Force send a mapblock to a player (#8140)

* Force send a mapblock to a player.

Send a single mapblock to a specific remote player.

This is badly needed for mods and games where players are teleported
into terrain which may be not generated, loaded, or modified
significantly since the last player visit.

In all these cases, the player currently ends up in void, air, or
inside blocks which not only looks bad, but has the effect that the
player might end up falling and then the server needs to correct for
the player position again later, which is a hack.

The best solution is to send at least the single mapblock that the
player will be teleported to. I've tested this with ITB which does this
all the time, and I can see it functioning as expected (it even shows
a half loaded entry hallway, as the further blocks aren't loaded yet).

The parameter is a blockpos (table of x, y, z), not a regular pos.

The function may return false if the call failed. This is most likely
due to the target position not being generated or emerged yet, or
another internal failure, such as the player not being initialized.

* Always send mapblock on teleport or respawn.

This avoids the need for mods to send a mapblock on teleport or
respawn, since any call to `player:set_pos()` will pass this code.

6 files changed, 55 insertions, 0 deletions

diff --git a/doc/lua_api.txt b/doc/lua_api.txt
index c7035bb72..c75800cf4 100644
--- a/doc/lua_api.txt
+++ b/doc/lua_api.txt
@@ -5382,6 +5382,11 @@ This is basically a reference to a C++ `ServerActiveObject`
     * in first person view
     * in third person view (max. values `{x=-10/10,y=-10,15,z=-5/5}`)
 * `get_eye_offset()`: returns `offset_first` and `offset_third`
+* `send_mapblock(blockpos)`:
+    * Sends a server-side loaded mapblock to the player.
+    * Returns `false` if failed.
+    * Resource intensive - use sparsely
+    * To get blockpos, integer divide pos by 16
 
 `PcgRandom`
 -----------
diff --git a/src/content_sao.cpp b/src/content_sao.cpp
index 7acf03684..cefdeb597 100644
--- a/src/content_sao.cpp
+++ b/src/content_sao.cpp
@@ -1205,6 +1205,10 @@ void PlayerSAO::setPos(const v3f &pos)
 	if(isAttached())
 		return;
 
+	// Send mapblock of target location
+	v3s16 blockpos = v3s16(pos.X / MAP_BLOCKSIZE, pos.Y / MAP_BLOCKSIZE, pos.Z / MAP_BLOCKSIZE);
+	m_env->getGameDef()->SendBlock(m_peer_id, blockpos);
+
 	setBasePosition(pos);
 	// Movement caused by this command is always valid
 	m_last_good_position = pos;
diff --git a/src/script/lua_api/l_object.cpp b/src/script/lua_api/l_object.cpp
index 2efcd894a..b1f4e3da5 100644
--- a/src/script/lua_api/l_object.cpp
+++ b/src/script/lua_api/l_object.cpp
@@ -584,6 +584,24 @@ int ObjectRef::l_get_eye_offset(lua_State *L)
 	return 2;
 }
 
+// send_mapblock(self, pos)
+int ObjectRef::l_send_mapblock(lua_State *L)
+{
+	NO_MAP_LOCK_REQUIRED;
+	ObjectRef *ref = checkobject(L, 1);
+
+	RemotePlayer *player = getplayer(ref);
+	if (!player)
+		return 0;
+	v3s16 p = read_v3s16(L, 2);
+
+	session_t peer_id = player->getPeerId();
+	bool r = getServer(L)->SendBlock(peer_id, p);
+
+	lua_pushboolean(L, r);
+	return 1;
+}
+
 // set_animation_frame_speed(self, frame_speed)
 int ObjectRef::l_set_animation_frame_speed(lua_State *L)
 {
@@ -1958,5 +1976,6 @@ luaL_Reg ObjectRef::methods[] = {
 	luamethod(ObjectRef, get_local_animation),
 	luamethod(ObjectRef, set_eye_offset),
 	luamethod(ObjectRef, get_eye_offset),
+	luamethod(ObjectRef, send_mapblock),
 	{0,0}
 };
diff --git a/src/script/lua_api/l_object.h b/src/script/lua_api/l_object.h
index c7d963d87..653d833f6 100644
--- a/src/script/lua_api/l_object.h
+++ b/src/script/lua_api/l_object.h
@@ -351,4 +351,6 @@ private:
 	// get_nametag_attributes(self)
 	static int l_get_nametag_attributes(lua_State *L);
 
+	// send_mapblock(pos)
+	static int l_send_mapblock(lua_State *L);
 };
diff --git a/src/server.cpp b/src/server.cpp
index 869498965..496170da3 100644
--- a/src/server.cpp
+++ b/src/server.cpp
@@ -2312,6 +2312,28 @@ void Server::SendBlocks(float dtime)
 	m_clients.unlock();
 }
 
+bool Server::SendBlock(session_t peer_id, const v3s16 &blockpos)
+{
+	MapBlock *block = nullptr;
+	try {
+		block = m_env->getMap().getBlockNoCreate(blockpos);
+	} catch (InvalidPositionException &e) {};
+	if (!block)
+		return false;
+
+	m_clients.lock();
+	RemoteClient *client = m_clients.lockedGetClientNoEx(peer_id, CS_Active);
+	if (!client || client->isBlockSent(blockpos)) {
+		m_clients.unlock();
+		return false;
+	}
+	SendBlockNoLock(peer_id, block, client->serialization_version,
+			client->net_proto_version);
+	m_clients.unlock();
+
+	return true;
+}
+
 void Server::fillMediaCache()
 {
 	infostream<<"Server: Calculating media file checksums"<<std::endl;
diff --git a/src/server.h b/src/server.h
index 5949d9bf1..5e58ac9e6 100644
--- a/src/server.h
+++ b/src/server.h
@@ -344,6 +344,9 @@ public:
 	bool sendModChannelMessage(const std::string &channel, const std::string &message);
 	ModChannel *getModChannel(const std::string &channel);
 
+	// Send block to specific player only
+	bool SendBlock(session_t peer_id, const v3s16 &blockpos);
+
 	// Bind address
 	Address m_bind_addr;