fix: make ParallelCompositeUploadBlobWriteSessionConfig.ExecutorSuppl…

…ier#cachedPool a singleton (#2691)

google-cloud-storage/src/main/java/com/google/cloud/storage/ParallelCompositeUploadBlobWriteSessionConfig.java

@@ -452,7 +452,7 @@ private ExecutorSupplier() {}
      */
     @BetaApi
     public static ExecutorSupplier cachedPool() {
-      return new CachedSupplier();
+      return CachedSupplier.INSTANCE;
     }
 
     /**
@@ -529,12 +529,18 @@ private void writeObject(ObjectOutputStream out) throws IOException {
 
     private static class CachedSupplier extends ExecutorSupplier implements Serializable {
       private static final long serialVersionUID = 7768210719775319260L;
+      private static final CachedSupplier INSTANCE = new CachedSupplier();
 
       @Override
       Executor get() {
         ThreadFactory threadFactory = newThreadFactory();
         return Executors.newCachedThreadPool(threadFactory);
       }
+
+      /** prevent java serialization from using a new instance */
+      private Object readResolve() {
+        return INSTANCE;
+      }
     }
 
     private static class FixedSupplier extends ExecutorSupplier implements Serializable {