Extract multiple commit prevention in DNS writers into a base class

This still retains the DnsWriter interface itself for better integration
with Dagger and to preserve the option of having a DNS writer that does
not have this requirement (e.g. because it is idempotent).

This also makes the commit check thread-safe, which is a nice-to-have.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168451114

java/google/registry/dns/writer/BaseDnsWriter.java

@@ -0,0 +1,36 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.dns.writer;
+
+import static com.google.common.base.Preconditions.checkState;
+
+import java.util.concurrent.atomic.AtomicBoolean;
+
+/**
+ * A base implementation of {@link DnsWriter} that protects against multiple calls to commit().
+ */
+public abstract class BaseDnsWriter implements DnsWriter {
+
+  private final AtomicBoolean committedAlready = new AtomicBoolean(false);
+
+  @Override
+  public final void commit() {
+    checkState(committedAlready.compareAndSet(false, true), "commit() has already been called");
+    commitUnchecked();
+  }
+
+  /** Commits DNS updates. This can never be called more than once. */
+  protected abstract void commitUnchecked();
+}