Allow tasks to be bound to the same thread for each run.

src/lwjgl/java/net/minestom/server/map/framebuffers/GLFWCapableBuffer.java

@@ -68,6 +68,7 @@ public abstract class GLFWCapableBuffer {
     }
 
     public void changeRenderingThreadToCurrent() {
+        System.out.println("Currently on thread "+Thread.currentThread().getId());
         glfwMakeContextCurrent(glfwWindow);
         GL.createCapabilities();
     }
@@ -90,6 +91,7 @@ public abstract class GLFWCapableBuffer {
                         render(rendering);
                     }
                 })
+                .bindToSingleThread()
                 .repeat(period)
                 .schedule();
     }

src/main/java/net/minestom/server/timer/SchedulerManager.java

@@ -7,6 +7,7 @@ import net.minestom.server.MinecraftServer;
 import net.minestom.server.extensions.Extension;
 import net.minestom.server.extensions.IExtensionObserver;
 import net.minestom.server.utils.thread.MinestomThread;
+import net.minestom.server.utils.thread.ThreadBindingExecutor;
 import org.jetbrains.annotations.NotNull;
 
 import java.util.Collection;
@@ -35,6 +36,8 @@ public final class SchedulerManager implements IExtensionObserver {
     private final AtomicInteger shutdownCounter;
     //A threaded execution
     private final ExecutorService batchesPool;
+    //Thread execution, which always uses the same Thread for a given Task
+    private final ExecutorService threadBindingPool;
     // A single threaded scheduled execution
     private final ScheduledExecutorService timerExecutionService;
     // All the registered tasks (task id = task)
@@ -66,6 +69,7 @@ public final class SchedulerManager implements IExtensionObserver {
         this.shutdownCounter = new AtomicInteger();
 
         this.batchesPool = new MinestomThread(MinecraftServer.THREAD_COUNT_SCHEDULER, MinecraftServer.THREAD_NAME_SCHEDULER);
+        this.threadBindingPool = new ThreadBindingExecutor(MinecraftServer.THREAD_COUNT_SCHEDULER, MinecraftServer.THREAD_NAME_SCHEDULER);
         this.timerExecutionService = Executors.newSingleThreadScheduledExecutor();
         this.tasks = new Int2ObjectOpenHashMap<>();
         this.shutdownTasks = new Int2ObjectOpenHashMap<>();
@@ -183,15 +187,26 @@ public final class SchedulerManager implements IExtensionObserver {
     }
 
     /**
-     * Gets the execution service for all the registered {@link Task}.
+     * Gets the execution service for all the registered {@link Task}, which are not marked as thread-bound.
      *
-     * @return the execution service for all the registered {@link Task}
+     * @return the execution service for all the registered {@link Task}, which are not marked as thread-bound
      */
     @NotNull
     public ExecutorService getBatchesPool() {
         return batchesPool;
     }
 
+    /**
+     * Gets the execution service for all the registered {@link Task}, which are marked as thread-bound.
+     * The thread to which tasks are assigned depends on their runnable hashcode. Two (or more) tasks can be bound to the same Thread.
+     *
+     * @return the execution service for all the registered {@link Task}, which are marked as thread-bound
+     */
+    @NotNull
+    public ExecutorService getThreadBindingPool() {
+        return threadBindingPool;
+    }
+
     /**
      * Gets the scheduled execution service for all the registered {@link Task}.
      *

src/main/java/net/minestom/server/timer/Task.java

@@ -6,6 +6,7 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 import java.util.Objects;
+import java.util.concurrent.ExecutorService;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 
@@ -42,6 +43,12 @@ public class Task implements Runnable {
     private ScheduledFuture<?> future;
     // The thread of the task
     private volatile Thread currentThreadTask;
+    // The executor service used for this task
+    private final ExecutorService executorService;
+    // Whether this task will always execute on the same thread
+    private final boolean boundToSingleThread;
+    // Action executed on the executor. Stored inside the Task to avoid changing the hashcode (which ThreadBindingExecutor relies on)
+    private final Runnable action;
 
     /**
      * Creates a task.
@@ -51,24 +58,21 @@ public class Task implements Runnable {
      * @param shutdown         Defines whether the task is a shutdown task
      * @param delay            The time to delay
      * @param repeat           The time until the repetition
+     * @param bindToSingleThread Whether to run the given task always on the same thread.
      */
-    public Task(@NotNull SchedulerManager schedulerManager, @NotNull Runnable runnable, boolean shutdown, long delay, long repeat, boolean isTransient, @Nullable String owningExtension) {
+    public Task(@NotNull SchedulerManager schedulerManager, @NotNull Runnable runnable, boolean shutdown, long delay, long repeat, boolean isTransient, @Nullable String owningExtension, boolean bindToSingleThread) {
         this.schedulerManager = schedulerManager;
         this.runnable = runnable;
         this.shutdown = shutdown;
         this.id = shutdown ? this.schedulerManager.getShutdownCounterIdentifier() : this.schedulerManager.getCounterIdentifier();
+        this.executorService = bindToSingleThread ? this.schedulerManager.getThreadBindingPool() : this.schedulerManager.getBatchesPool();
         this.delay = delay;
         this.repeat = repeat;
         this.isTransient = isTransient;
+        this.boundToSingleThread = bindToSingleThread;
         this.owningExtension = owningExtension;
-    }
 
-    /**
-     * Executes the task.
-     */
-    @Override
-    public void run() {
-        this.schedulerManager.getBatchesPool().execute(() -> {
+        this.action = () -> {
             this.currentThreadTask = Thread.currentThread();
             try {
                 this.runnable.run();
@@ -84,7 +88,15 @@ public class Task implements Runnable {
                 if (this.repeat == 0) this.finish();
                 this.currentThreadTask = null;
             }
-        });
+        };
+    }
+
+    /**
+     * Executes the task.
+     */
+    @Override
+    public void run() {
+        executorService.execute(action);
     }
 
     /**

src/main/java/net/minestom/server/timer/TaskBuilder.java

@@ -36,6 +36,12 @@ public class TaskBuilder {
      */
     private boolean isTransient;
 
+    /**
+     * Should this Task be run on the same thread for each run? Can be used for situations where context is bound to a
+     * single thread. (for instance OpenGL context)
+     */
+    private boolean boundToSingleThread = false;
+
     /**
      * Creates a task builder.
      * <br>
@@ -137,6 +143,15 @@ public class TaskBuilder {
         return this;
     }
 
+    /**
+     * Makes this Task be run on the same thread for each run. Can be used for situations where context is bound to a
+     * single thread. (for instance OpenGL context)
+     */
+    public TaskBuilder bindToSingleThread() {
+        boundToSingleThread = true;
+        return this;
+    }
+
     /**
      * Builds a {@link Task}.
      *
@@ -151,7 +166,8 @@ public class TaskBuilder {
                 this.delay,
                 this.repeat,
                 this.isTransient,
-                this.owningExtension);
+                this.owningExtension,
+                this.boundToSingleThread);
     }
 
     /**

src/main/java/net/minestom/server/utils/thread/ThreadBindingExecutor.java

@@ -0,0 +1,92 @@
+package net.minestom.server.utils.thread;
+
+import org.jetbrains.annotations.NotNull;
+
+import java.util.LinkedList;
+import java.util.List;
+import java.util.concurrent.AbstractExecutorService;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Executor service which will always give the same thread to a given Runnable.
+ * Uses {@link Runnable#hashCode()} to determine the thread to assign.
+ */
+public class ThreadBindingExecutor extends AbstractExecutorService {
+
+    private MinestomThread[] threadExecutors;
+
+    /**
+     * Creates a non-local thread-binding executor
+     *
+     * @param nThreads the number of threads
+     * @param name     the name of the thread pool
+     */
+    public ThreadBindingExecutor(int nThreads, String name) {
+        this(nThreads, name, false);
+    }
+
+    /**
+     * @param nThreads the number of threads
+     * @param name     the name of the thread pool
+     * @param local    set to true if this executor is only used inside a method and should *not* be kept in the internal list of executors
+     */
+    public ThreadBindingExecutor(int nThreads, String name, boolean local) {
+        threadExecutors = new MinestomThread[nThreads];
+        for (int i = 0; i < nThreads; i++) {
+            threadExecutors[i] = new MinestomThread(1, name, local);
+        }
+    }
+
+    @Override
+    public void shutdown() {
+        for (MinestomThread t : threadExecutors) {
+            t.shutdown();
+        }
+    }
+
+    @NotNull
+    @Override
+    public List<Runnable> shutdownNow() {
+        List<Runnable> allTasks = new LinkedList<>();
+        for (MinestomThread t : threadExecutors) {
+            allTasks.addAll(t.shutdownNow());
+        }
+        return allTasks;
+    }
+
+    @Override
+    public boolean isShutdown() {
+        for (MinestomThread t : threadExecutors) {
+            if(!t.isShutdown())
+                return false;
+        }
+        return true;
+    }
+
+    @Override
+    public boolean isTerminated() {
+        for (MinestomThread t : threadExecutors) {
+            if(!t.isShutdown())
+                return false;
+        }
+        return true;
+    }
+
+    @Override
+    public boolean awaitTermination(long timeout, @NotNull TimeUnit unit) throws InterruptedException {
+        boolean terminated = true;
+        for (MinestomThread t : threadExecutors) {
+            terminated &= t.awaitTermination(timeout, unit);
+        }
+        return terminated;
+    }
+
+    @Override
+    public void execute(@NotNull Runnable command) {
+        int hash = command.hashCode();
+        if(hash < 0) hash = -hash;
+        int bucket = hash % threadExecutors.length;
+
+        threadExecutors[bucket].execute(command);
+    }
+}