Fixes memory leak on JDK 16+

In newer versions of java, specifically 16 and over, the `ThreadLocal` can
be cleared before the Thread is killed. This breaks the assumption of
`ThreadLocalHolder` which expected the `initializeThread` method to be
executed only once per `Thread`.

This commit changes the `threadValues` to become a map that holds which
thread the value is associated with so we can avoid recreating new
values when one already exists.

Fixes logfellow#722

src/main/java/net/logstash/logback/util/ThreadLocalHolder.java

@@ -17,30 +17,31 @@
 
 import java.lang.ref.ReferenceQueue;
 import java.lang.ref.WeakReference;
+import java.util.Map;
 import java.util.Objects;
-import java.util.concurrent.CopyOnWriteArrayList;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Supplier;
 
 
 /**
  * Maintains a per-thread value created by the {@link Supplier} given to the constructor.
- * 
+ *
  * <p>A thread obtains the value by calling {@link #acquire()} and must release it after
  * use by calling {@link #release()}. If the value is not released, subsequent calls to
  * {@link #acquire()} will throw an {@link IllegalStateException}.
- * 
+ *
  * <p>Instances value may also implement the optional {@link ThreadLocalHolder.Lifecycle}
  * interface if they wish to be notified when they are recycled or disposed.
- * 
+ *
  * <p>The holder keeps track of each requesting thread and takes care of disposing the
  * allocated value when it dies.
- * 
+ *
  * All allocated values are automatically disposed when {@link ThreadLocalHolder#close()}
  * is called.
- * 
+ *
  * <p>Note: This class is for internal use only and subject to backward incompatible change
  * at any time.
- * 
+ *
  * @param <T> type of instances returned by this {@link ThreadLocalHolder}.
  *
  * @author brenuart
@@ -51,92 +52,92 @@ public class ThreadLocalHolder<T> {
      * The factory used to create new instances
      */
     private final Supplier<T> factory;
-    
+
     /**
      * ThreadLocal holding per-thread instances
      */
     private final ThreadLocal<Holder<T>> threadLocal = ThreadLocal.withInitial(this::initializeThread);
-    
+
     /**
      * Collection of values assigned to each thread
      */
-    protected final CopyOnWriteArrayList<HolderRef> threadValues = new CopyOnWriteArrayList<>(); /* visible for testing */
-    
+    protected final Map<Long, HolderRef> threadValues = new ConcurrentHashMap<>(); /* visible for testing */
+
     /**
      * Reference to dead threads
      */
     private final ReferenceQueue<Thread> deadThreads = new ReferenceQueue<>();
-    
+
     /**
      * {@code true} when the {@link ThreadLocalHolder} is closed.
      * When closed, values released by threads will be immediately disposed and the reference cleared.
      */
     private volatile boolean closed = false;
-    
-    
+
+
     /**
      * Create a new instance of the pool.
-     * 
+     *
      * @param factory the factory used to create new instances.
      */
     public ThreadLocalHolder(Supplier<T> factory) {
         this.factory = Objects.requireNonNull(factory);
     }
-    
-    
+
+
     /**
      * Get the value assigned to the current thread, creating a new one if none is assigned yet or the
      * previous has been disposed.
-     * 
+     *
      * The value must be {@link #release()} to ensure proper life cycle before it can be {@link #acquire()}
      * again.
-     * 
+     *
      * @return the value assigned to this thread
      * @throws IllegalStateException if the value is already in use and {@link #release()} was not yet invoked.
      */
     public final T acquire() {
         Holder<T> holder = this.threadLocal.get();
-        
+
         if (holder.leased) {
             throw new IllegalStateException("ThreadLocal value is already in use and not yet released.");
         }
-        
+
         if (holder.value == null) {
             holder.value = Objects.requireNonNull(createInstance());
         }
-        
+
         holder.leased = true;
         return holder.value;
     }
-    
-    
+
+
     /**
      * Release the value and recycle it if possible.
-     * 
+     *
      * @throws IllegalStateException if the value was not previously {@link #acquire()}.
      */
     public final void release() {
         Holder<T> holder = this.threadLocal.get();
-        
+
         if (!holder.leased) {
             throw new IllegalStateException("Invalid attempt at releasing a value that was not previously acquired.");
         }
         holder.leased = false;
-        
+
         /*
          * Dispose value if it cannot be recycled
          */
         if (this.closed || !safelyRecycleInstance(holder.value)) {
             disposeHolder(holder);
         }
-              
+
         /*
          * Dispose values assigned to threads that just died
          */
         processDeadThreads();
     }
-    
-      
+
+
     /**
      * Close the holder and dispose all values.
      * Threads are still able to {@link #acquire()} values after the holder is closed, but they will be disposed
@@ -148,39 +149,42 @@ public void close() {
          * immediately instead of being recycled.
          */
         this.closed = true;
-        
+
         /*
          * Dispose value assigned to running threads.
          * "inuse" values will be disposed by the owning thread when it releases it.
          */
-        for (HolderRef holderRef: this.threadValues) {
+        for (HolderRef holderRef: this.threadValues.values()) {
             Holder<T> holder = holderRef.getHolder();
             if (!holder.leased) {
                 disposeHolder(holder);
             }
         }
         this.threadValues.clear();
-     
+
         /*
          * Dispose values assigned to threads that just died
          */
         processDeadThreads();
     }
-    
-    
+
+
     /**
      * Create a new {@link Holder} and keep track of the asking thread for clearing when the thread
      * is gone.
-     * 
+     *
      * @return a {@link Holder} assigned to the current thread.
      */
     private Holder<T> initializeThread() {
-        Holder<T> holder = new Holder<>();
-        threadValues.add(new HolderRef(Thread.currentThread(), holder, deadThreads));
-        return holder;
+        final Thread currentThread = Thread.currentThread();
+        final long threadId = currentThread.getId();
+        return threadValues.computeIfAbsent(
+                threadId,
+                ignore -> new HolderRef(currentThread, new Holder<>(), deadThreads))
+                .holder;
     }
 
-    
+
     /**
      * Dispose values of dead threads
      */
@@ -194,45 +198,45 @@ private void processDeadThreads() {
             Holder<T> holder = ref.getHolder();
             disposeHolder(holder);
             threadValues.remove(ref);
-            
+
             ref = (HolderRef) deadThreads.poll();
         }
     }
-    
-   
+
+
     private void disposeHolder(Holder<T> holder) {
         safelyDisposeInstance(holder.value);
         holder.value = null;
     }
-    
-    
+
+
     /**
      * Create a new object instance (must be non-null).
      * Sub-classes may override this method to implement their own custom logic if needed.
-     * 
+     *
      * @return a new object instance
      */
     protected T createInstance() {
         return this.factory.get();
     }
-    
-    
+
+
     /**
      * Dispose the object instance by calling its life cycle methods.
      * Sub-classes may override this method if they wish to implement their own custom logic.
-     * 
+     *
      * @param instance the instance to dispose
      */
     protected void disposeInstance(T instance) {
         if (instance instanceof Lifecycle) {
             ((Lifecycle) instance).dispose();
         }
     }
-    
-    
+
+
     /**
      * Safely dispose the given instance, ignoring any exception that may be thrown.
-     * 
+     *
      * @param instance the instance to dispose
      */
     private void safelyDisposeInstance(T instance) {
@@ -242,12 +246,12 @@ private void safelyDisposeInstance(T instance) {
             // ignore
         }
     }
-    
-    
+
+
     /**
      * Recycle the instance before returning it to the pool.
      * Sub-classes may override this method if they wish to implement their own custom logic.
-     * 
+     *
      * @param instance the instance to recycle
      * @return {@code true} if the instance can be recycled and returned to the pool, {@code false} if not.
      */
@@ -258,12 +262,12 @@ protected boolean recycleInstance(T instance) {
             return true;
         }
     }
-    
-    
+
+
     /**
      * Safely call {@link ThreadLocalHolder#recycleInstance(Object)}, ignoring exceptions but returning
      * {@code false} to prevent reuse if any is thrown.
-     * 
+     *
      * @param instance the instance to recycle
      * @return {@code true} if the instance can be recycled, {@code false} otherwise.
      */
@@ -274,8 +278,8 @@ private boolean safelyRecycleInstance(T instance) {
             return false;
         }
     }
-    
-    
+
+
     /**
      * Optional interface that pooled instances may implement if they wish to be notified of
      * life cycle events.
@@ -284,26 +288,26 @@ public interface Lifecycle {
         /**
          * Indicate whether the instance can be recycled and returned to the pool and perform
          * the necessary recycling tasks.
-         * 
+         *
          * @return {@code true} if the instance can be returned to the pool, {@code false} if
          *         it must be disposed instead.
          */
         default boolean recycle() {
             return true;
         }
-        
+
         /**
          * Dispose the instance and free allocated resources.
          */
         default void dispose() {
             // noop
         }
     }
-    
-    
+
+
     /**
      * Holds the value assigned to a thread together with its "inuse" state.
-     * 
+     *
      * This class is static as to not have a reference to the outer {@link ThreadLocalHolder}
      * and prevent it from being garbage collected.
      */
@@ -313,29 +317,29 @@ private static class Holder<T> {
          * May be null if none is already assigned or when the previous is disposed.
          */
         private T value;
-        
+
         /**
          * Indicate whether the instance is in use (acquired).
          * Maintaining this flag helps to avoid recreating a new SoftReference every time
          * the instance is released.
          */
         private boolean leased;
     }
-    
-    
+
+
     /**
      * A {@link WeakReference} to a thread with the {@link Holder} assigned to it.
      * Used to detect the death of a thread and dispose the associated value.
      */
     /* visible for testing */
     protected class HolderRef extends WeakReference<Thread> {
         private final Holder<T> holder;
-        
+
         HolderRef(Thread owningThread, Holder<T> holder, ReferenceQueue<Thread> referenceQueue) {
             super(owningThread, referenceQueue);
             this.holder = holder;
         }
-        
+
         public Holder<T> getHolder() {
             return this.holder;
         }