This change implements a deep copy of the `Fields` map for each composition environment, ensuring thread safety.
  
  ### Issue
  Fixes #135757 ([#135759](https://github.com/kubernetes/kubernetes/pull/135759), [@Abhigyan-Shekhar](https://github.com/Abhigyan-Shekhar)) [SIG API Machinery and CLI]

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safe
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  - [beta] PodDisruptionBudget has been promoted to beta, can be used to safely drain nodes while respecting application SLO's ([docs](http://kubernetes.io/docs/tasks/administer-cluster/safely-drain-node/)) ([kubernetes/features#85](https://github.com/kubernetes/enhancements/issues/85))
- **UI**

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      return fromMap.containsKey(key);
    }

    // safe as long as the user followed the <b>Warning</b> in the javadoc
    @SuppressWarnings("unchecked")
    @Override
    public @Nullable V2 get(@Nullable Object key) {
      V1 value = fromMap.get(key);
      if (value != null || fromMap.containsKey(key)) {
        // The cast is safe because of the containsKey check.

      return getOrDefault(key, null);
    }

    // safe as long as the user followed the <b>Warning</b> in the javadoc
    @SuppressWarnings("unchecked")
    @Override
    public @Nullable V2 getOrDefault(@Nullable Object key, @Nullable V2 defaultValue) {
      V1 value = fromMap.get(key);
      if (value != null || fromMap.containsKey(key)) {
        // The cast is safe because of the containsKey check.

      * In 1.4, either that key or this key: `pods.beta.kubernetes.io/init-containers`,can be used.
    * When you GET an object, you will see both annotation keys with the same values.
      You can safely roll back from 1.4 to 1.3, and things with init-containers
      will still work (pods, deployments, etc).
    * If you are running 1.3, only use the alpha annotation, or it may be lost when
      rolling forward.

    Node* node = graph->graph.FindNodeId(*pos);
    // FindNodeId() returns nullptr for nodes that have been deleted.
    // We aren't currently allowing nodes to be deleted, but it is safer
    // to still check.
    if (node != nullptr) return ToOperation(node);
    *pos += 1;
  }

  // No more nodes.
  return nullptr;
}

      return getDone(future);
    }
  }

  private static class MyException extends Exception {}

  // Class hierarchy for generics sanity checks
  private static class Foo {}

  private static class FooChild extends Foo {}

  private static class Bar {}

  private static class BarChild extends Bar {}

      return getDone(future);
    }
  }

  private static class MyException extends Exception {}

  // Class hierarchy for generics sanity checks
  private static class Foo {}

  private static class FooChild extends Foo {}

  private static class Bar {}

  private static class BarChild extends Bar {}