Null out completed tasks to help with garbage collection (#9613)

Motivation

When ScheduledFutureTasks complete, there's no need to retain a ref to
the wrapped task. Clearing it could help in particular with the case
where many scheduled tasks have been cancelled but their queue removal
delayed (since it is done lazily).

Modifications

This comprises just the PromiseTask changes from #9580. Upon completion,
replace the task reference with a static sentinel depending on the type
of completion (so that it will be reflected by toString).

Result

More expedient collection of cancelled task objects

common/src/main/java/io/netty/util/concurrent/PromiseTask.java

@@ -45,7 +45,29 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
         }
     }
 
-    protected final Callable<V> task;
+    private static final Callable<?> COMPLETED = new SentinelCallable<Object>("COMPLETED");
+    private static final Callable<?> CANCELLED = new SentinelCallable<Object>("CANCELLED");
+    private static final Callable<?> FAILED = new SentinelCallable<Object>("FAILED");
+
+    private static class SentinelCallable<T> implements Callable<T> {
+        private final String name;
+
+        SentinelCallable(String name) {
+            this.name = name;
+        }
+
+        @Override
+        public T call() {
+            return null;
+        }
+
+        @Override
+        public String toString() {
+            return name;
+        }
+    }
+
+    protected Callable<V> task;
 
     PromiseTask(EventExecutor executor, Runnable runnable, V result) {
         this(executor, toCallable(runnable, result));
@@ -78,6 +100,18 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
         }
     }
 
+    @SuppressWarnings("unchecked")
+    private boolean clearTaskAfterCompletion(boolean done, Callable<?> result) {
+        if (done) {
+            // The only time where it might be possible for the sentinel task
+            // to be called is in the case of a periodic ScheduledFutureTask,
+            // in which case it's a benign race with cancellation and the (null)
+            // return value is not used.
+            task = (Callable<V>) result;
+        }
+        return done;
+    }
+
     @Override
     public final Promise<V> setFailure(Throwable cause) {
         throw new IllegalStateException();
@@ -85,6 +119,7 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
 
     protected final Promise<V> setFailureInternal(Throwable cause) {
         super.setFailure(cause);
+        clearTaskAfterCompletion(true, FAILED);
         return this;
     }
 
@@ -94,7 +129,7 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
     }
 
     protected final boolean tryFailureInternal(Throwable cause) {
-        return super.tryFailure(cause);
+        return clearTaskAfterCompletion(super.tryFailure(cause), FAILED);
     }
 
     @Override
@@ -104,6 +139,7 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
 
     protected final Promise<V> setSuccessInternal(V result) {
         super.setSuccess(result);
+        clearTaskAfterCompletion(true, COMPLETED);
         return this;
     }
 
@@ -113,7 +149,7 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
     }
 
     protected final boolean trySuccessInternal(V result) {
-        return super.trySuccess(result);
+        return clearTaskAfterCompletion(super.trySuccess(result), COMPLETED);
     }
 
     @Override
@@ -125,6 +161,11 @@ class PromiseTask<V> extends DefaultPromise<V> implements RunnableFuture<V> {
         return super.setUncancellable();
     }
 
+    @Override
+    public boolean cancel(boolean mayInterruptIfRunning) {
+        return clearTaskAfterCompletion(super.cancel(mayInterruptIfRunning), CANCELLED);
+    }
+
     @Override
     protected StringBuilder toStringBuilder() {
         StringBuilder buf = super.toStringBuilder();