A couple of minor tweaks

- Rename unload -> close to be a little more consistent
 - Make pollAndResetChanged be atomic, so we don't need to aquire a lock
 - Get the computer queue from the task owner, rather than a separate
   argument.

src/main/java/dan200/computercraft/core/computer/ApiWrapper.java

@@ -16,7 +16,7 @@
 /**
  * A wrapper for {@link ILuaAPI}s which cleans up after a {@link ComputerSystem} when the computer is shutdown.
  */
-public class ApiWrapper implements ILuaAPI
+final class ApiWrapper implements ILuaAPI
 {
     private final ILuaAPI delegate;
     private final ComputerSystem system;

src/main/java/dan200/computercraft/core/computer/Computer.java

@@ -20,10 +20,12 @@
 import dan200.computercraft.core.lua.ILuaMachine;
 import dan200.computercraft.core.terminal.Terminal;
 
+import javax.annotation.Nonnull;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
 
 public class Computer
 {
@@ -54,7 +56,7 @@ private enum State
     private FileSystem m_fileSystem = null;
     private IWritableMount m_rootMount = null;
 
-    private boolean m_externalOutputChanged;
+    private final AtomicBoolean externalOutputChanged = new AtomicBoolean();
 
     public Computer( IComputerEnvironment environment, Terminal terminal, int id )
     {
@@ -182,7 +184,7 @@ public void setLabel( String label )
         if( !Objects.equal( label, m_label ) )
         {
             m_label = label;
-            m_externalOutputChanged = true;
+            externalOutputChanged.set( true );
         }
     }
 
@@ -212,7 +214,7 @@ public void tick()
         }
 
         // Prepare to propagate the environment's output to the world.
-        if( m_internalEnvironment.updateOutput() ) m_externalOutputChanged = true;
+        if( m_internalEnvironment.updateOutput() ) externalOutputChanged.set( true );
 
         // Set output changed if the terminal has changed from blinking to not
         boolean blinking =
@@ -223,18 +225,13 @@ public void tick()
         if( blinking != m_blinking )
         {
             m_blinking = blinking;
-            m_externalOutputChanged = true;
+            externalOutputChanged.set( true );
         }
     }
 
     public boolean pollAndResetChanged()
     {
-        synchronized( this )
-        {
-            boolean changed = m_externalOutputChanged;
-            m_externalOutputChanged = false;
-            return changed;
-        }
+        return externalOutputChanged.getAndSet( false );
     }
 
     public boolean isBlinking()
@@ -327,7 +324,7 @@ private void initLua()
                 m_terminal.setCursorPos( 0, 1 );
                 m_terminal.write( "ComputerCraft may be installed incorrectly" );
 
-                machine.unload();
+                machine.close();
                 m_machine = null;
             }
             else
@@ -342,7 +339,7 @@ private void initLua()
             m_terminal.setCursorPos( 0, 1 );
             m_terminal.write( "ComputerCraft may be installed incorrectly" );
 
-            machine.unload();
+            machine.close();
             m_machine = null;
         }
     }
@@ -356,18 +353,18 @@ private void startComputer()
                 return;
             }
             m_state = State.Starting;
-            m_externalOutputChanged = true;
+            externalOutputChanged.set( true );
             m_ticksSinceStart = 0;
         }
 
-        // Turn the computercraft on
-        final Computer computer = this;
+        // Turn the computer on
         ComputerThread.queueTask( new ITask()
         {
+            @Nonnull
             @Override
             public Computer getOwner()
             {
-                return computer;
+                return Computer.this;
             }
 
             @Override
@@ -414,14 +411,14 @@ public void execute()
 
                     // Start a new state
                     m_state = State.Running;
-                    m_externalOutputChanged = true;
+                    externalOutputChanged.set( true );
                     synchronized( m_machine )
                     {
                         m_machine.handleEvent( null, null );
                     }
                 }
             }
-        }, computer );
+        } );
     }
 
     private void stopComputer( final boolean reboot )
@@ -433,17 +430,17 @@ private void stopComputer( final boolean reboot )
                 return;
             }
             m_state = State.Stopping;
-            m_externalOutputChanged = true;
+            externalOutputChanged.set( true );
         }
 
         // Turn the computercraft off
-        final Computer computer = this;
         ComputerThread.queueTask( new ITask()
         {
+            @Nonnull
             @Override
             public Computer getOwner()
             {
-                return computer;
+                return Computer.this;
             }
 
             @Override
@@ -468,7 +465,7 @@ public void execute()
                     // Shutdown terminal and filesystem
                     if( m_fileSystem != null )
                     {
-                        m_fileSystem.unload();
+                        m_fileSystem.close();
                         m_fileSystem = null;
                     }
 
@@ -478,7 +475,7 @@ public void execute()
 
                         synchronized( m_machine )
                         {
-                            m_machine.unload();
+                            m_machine.close();
                             m_machine = null;
                         }
                     }
@@ -487,14 +484,14 @@ public void execute()
                     m_internalEnvironment.resetOutput();
 
                     m_state = State.Off;
-                    m_externalOutputChanged = true;
+                    externalOutputChanged.set( true );
                     if( reboot )
                     {
                         m_startRequested = true;
                     }
                 }
             }
-        }, computer );
+        } );
     }
 
     public void queueEvent( final String event, final Object[] arguments )
@@ -507,13 +504,13 @@ public void queueEvent( final String event, final Object[] arguments )
             }
         }
 
-        final Computer computer = this;
-        ITask task = new ITask()
+        ComputerThread.queueTask( new ITask()
         {
+            @Nonnull
             @Override
             public Computer getOwner()
             {
-                return computer;
+                return Computer.this;
             }
 
             @Override
@@ -541,9 +538,7 @@ public void execute()
                     }
                 }
             }
-        };
-
-        ComputerThread.queueTask( task, computer );
+        } );
     }
 
     @Deprecated

src/main/java/dan200/computercraft/core/computer/ComputerThread.java

@@ -35,7 +35,7 @@ public class ComputerThread
     /**
      * Map of objects to task list
      */
-    private static final WeakHashMap<Object, BlockingQueue<ITask>> s_computerTaskQueues = new WeakHashMap<>();
+    private static final WeakHashMap<Computer, BlockingQueue<ITask>> s_computerTaskQueues = new WeakHashMap<>();
 
     /**
      * Active queues to execute
@@ -43,11 +43,6 @@ public class ComputerThread
     private static final BlockingQueue<BlockingQueue<ITask>> s_computerTasksActive = new LinkedBlockingQueue<>();
     private static final Set<BlockingQueue<ITask>> s_computerTasksActiveSet = new HashSet<>();
 
-    /**
-     * The default object for items which don't have an owner
-     */
-    private static final Object s_defaultOwner = new Object();
-
     /**
      * Whether the thread is stopped or should be stopped
      */
@@ -64,7 +59,7 @@ public class ComputerThread
     /**
      * Start the computer thread
      */
-    public static void start()
+    static void start()
     {
         synchronized( s_stateLock )
         {
@@ -116,20 +111,18 @@ public static void stop()
     /**
      * Queue a task to execute on the thread
      *
-     * @param task     The task to execute
-     * @param computer The computer to execute it on, use {@code null} to execute on the default object.
+     * @param task The task to execute
      */
-    public static void queueTask( ITask task, Computer computer )
+    static void queueTask( ITask task )
     {
-        Object queueObject = computer == null ? s_defaultOwner : computer;
-
+        Computer computer = task.getOwner();
         BlockingQueue<ITask> queue;
         synchronized( s_computerTaskQueues )
         {
-            queue = s_computerTaskQueues.get( queueObject );
+            queue = s_computerTaskQueues.get( computer );
             if( queue == null )
             {
-                s_computerTaskQueues.put( queueObject, queue = new LinkedBlockingQueue<>( QUEUE_LIMIT ) );
+                s_computerTaskQueues.put( computer, queue = new LinkedBlockingQueue<>( QUEUE_LIMIT ) );
             }
         }
 

src/main/java/dan200/computercraft/core/computer/ITask.java

@@ -6,8 +6,11 @@
 
 package dan200.computercraft.core.computer;
 
+import javax.annotation.Nonnull;
+
 public interface ITask
 {
+    @Nonnull
     Computer getOwner();
 
     void execute();

src/main/java/dan200/computercraft/core/filesystem/FileSystem.java

@@ -330,7 +330,7 @@ public FileSystem( String rootLabel, IWritableMount rootMount ) throws FileSyste
         mountWritable( rootLabel, "", rootMount );
     }
 
-    public void unload()
+    public void close()
     {
         // Close all dangling open files
         synchronized( m_openFiles )

src/main/java/dan200/computercraft/core/lua/CobaltLuaMachine.java

@@ -196,12 +196,12 @@ public void loadBios( InputStream bios )
         }
         catch( CompileException e )
         {
-            unload();
+            close();
         }
         catch( IOException e )
         {
             ComputerCraft.log.warn( "Could not load bios.lua ", e );
-            unload();
+            close();
         }
     }
 
@@ -229,11 +229,11 @@ public void handleEvent( String eventName, Object[] arguments )
             LuaValue filter = results.first();
             m_eventFilter = filter.isString() ? filter.toString() : null;
 
-            if( m_mainRoutine.getStatus().equals( "dead" ) ) unload();
+            if( m_mainRoutine.getStatus().equals( "dead" ) ) close();
         }
         catch( LuaError | HardAbortError | InterruptedException e )
         {
-            unload();
+            close();
             ComputerCraft.log.warn( "Top level coroutine errored", e );
         }
         finally
@@ -275,7 +275,7 @@ public boolean isFinished()
     }
 
     @Override
-    public void unload()
+    public void close()
     {
         if( m_state == null ) return;
 
@@ -348,6 +348,7 @@ public long issueMainThreadTask( @Nonnull final ILuaTask task ) throws LuaExcept
                                     final long taskID = MainThread.getUniqueTaskID();
                                     final ITask iTask = new ITask()
                                     {
+                                        @Nonnull
                                         @Override
                                         public Computer getOwner()
                                         {

src/main/java/dan200/computercraft/core/lua/ILuaMachine.java

@@ -29,5 +29,5 @@ public interface ILuaMachine
 
     boolean isFinished();
 
-    void unload();
+    void close();
 }