Avoid redundant volatile read in DefaultPromise#get() (#9547)

Motivation

Currently every call to get() on a promise results in two reads of the
volatile result field when one would suffice. Maybe this is optimized
away but it seems sensible not to rely on that.

Modification

Reimplement get() and get(...) in DefaultPromise to reduce volatile access.

Result

Fewer volatile reads.

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

@@ -16,7 +16,6 @@
 package io.netty.util.concurrent;
 
 import io.netty.util.internal.InternalThreadLocalMap;
-import io.netty.util.internal.PlatformDependent;
 import io.netty.util.internal.StringUtil;
 import io.netty.util.internal.SystemPropertyUtil;
 import io.netty.util.internal.ThrowableUtil;
@@ -25,7 +24,9 @@ import io.netty.util.internal.logging.InternalLoggerFactory;
 
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.CompletionException;
+import java.util.concurrent.ExecutionException;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 
 import static java.util.Objects.requireNonNull;
@@ -145,15 +146,21 @@ public class DefaultPromise<V> implements Promise<V> {
 
     @Override
     public Throwable cause() {
-        Object result = this.result;
+        return cause0(result);
-        if (result != CANCELLATION_CAUSE_HOLDER) {
-            return (result instanceof CauseHolder) ? ((CauseHolder) result).cause : null;
     }
+
+    private Throwable cause0(Object result) {
+        if (!(result instanceof CauseHolder)) {
+            return null;
+        }
+        if (result == CANCELLATION_CAUSE_HOLDER) {
             CancellationException ce = new LeanCancellationException();
             if (RESULT_UPDATER.compareAndSet(this, CANCELLATION_CAUSE_HOLDER, new CauseHolder(ce))) {
                 return ce;
             }
-        return ((CauseHolder) this.result).cause;
+            result = this.result;
+        }
+        return ((CauseHolder) result).cause;
     }
 
     @Override
@@ -313,6 +320,50 @@ public class DefaultPromise<V> implements Promise<V> {
         return (V) result;
     }
 
+    @SuppressWarnings("unchecked")
+    @Override
+    public V get() throws InterruptedException, ExecutionException {
+        Object result = this.result;
+        if (!isDone0(result)) {
+            await();
+            result = this.result;
+        }
+        if (result == SUCCESS || result == UNCANCELLABLE) {
+            return null;
+        }
+        Throwable cause = cause0(result);
+        if (cause == null) {
+            return (V) result;
+        }
+        if (cause instanceof CancellationException) {
+            throw (CancellationException) cause;
+        }
+        throw new ExecutionException(cause);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Override
+    public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
+        Object result = this.result;
+        if (!isDone0(result)) {
+            if (!await(timeout, unit)) {
+                throw new TimeoutException();
+            }
+            result = this.result;
+        }
+        if (result == SUCCESS || result == UNCANCELLABLE) {
+            return null;
+        }
+        Throwable cause = cause0(result);
+        if (cause == null) {
+            return (V) result;
+        }
+        if (cause instanceof CancellationException) {
+            throw (CancellationException) cause;
+        }
+        throw new ExecutionException(cause);
+    }
+
     /**
      * {@inheritDoc}
      *