyright (C) 2021  Gioacchino Mazzurco <gio@eigenlab.org>                  *
+ * Copyright (C) 2021  Asociación Civil Altermundi <info@altermundi.net>       *
  *                                                                             *
  * This program is free software: you can redistribute it and/or modify        *
  * it under the terms of the GNU Lesser General Public License as              *
@@ -21,7 +23,9 @@
  *******************************************************************************/
 #pragma once
 
-#include <stdint.h>
+#include <cstdint>
+
+using RsItemPriority = uint8_t;
 
 // This file centralises QoS priorities for all transfer RsItems
 //

libretroshare/src/rsitems/rsitem.h

@@ -3,7 +3,8 @@
  *                                                                             *
  * libretroshare: retroshare core library                                      *
  *                                                                             *
- * Copyright (C) 2018  Gioacchino Mazzurco <gio@eigenlab.org>                  *
+ * Copyright (C) 2018-2021  Gioacchino Mazzurco <gio@eigenlab.org>             *
+ * Copyright (C) 2021  Asociación Civil Altermundi <info@altermundi.net>       *
  *                                                                             *
  * This program is free software: you can redistribute it and/or modify        *
  * it under the terms of the GNU Lesser General Public License as              *
@@ -28,6 +29,9 @@
 #include "serialiser/rsserializer.h"
 #include "serialiser/rsserializable.h"
 #include "util/stacktrace.h"
+#include "rsitems/itempriorities.h"
+#include "rsitems/rsserviceids.h"
+
 
 #include <typeinfo>
 
@@ -42,8 +46,13 @@ struct RsItem : RsMemoryManagement::SmallObject, RsSerializable
 
 	virtual ~RsItem();
 
-	/// TODO: Do this make sense with the new serialization system?
+	/** TODO: Does the existence of this method make sense with the new
-	virtual void clear() = 0;
+	 * serialization system? **/
+	virtual void clear()
+	{
+		RS_ERR("Called without being overridden, report to developers");
+		print_stacktrace();
+	}
 
 	/// @deprecated use << ostream operator instead
 	RS_DEPRECATED_FOR("<< ostream operator")
@@ -70,14 +79,21 @@ struct RsItem : RsMemoryManagement::SmallObject, RsSerializable
 	uint8_t PacketType();
 	uint8_t PacketSubType() const;
 
+	/** For Service Packets, @deprecated use the costructor with priority
+	 * paramether instead */
+	RS_DEPRECATED RsItem(uint8_t ver, uint16_t service, uint8_t subtype);
+
 	/// For Service Packets
-	RsItem(uint8_t ver, uint16_t service, uint8_t subtype);
+	RsItem( uint8_t ver, RsServiceType service, uint8_t subtype,
+	        RsItemPriority prio );
+
 	uint16_t PacketService() const; /* combined Packet class/type (mid 16bits) */
 	void setPacketService(uint16_t service);
 
 	inline uint8_t priority_level() const { return _priority_level ;}
 	inline void setPriorityLevel(uint8_t l) { _priority_level = l ;}
 
+#ifdef RS_DEAD_CODE
 	/*
 	 * TODO: This default implementation should be removed and childs structs
 	 * implement ::serial_process(...) as soon as all the codebase is ported to
@@ -90,11 +106,12 @@ struct RsItem : RsMemoryManagement::SmallObject, RsSerializable
 		        "overriding Class is: ", typeid(*this).name() );
 		print_stacktrace();
 	}
+#endif //def RS_DEAD_CODE
 
 protected:
 	uint32_t type;
 	RsPeerId peerId;
-	uint8_t _priority_level;
+	RsItemPriority _priority_level;
 };
 
 /// TODO: Do this make sense with the new serialization system?
@@ -108,9 +125,17 @@ public:
 	uint32_t getRawLength() { return len; }
 	void * getRawData() { return data; }
 
-	virtual void clear() {}
+//	virtual void clear() override {}
 	virtual std::ostream &print(std::ostream &out, uint16_t indent = 0);
 
+	virtual void serial_process(RsGenericSerializer::SerializeJob,
+	                            RsGenericSerializer::SerializeContext&) override
+	{
+		RS_ERR( "called by an item using new serialization system ",
+		        typeid(*this).name() );
+		print_stacktrace();
+	}
+
 private:
 	void *data;
 	uint32_t len;

libretroshare/src/rsitems/rsnxsitems.h

@@ -3,7 +3,10 @@
  *                                                                             *
  * libretroshare: retroshare core library                                      *
  *                                                                             *
- * Copyright 2012 Christopher Evi-Parker,Robert Fernie<retroshare@lunamutt.com>*
+ * Copyright (C) 2012  Christopher Evi-Parker                                  *
+ * Copyright (C) 2012  Robert Fernie <retroshare@lunamutt.com>                 *
+ * Copyright (C) 2021  Gioacchino Mazzurco <gio@altermundi.net>                *
+ * Copyright (C) 2021  Asociación Civil Altermundi <info@altermundi.net>       *
  *                                                                             *
  * This program is free software: you can redistribute it and/or modify        *
  * it under the terms of the GNU Lesser General Public License as              *
@@ -19,8 +22,7 @@
  * along with this program. If not, see <https://www.gnu.org/licenses/>.       *
  *                                                                             *
  *******************************************************************************/
-#ifndef RSNXSITEMS_H
+#pragma once
-#define RSNXSITEMS_H
 
 #include <map>
 #include <openssl/ssl.h>
@@ -35,6 +37,7 @@
 
 // These items have "flag type" numbers, but this is not used.
 
+// TODO: refactor as C++11 enum class
 const uint8_t RS_PKT_SUBTYPE_NXS_SYNC_GRP_REQ_ITEM    = 0x01;
 const uint8_t RS_PKT_SUBTYPE_NXS_SYNC_GRP_ITEM        = 0x02;
 const uint8_t RS_PKT_SUBTYPE_NXS_SYNC_GRP_STATS_ITEM  = 0x03;
@@ -47,14 +50,19 @@ const uint8_t RS_PKT_SUBTYPE_NXS_MSG_ITEM             = 0x20;
 const uint8_t RS_PKT_SUBTYPE_NXS_TRANSAC_ITEM         = 0x40;
 const uint8_t RS_PKT_SUBTYPE_NXS_GRP_PUBLISH_KEY_ITEM = 0x80;
 
-// possibility create second service to deal with this functionality
+enum class RsNxsSubtype : uint8_t
+{
+	PULL_REQUEST = 0x90 /// @see RsNxsPullRequestItem
+};
 
+#ifdef RS_DEAD_CODE
+// possibility create second service to deal with this functionality
 const uint8_t RS_PKT_SUBTYPE_EXT_SEARCH_GRP   = 0x0001;
 const uint8_t RS_PKT_SUBTYPE_EXT_SEARCH_MSG   = 0x0002;
 const uint8_t RS_PKT_SUBTYPE_EXT_DELETE_GRP   = 0x0004;
 const uint8_t RS_PKT_SUBTYPE_EXT_DELETE_MSG   = 0x0008;
 const uint8_t RS_PKT_SUBTYPE_EXT_SEARCH_REQ   = 0x0010;
-
+#endif // def RS_DEAD_CODE
 
 /*!
  * Base class for Network exchange service
@@ -65,17 +73,14 @@ const uint8_t RS_PKT_SUBTYPE_EXT_SEARCH_REQ   = 0x0010;
  */
 class RsNxsItem : public RsItem
 {
-
 public:
-    RsNxsItem(uint16_t servtype, uint8_t subtype) : RsItem(RS_PKT_VERSION_SERVICE, servtype, subtype), transactionNumber(0)
+	RsNxsItem(uint16_t servtype, uint8_t subtype):
-    {
+	    RsItem(RS_PKT_VERSION_SERVICE, servtype, subtype), transactionNumber(0)
-        setPriorityLevel(QOS_PRIORITY_RS_GXS_NET);
+	{ setPriorityLevel(QOS_PRIORITY_RS_GXS_NET); }
-        return;
-    }
-    virtual ~RsNxsItem(){}
-    virtual void clear() = 0;
 
-    uint32_t transactionNumber; // set to zero if this is not a transaction item
+	virtual ~RsNxsItem() = default;
+
+	uint32_t transactionNumber; // set to zero if this is not a transaction item
 };
 
 
@@ -362,6 +367,22 @@ public:
 
 };
 
+/*!
+ * Used to request to a peer pull updates from us ASAP without waiting GXS sync
+ * timer */
+struct RsNxsPullRequestItem: RsItem
+{
+	explicit RsNxsPullRequestItem(RsServiceType servtype):
+	    RsItem( RS_PKT_VERSION_SERVICE,
+	            servtype,
+	            static_cast<uint8_t>(RsNxsSubtype::PULL_REQUEST),
+	            QOS_PRIORITY_RS_GXS_NET ) {}
+
+	/// @see RsSerializable
+	void serial_process( RsGenericSerializer::SerializeJob,
+	                     RsGenericSerializer::SerializeContext& ) override {}
+};
+
 
 /*!
  * Used to respond to a RsGrpMsgsReq
@@ -401,6 +422,7 @@ struct RsNxsMsg : RsNxsItem
 	RsGxsMsgMetaData* metaData;
 };
 
+#ifdef RS_DEAD_CODE
 /*!
  * Used to request a search of user data
  */
@@ -422,7 +444,7 @@ public:
     RsTlvBinaryData serviceSearchItem; // service aware of item class
     uint32_t expiration; // expiration date
 };
-
+#endif //def RS_DEAD_CODE
 
 #ifdef UNUSED_CODE
 
@@ -511,6 +533,3 @@ public:
 protected:
     const uint16_t SERVICE_TYPE;
 };
-
-
-#endif // RSNXSITEMS_H

libretroshare/src/serialiser/rsserial.cc

@@ -3,7 +3,9 @@
  *                                                                             *
  * libretroshare: retroshare core library                                      *
  *                                                                             *
- * Copyright 2007-2008 by Robert Fernie <retroshare@lunamutt.com>              *
+ * Copyright (C) 2007-2008  Robert Fernie <retroshare@lunamutt.com>            *
+ * Copyright (C) 2021  Gioacchino Mazzurco <gio@eigenlab.org>                  *
+ * Copyright (C) 2021  Asociación Civil Altermundi <info@altermundi.net>       *
  *                                                                             *
  * This program is free software: you can redistribute it and/or modify        *
  * it under the terms of the GNU Lesser General Public License as              *
@@ -20,21 +22,20 @@
  *                                                                             *
  *******************************************************************************/
 
-#include "serialiser/rsbaseserial.h"
+#include <cmath>
-
-#include "util/rsthreads.h"
-#include "util/rsstring.h"
-#include "util/rsprint.h"
-
-#include "rsitems/rsitem.h"
-#include "rsitems/itempriorities.h"
-
-#include <math.h>
 #include <map>
 #include <vector>
 #include <iostream>
 #include <typeinfo>
 
+#include "serialiser/rsbaseserial.h"
+#include "util/cxx23retrocompat.h"
+#include "util/rsthreads.h"
+#include "util/rsstring.h"
+#include "util/rsprint.h"
+#include "rsitems/rsitem.h"
+#include "rsitems/itempriorities.h"
+
 
 /***
  * #define RSSERIAL_DEBUG 		1
@@ -166,11 +167,17 @@ uint8_t    RsItem::PacketSubType() const
 	/* For Service Packets */	
 RsItem::RsItem(uint8_t ver, uint16_t service, uint8_t subtype)
 {
-	_priority_level = QOS_PRIORITY_UNKNOWN ;	// This value triggers PQIInterface to complain about undefined priorities
+	// This value triggers PQIInterface to complain about undefined priorities
+	_priority_level = QOS_PRIORITY_UNKNOWN;
 	type = (ver << 24) + (service << 8) + subtype;
-	return;
 }
 
+RsItem::RsItem( uint8_t ver, RsServiceType service, uint8_t subtype,
+        RsItemPriority prio ):
+    type(static_cast<uint32_t>(
+             (ver << 24) + (std::to_underlying(service) << 8) + subtype )),
+    _priority_level(prio) {}
+
 uint16_t    RsItem::PacketService() const
 {
 	return (type >> 8) & 0xFFFF;

libretroshare/src/util/cxx23retrocompat.h

@@ -0,0 +1,34 @@
+/*******************************************************************************
+ * RetroShare C++23 backwards compatibility utilities                          *
+ *                                                                             *
+ * libretroshare: retroshare core library                                      *
+ *                                                                             *
+ * Copyright (C) 2021  Gioacchino Mazzurco <gio@eigenlab.org>                  *
+ * Copyright (C) 2021  Asociación Civil Altermundi <info@altermundi.net>       *
+ *                                                                             *
+ * This program 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 3 of the          *
+ * License, or (at your option) any later version.                             *
+ *                                                                             *
+ * This program 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 program. If not, see <https://www.gnu.org/licenses/>.       *
+ *                                                                             *
+ *******************************************************************************/
+#pragma once
+
+#include <utility>
+
+#if ! defined(__cpp_lib_to_underlying)
+namespace std
+{
+template <class Enum>
+constexpr underlying_type_t<Enum> to_underlying(Enum e) noexcept
+{ return static_cast<std::underlying_type_t<Enum>>(e); }
+}
+#endif // ! defined(__cpp_lib_to_underlying)