Fix race condition in BufferedRequest

common/src/main/java/me/lucko/luckperms/common/buffers/BufferedRequest.java

@@ -101,19 +101,21 @@ public abstract class BufferedRequest<T> {
      */
     protected abstract T perform();
 
-    private static class Processor<R> implements Runnable {
+    private static class Processor<R> {
         private Supplier<R> supplier;
 
         private final long delay;
         private final TimeUnit unit;
 
         private final SchedulerAdapter schedulerAdapter;
-        private SchedulerTask task;
 
         private final Object[] mutex = new Object[0];
         private CompletableFuture<R> future = new CompletableFuture<>();
         private boolean usable = true;
 
+        private SchedulerTask scheduledTask;
+        private BoundTask boundTask = null;
+
         Processor(Supplier<R> supplier, long delay, TimeUnit unit, SchedulerAdapter schedulerAdapter) {
             this.supplier = supplier;
             this.delay = delay;
@@ -128,36 +130,14 @@ public abstract class BufferedRequest<T> {
                 if (!this.usable) {
                     throw new IllegalStateException("Processor not usable");
                 }
-                if (this.task != null) {
+                if (this.scheduledTask != null) {
-                    this.task.cancel();
+                    this.scheduledTask.cancel();
                 }
-                this.task = this.schedulerAdapter.asyncLater(this, this.delay, this.unit);
+                this.boundTask = new BoundTask();
+                this.scheduledTask = this.schedulerAdapter.asyncLater(this.boundTask, this.delay, this.unit);
             }
         }
 
-        @Override
-        public void run() {
-            synchronized (this.mutex) {
-                if (!this.usable) {
-                    throw new IllegalStateException("Task has already ran");
-                }
-                this.usable = false;
-            }
-
-            // compute result
-            try {
-                R result = this.supplier.get();
-                this.future.complete(result);
-            } catch (Exception e) {
-                this.future.completeExceptionally(e);
-            }
-
-            // allow supplier and future to be GCed
-            this.task = null;
-            this.supplier = null;
-            this.future = null;
-        }
-
         CompletableFuture<R> getFuture() {
             return this.future;
         }
@@ -167,6 +147,39 @@ public abstract class BufferedRequest<T> {
             return this.future;
         }
 
+        private final class BoundTask implements Runnable {
+            @Override
+            public void run() {
+                synchronized (Processor.this.mutex) {
+                    if (!Processor.this.usable) {
+                        throw new IllegalStateException("Task has already ran");
+                    }
+
+                    // check that we're still the bound task.
+                    // prevents a race condition between #run and #rescheduleTask
+                    if (Processor.this.boundTask != this) {
+                        return;
+                    }
+
+                    Processor.this.usable = false;
+                }
+
+                // compute result
+                try {
+                    R result = Processor.this.supplier.get();
+                    Processor.this.future.complete(result);
+                } catch (Exception e) {
+                    Processor.this.future.completeExceptionally(e);
+                }
+
+                // allow supplier and future to be GCed
+                Processor.this.supplier = null;
+                Processor.this.future = null;
+                Processor.this.scheduledTask = null;
+                Processor.this.boundTask = null;
+            }
+        }
+
     }
 
 }