src/main/java/com/google/devtools/build/skyframe/SkyframeIterableResult.java

// Copyright 2022 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,
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package com.google.devtools.build.skyframe;

import java.util.NoSuchElementException;
import javax.annotation.Nullable;

/**
 * An iterable-like result of getting Skyframe dependencies via {@link
 * SkyFunction.Environment#getOrderedValuesAndExceptions}. Callers can use the {@link #next} and
 * {@link #nextOrThrow} methods to obtain successive elements.
 *
 * <p>Can only be iterated through once: if an iterable of size k was passed to {@link
 * SkyFunction.Environment#getOrderedValuesAndExceptions}, then the returned {@link
 * SkyframeIterableResult} can have {@link #next} and {@link #nextOrThrow} called a total of k
 * times.
 *
 * <p>Note that a {@link SkyFunction} cannot guarantee that {@link
 * SkyFunction.Environment#valuesMissing} will be true upon receipt of a {@code
 * SkyframeIterableResult}. The elements must be iterated over. If {@link #next} or {@link
 * #nextOrThrow} returns {@code null}, only then will {@link SkyFunction.Environment#valuesMissing}
 * be guaranteed to return true.
 */
public interface SkyframeIterableResult {

  /**
   * Returns true if {@link SkyframeIterableResult} has more elements. In other words, returns true
   * if {@link #next} or {@link #nextOrThrow} would return an element rather than throwing a {@link
   * NoSuchElementException}.
   */
  boolean hasNext();

  /**
   * Returns the next direct dependency in {@link SkyframeIterableResult}. If the dependency is not
   * in the set of already evaluated direct dependencies, returns {@code null}. Also returns {@code
   * null} if the dependency has already been evaluated and found to be in error. In either of these
   * cases, {@link SkyFunction.Environment#valuesMissing} will subsequently return true.
   */
  @Nullable
  default SkyValue next() {
    return nextOrThrow(null, null, null);
  }

  /**
   * Returns the next direct dependency in {@link SkyframeIterableResult}. If the dependency is not
   * in the set of already evaluated direct dependencies, returns {@code null}. If the dependency
   * has already been evaluated and found to be in error, throws the exception coming from the
   * error, so long as the exception is of one of the specified types. SkyFunction implementations
   * may use this method to catch and rethrow a more informative exception or to continue
   * evaluation. The caller must specify the exception type(s) that might be thrown using the {@code
   * exceptionClass} argument(s). If the dependency's exception is not an instance of {@code
   * exceptionClass}, {@code null} is returned. In this case, {@link
   * SkyFunction.Environment#valuesMissing} will subsequently return true.
   *
   * <p>The exception class given cannot be a supertype or a subtype of {@link RuntimeException}, or
   * a subtype of {@link InterruptedException}. See {@link
   * SkyFunctionException#validateExceptionType} for details.
   */
  @Nullable
  default <E extends Exception> SkyValue nextOrThrow(Class<E> exceptionClass) throws E {
    return nextOrThrow(exceptionClass, null, null, null);
  }

  @Nullable
  default <E1 extends Exception, E2 extends Exception> SkyValue nextOrThrow(
      Class<E1> exceptionClass1, Class<E2> exceptionClass2) throws E1, E2 {
    return nextOrThrow(exceptionClass1, exceptionClass2, null, null);
  }

  @Nullable
  default <E1 extends Exception, E2 extends Exception, E3 extends Exception> SkyValue nextOrThrow(
      Class<E1> exceptionClass1, Class<E2> exceptionClass2, Class<E3> exceptionClass3)
      throws E1, E2, E3 {
    return nextOrThrow(exceptionClass1, exceptionClass2, exceptionClass3, null);
  }

  @Nullable
  <E1 extends Exception, E2 extends Exception, E3 extends Exception, E4 extends Exception>
      SkyValue nextOrThrow(
          @Nullable Class<E1> exceptionClass1,
          @Nullable Class<E2> exceptionClass2,
          @Nullable Class<E3> exceptionClass3,
          @Nullable Class<E4> exceptionClass4)
          throws E1, E2, E3, E4;
}