// Copyright 2019 The Bazel 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 com.google.devtools.build.lib.actions;

import com.google.devtools.build.lib.util.Fingerprint;
import javax.annotation.Nullable;

/**
 * An implementation of {@link ActionAnalysisMetadata} that caches its {@linkplain #getKey key} so
 * that it is only computed once.
 */
public abstract class ActionKeyCacher implements ActionAnalysisMetadata {

  @Nullable private String cachedKey = null;

  @Override
  public final synchronized String getKey(ActionKeyContext actionKeyContext) {
    if (cachedKey == null) {
      try {
        Fingerprint fp = new Fingerprint();
        computeKey(actionKeyContext, fp);

        // Add a bool indicating whether the execution platform was set.
        fp.addBoolean(getExecutionPlatform() != null);
        if (getExecutionPlatform() != null) {
          // Add the execution platform information.
          getExecutionPlatform().addTo(fp);
        }

        // Compute the actual key and store it.
        cachedKey = fp.hexDigestAndReset();
      } catch (CommandLineExpansionException e) {
        cachedKey = KEY_ERROR;
      }
    }
    return cachedKey;
  }

  /**
   * See the javadoc for {@link Action} and {@link ActionAnalysisMetadata#getKey} for the contract
   * of this method.
   */
  protected abstract void computeKey(ActionKeyContext actionKeyContext, Fingerprint fp)
      throws CommandLineExpansionException;
}

// Copyright 2019 The Bazel 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 com.google.devtools.build.lib.actions;

import com.google.devtools.build.lib.util.Fingerprint;
import javax.annotation.Nullable;

/**
 * An implementation of {@link ActionAnalysisMetadata} that caches its {@linkplain #getKey key} so
 * that it is only computed once.
 */
public abstract class ActionKeyCacher implements ActionAnalysisMetadata {

  @Nullable private volatile String cachedKey = null;

  @Override
  public final String getKey(ActionKeyContext actionKeyContext) {
    if (cachedKey == null) {
      synchronized (this) {
        if (cachedKey == null) {
          try {
            Fingerprint fp = new Fingerprint();
            computeKey(actionKeyContext, fp);

            // Add a bool indicating whether the execution platform was set.
            fp.addBoolean(getExecutionPlatform() != null);
            if (getExecutionPlatform() != null) {
              // Add the execution platform information.
              getExecutionPlatform().addTo(fp);
            }

            // Compute the actual key and store it.
            cachedKey = fp.hexDigestAndReset();
          } catch (CommandLineExpansionException e) {
            cachedKey = KEY_ERROR;
          }
        }
      }
    }
    return cachedKey;
  }

  /**
   * See the javadoc for {@link Action} and {@link ActionAnalysisMetadata#getKey} for the contract
   * of this method.
   */
  protected abstract void computeKey(ActionKeyContext actionKeyContext, Fingerprint fp)
      throws CommandLineExpansionException;
}

Use a double check in getKey instead of synchronizing every call.