Fix handling interruptedException in resource pool (#9044)

core/src/main/java/org/apache/druid/java/util/http/client/pool/ResourcePool.java

@@ -28,6 +28,7 @@ import org.apache.druid.java.util.common.StringUtils;
 import org.apache.druid.java.util.common.io.Closer;
 import org.apache.druid.java.util.common.logger.Logger;
 
+import javax.annotation.Nullable;
 import java.io.Closeable;
 import java.io.IOException;
 import java.util.ArrayDeque;
@@ -38,6 +39,16 @@ import java.util.concurrent.ExecutionException;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 /**
+ * A resource pool based on {@link LoadingCache}. When a resource is first requested for a new key,
+ * all {@link ResourcePoolConfig#getMaxPerKey()} resources are initialized and cached in the {@link #pool}.
+ * The individual resource in {@link ImmediateCreationResourceHolder} is valid while (current time - last access time)
+ * <= {@link ResourcePoolConfig#getUnusedConnectionTimeoutMillis()}.
+ *
+ * A resource is closed and reinitialized if {@link ResourceFactory#isGood} returns false or it's expired based on
+ * {@link ResourcePoolConfig#getUnusedConnectionTimeoutMillis()}.
+ *
+ * {@link ResourcePoolConfig#getMaxPerKey() is a hard limit for the max number of resources per cache entry. The total
+ * number of resources in {@link ImmediateCreationResourceHolder} cannot be larger than the limit in any case.
  */
 public class ResourcePool<K, V> implements Closeable
 {
@@ -45,10 +56,7 @@ public class ResourcePool<K, V> implements Closeable
   private final LoadingCache<K, ImmediateCreationResourceHolder<K, V>> pool;
   private final AtomicBoolean closed = new AtomicBoolean(false);
 
-  public ResourcePool(
+  public ResourcePool(final ResourceFactory<K, V> factory, final ResourcePoolConfig config)
-      final ResourceFactory<K, V> factory,
-      final ResourcePoolConfig config
-  )
   {
     this.pool = CacheBuilder.newBuilder().build(
         new CacheLoader<K, ImmediateCreationResourceHolder<K, V>>()
@@ -56,7 +64,7 @@ public class ResourcePool<K, V> implements Closeable
           @Override
           public ImmediateCreationResourceHolder<K, V> load(K input)
           {
-            return new ImmediateCreationResourceHolder<K, V>(
+            return new ImmediateCreationResourceHolder<>(
                 config.getMaxPerKey(),
                 config.getUnusedConnectionTimeoutMillis(),
                 input,
@@ -67,6 +75,10 @@ public class ResourcePool<K, V> implements Closeable
     );
   }
 
+  /**
+   * Returns a {@link ResourceContainer} for the given key or null if this pool is already closed.
+   */
+  @Nullable
   public ResourceContainer<V> take(final K key)
   {
     if (closed.get()) {
@@ -166,16 +178,22 @@ public class ResourcePool<K, V> implements Closeable
       this.resourceHolderList = new ArrayDeque<>();
 
       for (int i = 0; i < maxSize; ++i) {
-        resourceHolderList.add(new ResourceHolder<>(
+        resourceHolderList.add(
+            new ResourceHolder<>(
                 System.currentTimeMillis(),
                 Preconditions.checkNotNull(
                     factory.generate(key),
                     "factory.generate(key)"
                 )
-        ));
+            )
+        );
       }
     }
 
+    /**
+     * Returns a resource or null if this holder is already closed or the current thread is interrupted.
+     */
+    @Nullable
     V get()
     {
       // resourceHolderList can't have nulls, so we'll use a null to signal that we need to create a new resource.
@@ -186,7 +204,7 @@ public class ResourcePool<K, V> implements Closeable
             this.wait();
           }
           catch (InterruptedException e) {
-            Thread.interrupted();
+            Thread.currentThread().interrupt();
             return null;
           }
         }
@@ -293,16 +311,16 @@ public class ResourcePool<K, V> implements Closeable
 
   private static class ResourceHolder<V>
   {
-    private long lastAccessedTime;
+    private final long lastAccessedTime;
-    private V resource;
+    private final V resource;
 
-    public ResourceHolder(long lastAccessedTime, V resource)
+    private ResourceHolder(long lastAccessedTime, V resource)
     {
       this.resource = resource;
       this.lastAccessedTime = lastAccessedTime;
     }
 
-    public long getLastAccessedTime()
+    private long getLastAccessedTime()
     {
       return lastAccessedTime;
     }