Fix pocket speakers incorrectly detaching computers

- Fix UpgradeSpeakerPeripheral not calling super.detach (so old
   computers were never cleaned up)
 - Correctly lock computer accesses inside SpeakerPeripheral

Fixes #1003.

Fingers crossed this is the last bug. Then I can bump the year and push
a new release tomorrow.

src/main/java/dan200/computercraft/core/apis/PeripheralAPI.java

@@ -39,14 +39,13 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
         private final String type;
         private final Set<String> additionalTypes;
         private final Map<String, PeripheralMethod> methodMap;
-        private boolean attached;
+        private boolean attached = false;
 
         PeripheralWrapper( IPeripheral peripheral, String side )
         {
             super( environment );
             this.side = side;
             this.peripheral = peripheral;
-            attached = false;
 
             type = Objects.requireNonNull( peripheral.getType(), "Peripheral type cannot be null" );
             additionalTypes = peripheral.getAdditionalTypes();

src/main/java/dan200/computercraft/shared/peripheral/speaker/SpeakerPeripheral.java

@@ -53,7 +53,7 @@ public abstract class SpeakerPeripheral implements IPeripheral
     public static final int SAMPLE_RATE = 48000;
 
     private final UUID source = UUID.randomUUID();
-    private final Set<IComputerAccess> computers = Collections.newSetFromMap( new HashMap<>() );
+    private final Set<IComputerAccess> computers = new HashSet<>();
 
     private long clock = 0;
     private long lastPositionTime;
@@ -141,9 +141,12 @@ public abstract class SpeakerPeripheral implements IPeripheral
             syncedPosition( pos );
 
             // And notify computers that we have space for more audio.
-            for( IComputerAccess computer : computers )
+            synchronized( computers )
             {
-                computer.queueEvent( "speaker_audio_empty", computer.getAttachmentName() );
+                for( IComputerAccess computer : computers )
+                {
+                    computer.queueEvent( "speaker_audio_empty", computer.getAttachmentName() );
+                }
             }
         }
 
@@ -379,13 +382,19 @@ public abstract class SpeakerPeripheral implements IPeripheral
     @Override
     public void attach( @Nonnull IComputerAccess computer )
     {
-        computers.add( computer );
+        synchronized( computers )
+        {
+            computers.add( computer );
+        }
     }
 
     @Override
     public void detach( @Nonnull IComputerAccess computer )
     {
-        computers.remove( computer );
+        synchronized( computers )
+        {
+            computers.remove( computer );
+        }
     }
 
     private static final class PendingSound

src/main/java/dan200/computercraft/shared/peripheral/speaker/UpgradeSpeakerPeripheral.java

@@ -21,6 +21,8 @@ public abstract class UpgradeSpeakerPeripheral extends SpeakerPeripheral
     @Override
     public void detach( @Nonnull IComputerAccess computer )
     {
+        super.detach( computer );
+
         // We could be in the process of shutting down the server, so we can't send packets in this case.
         MinecraftServer server = ServerLifecycleHooks.getCurrentServer();
         if( server == null || server.isStopped() ) return;