2014-04-19 10:15:57 -04:00
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2015-08-11 16:48:55 -04:00
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<title>Cuberite - Using ChunkStays</title>
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2014-04-19 10:15:57 -04:00
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<h1>Using ChunkStays</h1>
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<p>
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A plugin may need to manipulate data in arbitrary chunks, and it needs a way to make the server
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guarantee that the chunks are available in memory.</p>
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<h2>The problem</h2>
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<p>
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Usually when plugins want to manipulate larger areas of world data, they need to make sure that the
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server has the appropriate chunks loaded in the memory. When the data being manipulated can be further
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away from the connected players, or the data is being manipulated from a console handler, there is a
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real chance that the chunks are not loaded.</p>
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<p>
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This gets even more important when using the <a href="cBlockArea.html">cBlockArea</a> class for reading
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and writing. Those functions will fail when any of the required chunks aren't valid. This means that
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either the block area has incomplete data (Read() failed) or incomplete data has been written to the
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world (Write() failed). Recovery from this is near impossible - you can't simply read or write again
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later, because the world may have changed in the meantime.</p>
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<h2>The solution</h2>
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<p>
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The naive solution would be to monitor chunk loads and unloads, and postpone the operations until all
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the chunks are available. This would be quite ineffective and also very soon it would become very
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difficult to maintain, if there were multiple code paths requiring this handling.</p>
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<p>
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An alternate approach has been implemented, accessible through a single (somewhat hidden) function
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call: <a href="cWorld.html">cWorld:ChunkStay()</a>. All that this call basically does is, it tells the
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server "Load these chunks for me, and call this callback function once you have them all." And the
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server does exactly that - it remembers the callback and asks the world loader / generator to provide
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the chunks. Once the chunks become available, it calls the callback function for the plugin.</p>
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<p>
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There are a few gotcha-s, though. If the code that was requesting the read or write had access to some
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of the volatile objects, such as <a href="cPlayer.html">cPlayer</a> or
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<a href="cEntity.html">cEntity</a> objects, those cannot be accessed by the callback anymore, because
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they may have become invalid in the meantime - the player may have disconnected, the entity may have
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despawned. So the callback must use the longer way to access such objects, such as calling
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<a href="cWorld.html">cWorld:DoWithEntityByID()</a> or
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<a href="cWorld.html">cWorld:DoWithPlayer()</a>.</p>
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<h2>The example</h2>
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<p>
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As a simple example, consider a theoretical plugin that allows a player to save the immediate
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surroundings of the spawn into a schematic file. The player issues a command to initiate the save, and
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the plugin reads a 50 x 50 x 50 block area around the spawn into a cBlockArea and saves it on the disk
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as "<PlayerName>_spawn.schematic". When it's done with the saving, it wants to send a message to the
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player to let them know the command has succeeded.</p>
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<p>
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The first attempt shows the naive approach. It simply reads the block area and saves it, then sends the
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message. I'll repeat once more, this code is <b>the wrong way</b> to do it!</p>
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<pre class="prettyprint lang-lua">
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function HandleCommandSaveSpawn(a_Split, a_Player)
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-- Get the coords for the spawn:
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local SpawnX = a_Player:GetWorld():GetSpawnX()
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local SpawnY = a_Player:GetWorld():GetSpawnY()
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local SpawnZ = a_Player:GetWorld():GetSpawnZ()
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local Bounds = cCuboid(SpawnX - 25, SpawnY - 25, SpawnZ - 25, SpawnX + 25, SpawnY + 25, SpawnZ + 25)
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Bounds:ClampY(0, 255)
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-- Read the area around spawn into a cBlockArea, save to file:
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local Area = cBlockArea()
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local FileName = a_Player:GetName() .. "_spawn.schematic"
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Area:Read(a_Player:GetWorld(), Bounds, cBlockArea.baTypes + cBlockArea.baMetas)
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Area:SaveToSchematicFile(FileName)
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-- Notify the player:
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a_Player:SendMessage(cCompositeChat("The spawn has been saved", mtInfo))
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return true
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end
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</pre>
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<p>
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Now if the player goes exploring far and uses the command to save their spawn, the chunks aren't
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loaded, so the BlockArea reading fails, the BlockArea contains bad data. Note that the plugin fails to
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do any error checking and if the area isn't read from the world, it happily saves the incomplete data
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and says "hey, everything's right", althought it has just trashed any previous backup of the spawn
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schematic with nonsense data.</p>
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<hr/>
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<p>
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The following script uses the ChunkStay method to alleviate chunk-related problems. This is <b>the
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right way</b> of doing it:</p>
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<pre class="prettyprint lang-lua">
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function HandleCommandSaveSpawn(a_Split, a_Player)
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-- Get the coords for the spawn:
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local SpawnX = a_Player:GetWorld():GetSpawnX()
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local SpawnY = a_Player:GetWorld():GetSpawnY()
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local SpawnZ = a_Player:GetWorld():GetSpawnZ()
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local Bounds = cCuboid(SpawnX - 25, SpawnY - 25, SpawnZ - 25, SpawnX + 25, SpawnY + 25, SpawnZ + 25)
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Bounds:ClampY(0, 255)
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-- Get a list of chunks that we need loaded:
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local MinChunkX = math.floor((SpawnX - 25) / 16)
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local MaxChunkX = math.ceil ((SpawnX + 25) / 16)
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local MinChunkZ = math.floor((SpawnZ - 25) / 16)
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local MaxChunkZ = math.ceil ((SpawnZ + 25) / 16)
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local Chunks = {}
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for x = MinChunkX, MaxChunkX do
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for z = MinChunkZ, MaxChunkZ do
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table.insert(Chunks, {x, z})
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end
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end -- for x
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-- Store the player's name and world to use in the callback, because the a_Player object may no longer be valid:
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local PlayerName = a_Player:GetName()
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local World = a_Player:GetWorld()
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-- This is the callback that is executed once all the chunks are loaded:
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local OnAllChunksAvailable = function()
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-- Read the area around spawn into a cBlockArea, save to file:
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local Area = cBlockArea()
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local FileName = PlayerName .. "_spawn.schematic"
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if (Area:Read(World, Bounds, cBlockArea.baTypes + cBlockArea.baMetas)) then
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Area:SaveToSchematicFile(FileName)
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Msg = cCompositeChat("The spawn has been saved", mtInfo)
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else
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Msg = cCompositeChat("Cannot save the spawn", mtFailure)
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end
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-- Notify the player:
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-- Note that we cannot use a_Player here, because it may no longer be valid (if the player disconnected before the command completes)
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World:DoWithPlayer(PlayerName,
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function (a_CBPlayer)
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a_CBPlayer:SendMessage(Msg)
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end
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)
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end
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-- Ask the server to load our chunks and notify us once it's done:
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World:ChunkStay(Chunks, nil, OnAllChunksAvailable)
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-- Note that code here may get executed before the callback is called!
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-- The ChunkStay says "once you have the chunks", not "wait until you have the chunks"
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-- So you can't notify the player here, because the saving needn't have occurred yet.
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return true
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end
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</pre>
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<p>
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Note that this code does its error checking of the Area:Read() function, and it will not overwrite the
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previous file unless it actually has the correct data. If you're wondering how the reading could fail
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when we've got the chunks loaded, there's still the issue of free RAM - if the memory for the area
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cannot be allocated, it cannot be read even with all the chunks present. So we still do need that
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check.</p>
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<h2>The conclusion</h2>
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<p>
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Although it makes the code a little bit longer and is a bit more difficult to grasp at first, the
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ChunkStay is a useful technique to add to your repertoire. It is to be used whenever you need access to
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chunks that may potentially be inaccessible, and you really need the data.</p>
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<p>Possibly the biggest hurdle in using the ChunkStay is the fact that it does its work in the
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background, thus invalidating all cPlayer and cEntity objects your function may hold, so you need to
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re-acquire them from their IDs and names. This is the penalty for using multi-threaded code.</p>
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