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

// Copyright 2016 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.skyframe;

import static com.google.common.base.Preconditions.checkState;

import com.google.common.base.MoreObjects;
import com.google.common.collect.ImmutableSet;
import com.google.devtools.build.skyframe.NodeEntry.DirtyType;
import com.google.devtools.build.skyframe.NodeEntry.LifecycleState;
import java.util.List;
import javax.annotation.Nullable;

/**
 * State for a node that either has not been built yet or has been dirtied.
 *
 * <p>If the node has previously been built and the state tracks the previous value and dependencies
 * for purposes of pruning, {@link #isIncremental} returns true. Deps are checked to see if
 * re-evaluation is needed, and the node will either marked clean or re-evaluated.
 *
 * <p>This class does not attempt to synchronize operations. It is assumed that the calling {@link
 * InMemoryNodeEntry} performs the appropriate synchronization when necessary.
 *
 * <p>This class is public only for the benefit of alternative graph implementations outside of the
 * package.
 */
public abstract class DirtyBuildingState {
  private static final int NOT_EVALUATING_SENTINEL = -1;

  /**
   * The state of a dirty node.
   *
   * <p>Initialized to either {@link LifecycleState#CHECK_DEPENDENCIES} or {@link
   * LifecycleState#NEEDS_REBUILDING} depending on the {@link DirtyType} (see {@link
   * #initialState}). May take on any {@link LifecycleState} value except {@link
   * LifecycleState#NOT_YET_EVALUATING} and {@link LifecycleState#DONE}.
   */
  private LifecycleState state;

  /**
   * The number of dependencies that are known to be done in a {@link NodeEntry}.
   *
   * <p>There is a potential check-then-act race here during evaluation, so we need to make sure
   * that when this is increased, we always check if the new value is equal to the number of
   * required dependencies, and if so, we must re-schedule the node for evaluation.
   *
   * <p>There are two potential pitfalls here: 1) If multiple dependencies signal this node in close
   * succession, this node should be scheduled exactly once. 2) If a thread is still working on this
   * node, it should not be scheduled.
   *
   * <p>To solve the first problem, the {@link NodeEntry#signalDep} method also returns if the node
   * needs to be re-scheduled, and ensures that only one thread gets a true return value.
   *
   * <p>The second problem is solved by first adding the newly discovered deps to a node's {@link
   * IncrementalInMemoryNodeEntry#directDeps}, and then looping through the direct deps and
   * registering this node as a reverse dependency. This ensures that the signaledDeps counter can
   * only reach {@link GroupedDeps#numElements} on the very last iteration of the loop, i.e., the
   * thread is not working on the node anymore. Note that this requires that there is no code after
   * the loop in {@link ParallelEvaluator.Evaluate#run}.
   */
  private int signaledDeps = NOT_EVALUATING_SENTINEL;

  /**
   * The number of external dependencies (in contrast to the number of internal dependencies which
   * are tracked in NodeEntry). We never keep information about external dependencies across
   * Skyframe calls.
   */
  // We do not strictly require a counter here; all external deps from one SkyFunction evaluation
  // pass are registered as a single logical dependency, and the SkyFunction is only re-evaluated if
  // all of them complete. Therefore, we only need a single bit to track this fact. If the mere
  // existence of this field turns out to be a significant memory burden, we could change the
  // implementation by moving to a single-bit approach, and then store that bit as part of the
  // state field, e.g., by adding a REBUILDING_WAITING_FOR_EXTERNAL_DEPS enum value, as this can
  // only happen during evaluation.
  private int externalDeps;

  /**
   * Returns the {@link GroupedDeps} requested last time the node was built, or {@code null} if on
   * its initial build.
   *
   * <p>Dependencies from the last build are be compared to dependencies requested on this build to
   * check whether this node has changed in {@link NodeEntry#setValue}. See {@link
   * IncrementalInMemoryNodeEntry#directDeps} for more on dependency group storage.
   *
   * <p>Public only for the use of alternative graph implementations.
   */
  @Nullable
  public abstract GroupedDeps getLastBuildDirectDeps() throws InterruptedException;

  /**
   * The number of groups of the dependencies requested last time when the node was built, or {@code
   * 0} if on its initial build.
   *
   * <p>Getting the number of last-built dependencies should not throw {@link InterruptedException}.
   */
  protected abstract int getNumOfGroupsInLastBuildDirectDeps();

  /**
   * The value of the node the last time it was built, or {@code null} if on its initial build.
   *
   * <p>Public only for the use of alternative graph implementations.
   */
  @Nullable
  public abstract SkyValue getLastBuildValue() throws InterruptedException;

  /**
   * Group of children to be checked next in the process of determining if this entry needs to be
   * re-evaluated. Used by {@link DirtyBuildingState#getNextDirtyDirectDeps} and {@link #signalDep}.
   */
  protected int dirtyDirectDepIndex = 0;

  protected DirtyBuildingState(DirtyType dirtyType) {
    state = initialState(dirtyType);
  }

  private static LifecycleState initialState(DirtyType dirtyType) {
    switch (dirtyType) {
      case DIRTY:
        return LifecycleState.CHECK_DEPENDENCIES;
      case CHANGE:
      case REWIND:
        return LifecycleState.NEEDS_REBUILDING;
    }
    throw new AssertionError(dirtyType);
  }

  /** Returns true if this state has information about a previously built version. */
  protected abstract boolean isIncremental();

  final void markChanged() {
    checkState(state == LifecycleState.CHECK_DEPENDENCIES, this);
    checkState(dirtyDirectDepIndex == 0, "Unexpected evaluation: %s", this);
    state = LifecycleState.NEEDS_REBUILDING;
  }

  final void forceRebuild(int numTemporaryDirectDeps) {
    checkState(state == LifecycleState.CHECK_DEPENDENCIES, this);
    checkState(numTemporaryDirectDeps + externalDeps == signaledDeps, this);
    checkState(getNumOfGroupsInLastBuildDirectDeps() == dirtyDirectDepIndex, this);
    state = LifecycleState.REBUILDING;
  }

  final boolean isEvaluating() {
    return signaledDeps > NOT_EVALUATING_SENTINEL;
  }

  final boolean isChanged() {
    return state == LifecycleState.NEEDS_REBUILDING || state == LifecycleState.REBUILDING;
  }

  private void checkFinishedBuildingWhenAboutToSetValue() {
    checkState(
        state == LifecycleState.VERIFIED_CLEAN || state == LifecycleState.REBUILDING,
        "not done building %s",
        this);
  }

  /**
   * Signals that a child is done.
   *
   * <p>If this node is not yet known to need rebuilding, sets {@link #state} to {@link
   * LifecycleState#NEEDS_REBUILDING} if the child has changed, and {@link
   * LifecycleState#VERIFIED_CLEAN} if the child has not changed and this was the last child to be
   * checked (as determined by {@code isReady} and comparing {@link #dirtyDirectDepIndex} and {@link
   * DirtyBuildingState#getNumOfGroupsInLastBuildDirectDeps()}.
   */
  final void signalDep(
      AbstractInMemoryNodeEntry<?> entry,
      NodeVersion version,
      Version childVersion,
      @Nullable SkyKey childForDebugging) {
    checkState(isEvaluating(), "%s %s", entry, childForDebugging);
    signaledDeps++;
    if (isChanged()) {
      return;
    }

    // childVersion > version.lastEvaluated() means the child has changed since the last evaluation.
    boolean childChanged = !childVersion.atMost(version.lastEvaluated());
    if (childChanged) {
      state = LifecycleState.NEEDS_REBUILDING;
    } else if (state == LifecycleState.CHECK_DEPENDENCIES
        && isReady(entry.getNumTemporaryDirectDeps())
        && getNumOfGroupsInLastBuildDirectDeps() == dirtyDirectDepIndex) {
      // No other dep already marked this as NEEDS_REBUILDING, no deps outstanding, and this was the
      // last block of deps to be checked.
      state = LifecycleState.VERIFIED_CLEAN;
    }
  }

  final void addExternalDep() {
    checkState(isEvaluating());
    externalDeps++;
  }

  /**
   * Returns true if {@code newValue}.equals the value from the last time this node was built.
   * Should only be used by {@link NodeEntry#setValue}.
   *
   * <p>Changes in direct deps do <i>not</i> force this to return false. Only the value is
   * considered.
   */
  public final boolean unchangedFromLastBuild(SkyValue newValue) throws InterruptedException {
    checkFinishedBuildingWhenAboutToSetValue();
    return !(newValue instanceof NotComparableSkyValue)
        && getLastBuildValue() != null
        && getLastBuildValue().equals(newValue);
  }

  /**
   * Returns true if the deps requested during this evaluation ({@code directDeps}) are exactly
   * those requested the last time this node was built, in the same order.
   */
  final boolean depsUnchangedFromLastBuild(GroupedDeps directDeps) throws InterruptedException {
    checkFinishedBuildingWhenAboutToSetValue();
    return getLastBuildDirectDeps().equals(directDeps);
  }

  final boolean noDepsLastBuild() {
    return getNumOfGroupsInLastBuildDirectDeps() == 0;
  }

  /** Returns the {@link LifecycleState} as documented by {@link NodeEntry#getLifecycleState}. */
  final LifecycleState getLifecycleState() {
    return state;
  }

  /**
   * Gets the next children to be re-evaluated to see if this dirty node needs to be re-evaluated.
   *
   * <p>See {@link NodeEntry#getNextDirtyDirectDeps}.
   */
  final List<SkyKey> getNextDirtyDirectDeps() throws InterruptedException {
    checkState(state == LifecycleState.CHECK_DEPENDENCIES, this);
    checkState(dirtyDirectDepIndex < getNumOfGroupsInLastBuildDirectDeps(), this);
    return getLastBuildDirectDeps().getDepGroup(dirtyDirectDepIndex++);
  }

  /**
   * Returns the remaining direct deps that have not been checked. If {@code preservePosition} is
   * true, this method is non-mutating. If {@code preservePosition} is false, the caller must
   * process the returned set, and so subsequent calls to this method will return the empty set.
   */
  final ImmutableSet<SkyKey> getAllRemainingDirtyDirectDeps(boolean preservePosition)
      throws InterruptedException {
    if (getLastBuildDirectDeps() == null) {
      return ImmutableSet.of();
    }
    ImmutableSet.Builder<SkyKey> result =
        dirtyDirectDepIndex == 0
            ? ImmutableSet.builderWithExpectedSize(getLastBuildDirectDeps().numElements())
            : ImmutableSet.builder();
    for (int ind = dirtyDirectDepIndex; ind < getNumOfGroupsInLastBuildDirectDeps(); ind++) {
      result.addAll(getLastBuildDirectDeps().getDepGroup(ind));
    }
    if (!preservePosition) {
      dirtyDirectDepIndex = getNumOfGroupsInLastBuildDirectDeps();
    }
    return result.build();
  }

  ImmutableSet<SkyKey> getResetDirectDeps() {
    return ImmutableSet.of();
  }

  protected void markRebuilding() {
    checkState(state == LifecycleState.NEEDS_REBUILDING, this);
    state = LifecycleState.REBUILDING;
  }

  final void startEvaluating() {
    checkState(!isEvaluating(), this);
    signaledDeps = 0;
  }

  /** Returns whether all known children of this node have signaled that they are done. */
  final boolean isReady(int numDirectDeps) {
    // Avoids calling Preconditions.checkState because it showed up in garbage profiles due to
    // boxing of the int format args.
    if (signaledDeps > numDirectDeps + externalDeps) {
      throw new IllegalStateException(String.format("%s %s %s", numDirectDeps, externalDeps, this));
    }
    return signaledDeps == numDirectDeps + externalDeps;
  }

  protected MoreObjects.ToStringHelper getStringHelper() {
    return MoreObjects.toStringHelper(this)
        .add("state", state)
        .add("signaledDeps", signaledDeps)
        .add("externalDeps", externalDeps)
        .add("dirtyDirectDepIndex", dirtyDirectDepIndex);
  }

  @Override
  public final String toString() {
    return getStringHelper().toString();
  }
}