Skip main-thread tasks if peripheral is detached

Due to the asynchronous nature of main-thread tasks, it's possible for
them to be executed on peripherals which have been detached. This has
been known for a long time (#893 was opened back in 2021), but finding a
good solution here is tricky.

Most of the time the method will silently succeed, but if we try to
interact with an IComputerAccess (such as in inventory methods, as seen
in #1750), we throw a NotAttachedException exception and spam the logs!

This is an initial step towards fixing this - when calling a peripheral
method via peripheral.call/modem.callRemote, we now wrap any enqueued
main-thread tasks and silently skip them if the peripheral has been
detached since.

This means that peripheral methods may start to return nil when they
didn't before. I think this is *fine* (though not ideal for sure!) - we
return nil if the peripheral has been detached, so it's largely
equivalent to that.

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

@@ -11,6 +11,7 @@ import dan200.computercraft.api.peripheral.IPeripheral;
 import dan200.computercraft.api.peripheral.NotAttachedException;
 import dan200.computercraft.api.peripheral.WorkMonitor;
 import dan200.computercraft.core.computer.ComputerSide;
+import dan200.computercraft.core.computer.GuardedLuaContext;
 import dan200.computercraft.core.methods.MethodSupplier;
 import dan200.computercraft.core.methods.PeripheralMethod;
 import dan200.computercraft.core.metrics.Metrics;
@@ -26,7 +27,7 @@ import java.util.*;
  * @hidden
  */
 public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChangeListener {
-    private class PeripheralWrapper extends ComputerAccess {
+    private class PeripheralWrapper extends ComputerAccess implements GuardedLuaContext.Guard {
         private final String side;
         private final IPeripheral peripheral;
 
@@ -35,6 +36,8 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
         private final Map<String, PeripheralMethod> methodMap;
         private boolean attached = false;
 
+        private @Nullable GuardedLuaContext contextWrapper;
+
         PeripheralWrapper(IPeripheral peripheral, String side) {
             super(environment);
             this.side = side;
@@ -91,9 +94,21 @@ public class PeripheralAPI implements ILuaAPI, IAPIEnvironment.IPeripheralChange
 
             if (method == null) throw new LuaException("No such method " + methodName);
 
-            try (var ignored = environment.time(Metrics.PERIPHERAL_OPS)) {
-                return method.apply(peripheral, context, this, arguments);
+            // Wrap the ILuaContext. We try to reuse the previous context where possible to avoid allocations - this
+            // should be pretty common as ILuaMachine uses a constant context.
+            var contextWrapper = this.contextWrapper;
+            if (contextWrapper == null || !contextWrapper.wraps(context)) {
+                contextWrapper = this.contextWrapper = new GuardedLuaContext(context, this);
             }
+
+            try (var ignored = environment.time(Metrics.PERIPHERAL_OPS)) {
+                return method.apply(peripheral, contextWrapper, this, arguments);
+            }
+        }
+
+        @Override
+        public boolean checkValid() {
+            return isAttached();
         }
 
         // IComputerAccess implementation

projects/core/src/main/java/dan200/computercraft/core/computer/GuardedLuaContext.java

@@ -0,0 +1,55 @@
+// SPDX-FileCopyrightText: 2023 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
+package dan200.computercraft.core.computer;
+
+import dan200.computercraft.api.lua.ILuaContext;
+import dan200.computercraft.api.lua.LuaException;
+import dan200.computercraft.api.lua.LuaTask;
+
+/**
+ * A {@link ILuaContext} which checks if context is valid when before executing
+ * {@linkplain #issueMainThreadTask(LuaTask) main-thread tasks}.
+ */
+public final class GuardedLuaContext implements ILuaContext {
+    private final ILuaContext original;
+    private final Guard guard;
+
+    public GuardedLuaContext(ILuaContext original, Guard guard) {
+        this.original = original;
+        this.guard = guard;
+    }
+
+    /**
+     * Determine if this {@link GuardedLuaContext} wraps another context.
+     * <p>
+     * This may be used to avoid constructing new guarded contexts, in a pattern something like:
+     *
+     * <pre>{@code
+     * var contextWrapper = this.contextWrapper;
+     * if(contextWrapper == null || !contextWrapper.wraps(context)) {
+     *     contextWrapper = this.contextWrapper = new GuardedLuaContext(context, this);
+     * }
+     * }</pre>
+     *
+     * @param context The original context.
+     * @return Whether {@code this} wraps {@code context}.
+     */
+    public boolean wraps(ILuaContext context) {
+        return original == context;
+    }
+
+    @Override
+    public long issueMainThreadTask(LuaTask task) throws LuaException {
+        return original.issueMainThreadTask(() -> guard.checkValid() ? task.execute() : null);
+    }
+
+    /**
+     * The function which checks if the context is still valid.
+     */
+    @FunctionalInterface
+    public interface Guard {
+        boolean checkValid();
+    }
+}