Remove default getRegister impl

This made sense in the early days of the blob-store register work but
now every blob store has its own implementation of `getRegister` there's
no need to include a default here.

Author: DaveCTurner

modules/repository-url/src/main/java/org/elasticsearch/common/blobstore/url/URLBlobContainer.java

@@ -169,4 +169,8 @@ public void compareAndExchangeRegister(
         listener.onFailure(new UnsupportedOperationException("URL repositories do not support this operation"));
     }
 
+    @Override
+    public void getRegister(OperationPurpose purpose, String key, ActionListener<OptionalBytesReference> listener) {
+        listener.onFailure(new UnsupportedOperationException("URL repositories do not support this operation"));
+    }
 }

plugins/repository-hdfs/src/main/java/org/elasticsearch/repositories/hdfs/HdfsBlobContainer.java

@@ -334,6 +334,11 @@ public void compareAndExchangeRegister(
         listener.onFailure(new UnsupportedOperationException("HDFS repositories do not support this operation"));
     }
 
+    @Override
+    public void getRegister(OperationPurpose purpose, String key, ActionListener<OptionalBytesReference> listener) {
+        listener.onFailure(new UnsupportedOperationException("HDFS repositories do not support this operation"));
+    }
+
     private static CreateOpts[] addOptionToArray(final CreateOpts[] opts, final CreateOpts opt) {
         if (opts == null) {
             return new CreateOpts[] { opt };

server/src/main/java/org/elasticsearch/common/blobstore/BlobContainer.java

@@ -11,7 +11,6 @@
 
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.common.blobstore.support.BlobMetadata;
-import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.core.CheckedConsumer;
 import org.elasticsearch.repositories.blobstore.BlobStoreRepository;
@@ -357,9 +356,7 @@ default void compareAndSetRegister(
      * @param listener a listener, completed with the value read from the register or {@code OptionalBytesReference#MISSING} if the value
      *                 could not be read due to concurrent activity (which should not happen).
      */
-    default void getRegister(OperationPurpose purpose, String key, ActionListener<OptionalBytesReference> listener) {
-        compareAndExchangeRegister(purpose, key, BytesArray.EMPTY, BytesArray.EMPTY, listener);
-    }
+    void getRegister(OperationPurpose purpose, String key, ActionListener<OptionalBytesReference> listener);
 
     /**
      * Verify that the {@link OperationPurpose} is (somewhat) suitable for the name of the blob to which it applies:

x-pack/plugin/searchable-snapshots/src/test/java/org/elasticsearch/xpack/searchablesnapshots/cache/common/TestUtils.java

@@ -269,6 +269,11 @@ public void compareAndExchangeRegister(
             listener.onFailure(unsupportedException());
         }
 
+        @Override
+        public void getRegister(OperationPurpose purpose, String key, ActionListener<OptionalBytesReference> listener) {
+            listener.onFailure(unsupportedException());
+        }
+
         private UnsupportedOperationException unsupportedException() {
             assert false : "this operation is not supported and should have not be called";
             return new UnsupportedOperationException("This operation is not supported");