src/main/java/com/google/devtools/build/lib/skyframe/actiongraph/v2/BaseCache.java

// Copyright 2018 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.skyframe.actiongraph.v2;

import java.io.IOException;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * Basic class to abstract action graph cache functionality.
 */
abstract class BaseCache<K, P> {
  private final Map<K, Integer> cache = new ConcurrentHashMap<>();
  protected final AqueryOutputHandler aqueryOutputHandler;
  // protobuf interprets the value 0 as "default value" for uint64, thus treating the field as
  // "unset". We should start from 1 instead.
  private final AtomicInteger nextId = new AtomicInteger(1);

  BaseCache(AqueryOutputHandler aqueryOutputHandler) {
    this.aqueryOutputHandler = aqueryOutputHandler;
  }

  protected K transformToKey(K data) {
    // In most cases, the data is the key but it can be overridden by subclasses.
    return data;
  }

  /**
   * Store the data in the internal cache, if it's not yet present. Return the generated id. Ids are
   * positive and unique.
   *
   * <p>Stream the proto to output, the first time it's generated.
   */
  int dataToIdAndStreamOutputProto(K data) throws IOException, InterruptedException {
    int id = -1;
    K key = transformToKey(data);
    boolean shouldOutputProto = false;

    // Double-checked locking here:
    // Once cache.get(key) != null it won't be changed again.
    if (cache.get(key) == null) {
      synchronized (this) {
        if (cache.get(key) == null) {
          id = nextId.getAndIncrement();
          // Note that this cannot be replaced by computeIfAbsent since createProto is a recursive
          // operation for the case of nested sets which will call dataToId on the same object and
          // thus computeIfAbsent again.
          cache.put(key, id);
          shouldOutputProto = true;
        }
      }
    }
    if (shouldOutputProto) {
      P proto = createProto(data, id);
      toOutput(proto);
    }
    return cache.get(key);
  }

  abstract P createProto(K key, int id) throws IOException, InterruptedException;

  abstract void toOutput(P proto) throws IOException;
}