Introduce SchedulerBackend to fix VelocitySchedulerTest intermittent failure (#1728)

proxy/src/main/java/com/velocitypowered/proxy/scheduler/ExecutorSchedulerBackend.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright (C) 2018-2026 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.scheduler;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import com.google.common.util.concurrent.ThreadFactoryBuilder;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * A {@link SchedulerBackend} backed by a real {@link ScheduledExecutorService}.
+ */
+public class ExecutorSchedulerBackend implements SchedulerBackend {
+
+  private final ScheduledExecutorService executor;
+
+  /**
+   * Creates a ExecutorSchedulerBackend with a default executor.
+   */
+  public ExecutorSchedulerBackend() {
+    this(Executors.newSingleThreadScheduledExecutor(
+        new ThreadFactoryBuilder()
+            .setDaemon(true)
+            .setNameFormat("Velocity Task Scheduler Timer")
+            .build()
+    ));
+  }
+
+  /**
+   * Creates a ExecutorSchedulerBackend with a given executor.
+   *
+   * @param executor The executor to use.
+   */
+  public ExecutorSchedulerBackend(ScheduledExecutorService executor) {
+    this.executor = checkNotNull(executor, "executor");
+  }
+
+  @Override
+  public ScheduledFuture<?> schedule(Runnable task, long delay, TimeUnit unit) {
+    return executor.schedule(task, delay, unit);
+  }
+
+  @Override
+  public ScheduledFuture<?> scheduleAtFixedRate(Runnable task, long initialDelay, long period, TimeUnit unit) {
+    return executor.scheduleAtFixedRate(task, initialDelay, period, unit);
+  }
+
+  @Override
+  public void shutdown() {
+    executor.shutdown();
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/scheduler/SchedulerBackend.java

@@ -0,0 +1,56 @@
+/*
+ * Copyright (C) 2018-2026 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.scheduler;
+
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Backend interface used by {@link VelocityScheduler} to schedule timer callbacks.
+ *
+ * <p>This is an internal abstraction that allows tests to replace the real-time scheduler
+ * with a deterministic implementation.
+ */
+interface SchedulerBackend {
+
+  /**
+   * Schedules a task to run once after the given delay.
+   *
+   * @param task the task to run
+   * @param delay the delay
+   * @param unit the delay unit
+   * @return a future representing the scheduled task
+   */
+  ScheduledFuture<?> schedule(Runnable task, long delay, TimeUnit unit);
+
+  /**
+   * Schedules a task to run at a fixed rate.
+   *
+   * @param task the task to run
+   * @param initialDelay the initial delay
+   * @param period the period between runs
+   * @param unit the time unit
+   * @return a future representing the scheduled task
+   */
+  ScheduledFuture<?> scheduleAtFixedRate(Runnable task, long initialDelay, long period, TimeUnit unit);
+
+  /**
+   * Shuts down the backend.
+   */
+  void shutdown();
+}

proxy/src/main/java/com/velocitypowered/proxy/scheduler/VelocityScheduler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (C) 2018-2023 Velocity Contributors
+ * Copyright (C) 2018-2026 Velocity Contributors
  *
  * This program is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -23,7 +23,6 @@ import static com.google.common.base.Preconditions.checkNotNull;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Multimap;
 import com.google.common.collect.Multimaps;
-import com.google.common.util.concurrent.ThreadFactoryBuilder;
 import com.velocitypowered.api.plugin.PluginContainer;
 import com.velocitypowered.api.plugin.PluginManager;
 import com.velocitypowered.api.scheduler.ScheduledTask;
@@ -40,7 +39,6 @@ import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
-import java.util.concurrent.Executors;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
@@ -60,20 +58,23 @@ import org.jetbrains.annotations.VisibleForTesting;
 public class VelocityScheduler implements Scheduler {
 
   private final PluginManager pluginManager;
-  private final ScheduledExecutorService timerExecutionService;
+  private final SchedulerBackend backend;
   private final Multimap<Object, ScheduledTask> tasksByPlugin = Multimaps.synchronizedMultimap(
       Multimaps.newSetMultimap(new IdentityHashMap<>(), HashSet::new));
 
   /**
-   * Initalizes the scheduler.
+   * Initializes the scheduler.
    *
    * @param pluginManager the Velocity plugin manager
    */
   public VelocityScheduler(PluginManager pluginManager) {
+    this(pluginManager, new ExecutorSchedulerBackend());
+  }
+
+  @VisibleForTesting
+  VelocityScheduler(PluginManager pluginManager, SchedulerBackend backend) {
     this.pluginManager = pluginManager;
-    this.timerExecutionService = Executors
-        .newSingleThreadScheduledExecutor(new ThreadFactoryBuilder().setDaemon(true)
-            .setNameFormat("Velocity Task Scheduler Timer").build());
+    this.backend = backend;
   }
 
   @Override
@@ -118,7 +119,7 @@ public class VelocityScheduler implements Scheduler {
     for (ScheduledTask task : terminating) {
       task.cancel();
     }
-    timerExecutionService.shutdown();
+    backend.shutdown();
     final List<PluginContainer> plugins = new ArrayList<>(this.pluginManager.getPlugins());
     final Iterator<PluginContainer> pluginIterator = plugins.iterator();
     while (pluginIterator.hasNext()) {
@@ -232,10 +233,9 @@ public class VelocityScheduler implements Scheduler {
 
     void schedule() {
       if (repeat == 0) {
-        this.future = timerExecutionService.schedule(this, delay, TimeUnit.MILLISECONDS);
+        this.future = backend.schedule(this, delay, TimeUnit.MILLISECONDS);
       } else {
-        this.future = timerExecutionService
-            .scheduleAtFixedRate(this, delay, repeat, TimeUnit.MILLISECONDS);
+        this.future = backend.scheduleAtFixedRate(this, delay, repeat, TimeUnit.MILLISECONDS);
       }
     }