Defer sending monitor updates until tick end

We send monitor updates when a player starts watching a chunk. However,
the block/tile data has not been sent when this event is fired, and so
the packet is entirely ignored.

Instead, we now queue a "send this" task, which is then dispatched on
the next tick end.

I have memories of this working on 1.12, so either something changed in
an update or I'm a complete idiot. Both are possible.

Fixes #687

src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorWatcher.java

@@ -9,6 +9,7 @@ import dan200.computercraft.ComputerCraft;
 import dan200.computercraft.shared.network.NetworkHandler;
 import dan200.computercraft.shared.network.client.MonitorClientMessage;
 import dan200.computercraft.shared.network.client.TerminalState;
+import net.minecraft.entity.player.ServerPlayerEntity;
 import net.minecraft.tileentity.TileEntity;
 import net.minecraft.util.math.BlockPos;
 import net.minecraft.util.math.ChunkPos;
@@ -28,6 +29,7 @@ import java.util.Queue;
 public final class MonitorWatcher
 {
     private static final Queue<TileMonitor> watching = new ArrayDeque<>();
+    private static final Queue<PlayerUpdate> playerUpdates = new ArrayDeque<>();
 
     private MonitorWatcher()
     {
@@ -58,10 +60,8 @@ public final class MonitorWatcher
             ServerMonitor serverMonitor = getMonitor( monitor );
             if( serverMonitor == null || monitor.enqueued ) continue;
 
-            // We use the cached terminal state if available - this is guaranteed to
+            // The chunk hasn't been sent to the client yet, so we can't send an update. Do it on tick end.
-            TerminalState state = monitor.cached;
+            playerUpdates.add( new PlayerUpdate( event.getPlayer(), monitor ) );
-            if( state == null ) state = monitor.cached = serverMonitor.write();
-            NetworkHandler.sendToPlayer( event.getPlayer(), new MonitorClientMessage( monitor.getBlockPos(), state ) );
         }
     }
 
@@ -70,6 +70,23 @@ public final class MonitorWatcher
     {
         if( event.phase != TickEvent.Phase.END ) return;
 
+        PlayerUpdate playerUpdate;
+        while( (playerUpdate = playerUpdates.poll()) != null )
+        {
+            TileMonitor tile = playerUpdate.monitor;
+            if( tile.enqueued || tile.isRemoved() ) continue;
+
+            ServerMonitor monitor = getMonitor( tile );
+            if( monitor == null ) continue;
+
+            // Some basic sanity checks to the player. It's possible they're no longer within range, but that's harder
+            // to track efficiently.
+            ServerPlayerEntity player = playerUpdate.player;
+            if( !player.isAlive() || player.getLevel() != tile.getLevel() ) continue;
+
+            NetworkHandler.sendToPlayer( playerUpdate.player, new MonitorClientMessage( tile.getBlockPos(), getState( tile, monitor ) ) );
+        }
+
         long limit = ComputerCraft.monitorBandwidth;
         boolean obeyLimit = limit > 0;
 
@@ -90,7 +107,7 @@ public final class MonitorWatcher
                 continue;
             }
 
-            TerminalState state = tile.cached = monitor.write();
+            TerminalState state = getState( tile, monitor );
             NetworkHandler.sendToAllTracking( new MonitorClientMessage( pos, state ), chunk );
 
             limit -= state.size();
@@ -101,4 +118,23 @@ public final class MonitorWatcher
     {
         return !monitor.isRemoved() && monitor.getXIndex() == 0 && monitor.getYIndex() == 0 ? monitor.getCachedServerMonitor() : null;
     }
+
+    private static TerminalState getState( TileMonitor tile, ServerMonitor monitor )
+    {
+        TerminalState state = tile.cached;
+        if( state == null ) state = tile.cached = monitor.write();
+        return state;
+    }
+
+    private static final class PlayerUpdate
+    {
+        final ServerPlayerEntity player;
+        final TileMonitor monitor;
+
+        private PlayerUpdate( ServerPlayerEntity player, TileMonitor monitor )
+        {
+            this.player = player;
+            this.monitor = monitor;
+        }
+    }
 }