// Copyright 2014 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;

import com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Iterables;
import com.google.common.collect.Maps;
import com.google.devtools.build.lib.actions.Action;
import com.google.devtools.build.lib.actions.ActionCacheChecker.Token;
import com.google.devtools.build.lib.actions.ActionCompletionEvent;
import com.google.devtools.build.lib.actions.ActionExecutedEvent;
import com.google.devtools.build.lib.actions.ActionExecutedEvent.ErrorTiming;
import com.google.devtools.build.lib.actions.ActionExecutionContext;
import com.google.devtools.build.lib.actions.ActionExecutionException;
import com.google.devtools.build.lib.actions.ActionInput;
import com.google.devtools.build.lib.actions.ActionInputDepOwnerMap;
import com.google.devtools.build.lib.actions.ActionInputDepOwners;
import com.google.devtools.build.lib.actions.ActionInputMap;
import com.google.devtools.build.lib.actions.ActionInputMapSink;
import com.google.devtools.build.lib.actions.ActionLookupData;
import com.google.devtools.build.lib.actions.ActionLookupValue;
import com.google.devtools.build.lib.actions.ActionRewoundEvent;
import com.google.devtools.build.lib.actions.AlreadyReportedActionExecutionException;
import com.google.devtools.build.lib.actions.Artifact;
import com.google.devtools.build.lib.actions.ArtifactPathResolver;
import com.google.devtools.build.lib.actions.FileArtifactValue;
import com.google.devtools.build.lib.actions.FilesetOutputSymlink;
import com.google.devtools.build.lib.actions.LostInputsExecException.LostInputsActionExecutionException;
import com.google.devtools.build.lib.actions.MissingDepException;
import com.google.devtools.build.lib.actions.MissingInputFileException;
import com.google.devtools.build.lib.actions.NotifyOnActionCacheHit;
import com.google.devtools.build.lib.actions.PackageRootResolver;
import com.google.devtools.build.lib.actionsketch.ActionSketch;
import com.google.devtools.build.lib.analysis.BlazeDirectories;
import com.google.devtools.build.lib.causes.Cause;
import com.google.devtools.build.lib.causes.LabelCause;
import com.google.devtools.build.lib.clock.BlazeClock;
import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
import com.google.devtools.build.lib.cmdline.PackageIdentifier;
import com.google.devtools.build.lib.collect.nestedset.NestedSet;
import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
import com.google.devtools.build.lib.collect.nestedset.NestedSetView;
import com.google.devtools.build.lib.collect.nestedset.Order;
import com.google.devtools.build.lib.events.Event;
import com.google.devtools.build.lib.events.ExtendedEventHandler;
import com.google.devtools.build.lib.profiler.Profiler;
import com.google.devtools.build.lib.profiler.ProfilerTask;
import com.google.devtools.build.lib.profiler.SilentCloseable;
import com.google.devtools.build.lib.rules.cpp.IncludeScannable;
import com.google.devtools.build.lib.skyframe.ActionRewindStrategy.RewindPlan;
import com.google.devtools.build.lib.skyframe.ArtifactFunction.MissingFileArtifactValue;
import com.google.devtools.build.lib.skyframe.SkyframeActionExecutor.ActionPostprocessing;
import com.google.devtools.build.lib.util.io.FileOutErr;
import com.google.devtools.build.lib.util.io.TimestampGranularityMonitor;
import com.google.devtools.build.lib.vfs.FileSystem;
import com.google.devtools.build.lib.vfs.PathFragment;
import com.google.devtools.build.lib.vfs.Root;
import com.google.devtools.build.skyframe.SkyFunction;
import com.google.devtools.build.skyframe.SkyFunctionException;
import com.google.devtools.build.skyframe.SkyKey;
import com.google.devtools.build.skyframe.SkyValue;
import com.google.devtools.build.skyframe.ValueOrException;
import com.google.devtools.build.skyframe.ValueOrException2;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.IntFunction;
import javax.annotation.Nullable;

/**
 * A {@link SkyFunction} that creates {@link ActionExecutionValue}s. There are four points where
 * this function can abort due to missing values in the graph:
 *
 * <ol>
 *   <li>For actions that discover inputs, if missing metadata needed to resolve an artifact from a
 *       string input in the action cache.
 *   <li>If missing metadata for artifacts in inputs (including the artifacts above).
 *   <li>For actions that discover inputs, if missing metadata for inputs discovered prior to
 *       execution.
 *   <li>For actions that discover inputs, but do so during execution, if missing metadata for
 *       inputs discovered during execution.
 * </ol>
 */
public class ActionExecutionFunction implements SkyFunction {

  private final ActionRewindStrategy actionRewindStrategy = new ActionRewindStrategy();
  private final SkyframeActionExecutor skyframeActionExecutor;
  private final BlazeDirectories directories;
  private final AtomicReference<TimestampGranularityMonitor> tsgm;
  private ConcurrentMap<Action, ContinuationState> stateMap;

  public ActionExecutionFunction(
      SkyframeActionExecutor skyframeActionExecutor,
      BlazeDirectories directories,
      AtomicReference<TimestampGranularityMonitor> tsgm) {
    this.skyframeActionExecutor = skyframeActionExecutor;
    this.directories = directories;
    this.tsgm = tsgm;
    // TODO(b/136156191): This stays in RAM while the SkyFunction of the action is pending, which
    // can result in a lot of memory pressure if a lot of actions are pending.
    stateMap = Maps.newConcurrentMap();
  }

  @Override
  public SkyValue compute(SkyKey skyKey, Environment env)
      throws ActionExecutionFunctionException, InterruptedException {
    ActionLookupData actionLookupData = (ActionLookupData) skyKey.argument();
    Action action = getActionForLookupData(env, actionLookupData);
    if (action == null) {
      return null;
    }
    skyframeActionExecutor.noteActionEvaluationStarted(actionLookupData, action);
    if (actionDependsOnBuildId(action)) {
      PrecomputedValue.BUILD_ID.get(env);
    }

    if (skyframeActionExecutor.isBazelRemoteExecutionEnabled()) {
      // Declaring a dependency on the precomputed value so that all actions are invalidated if
      // the value of the flag changes. We are doing this conditionally only in Bazel if remote
      // execution is available in order to not introduce additional skyframe edges in Blaze.
      PrecomputedValue.REMOTE_OUTPUTS_MODE.get(env);
      PrecomputedValue.REMOTE_DEFAULT_PLATFORM_PROPERTIES.get(env);
    }

    // Look up the parts of the environment that influence the action.
    Map<SkyKey, SkyValue> clientEnvLookup =
        env.getValues(
            Iterables.transform(
                action.getClientEnvironmentVariables(), ClientEnvironmentFunction::key));
    if (env.valuesMissing()) {
      return null;
    }
    Map<String, String> clientEnv = new HashMap<>();
    for (Map.Entry<SkyKey, SkyValue> entry : clientEnvLookup.entrySet()) {
      ClientEnvironmentValue envValue = (ClientEnvironmentValue) entry.getValue();
      if (envValue.getValue() != null) {
        clientEnv.put((String) entry.getKey().argument(), envValue.getValue());
      }
    }

    ActionSketch sketch = null;
    TopDownActionCache topDownActionCache = skyframeActionExecutor.getTopDownActionCache();
    if (topDownActionCache != null) {
      sketch = (ActionSketch) env.getValue(ActionSketchFunction.key(actionLookupData));
      if (sketch == null) {
        return null;
      }
      ActionExecutionValue actionExecutionValue = topDownActionCache.get(sketch);
      if (actionExecutionValue != null) {
        return actionExecutionValue.transformForSharedAction(action.getOutputs());
      }
    }

    // For restarts of this ActionExecutionFunction we use a ContinuationState variable, below, to
    // avoid redoing work.
    //
    // However, if two actions are shared and the first one executes, when the
    // second one goes to execute, we should detect that and short-circuit, even without taking
    // ContinuationState into account.
    //
    // Additionally, if an action restarted (in the Skyframe sense) after it executed because it
    // discovered new inputs during execution, we should detect that and short-circuit.
    ActionExecutionState previousExecution = skyframeActionExecutor.probeActionExecution(action);

    // If this action was previously completed this build, then this evaluation must be happening
    // because of rewinding. Prevent any ProgressLike events from being published a second time for
    // this action; downstream consumers of action events reasonably don't expect them.
    env = getProgressEventSuppressingEnvironmentIfPreviouslyCompleted(action, env);

    if (action.discoversInputs()) {
      // If this action previously failed due to a lost input found during input discovery, ensure
      // that the input is regenerated before attempting discovery again.
      if (declareDepsOnLostDiscoveredInputsIfAny(env, action)) {
        return null;
      }
    }

    ContinuationState state;
    if (action.discoversInputs()) {
      state = getState(action);
    } else {
      // Because this is a new state, all conditionals below about whether state has already done
      // something will return false, and so we will execute all necessary steps.
      state = new ContinuationState();
    }
    if (!state.hasCollectedInputs()) {
      state.allInputs = collectInputs(action, env);
      if (state.allInputs == null) {
        // Missing deps.
        return null;
      }
    } else if (state.allInputs.keysRequested != null) {
      // Preserve the invariant that we ask for the same deps each build.
      env.getValues(state.allInputs.keysRequested);
      Preconditions.checkState(!env.valuesMissing(), "%s %s", action, state);
    }
    CheckInputResults checkedInputs = null;
    @Nullable
    ImmutableSet<Artifact> mandatoryInputs =
        action.discoversInputs() ? ImmutableSet.copyOf(action.getMandatoryInputs()) : null;

    int nestedSetSizeThreshold = skyframeActionExecutor.getNestedSetAsSkyKeyThreshold();
    Iterable<Artifact> allInputs =
        state.allInputs.getAllInputs(/* maybeAsNestedSet= */ nestedSetSizeThreshold > 0);

    Map<SkyKey, ValueOrException2<IOException, ActionExecutionException>> inputDeps =
        getInputDeps(env, nestedSetSizeThreshold, allInputs);
    // If there's a missing value.
    if (inputDeps == null) {
      return null;
    }

    try {
      if (previousExecution == null && !state.hasArtifactData()) {
        // Do we actually need to find our metadata?
        checkedInputs = checkInputs(env, action, inputDeps, allInputs, mandatoryInputs);
      }
    } catch (ActionExecutionException e) {
      // Remove action from state map in case it's there (won't be unless it discovers inputs).
      stateMap.remove(action);
      throw new ActionExecutionFunctionException(e);
    }

    if (env.valuesMissing()) {
      // There was missing artifact metadata in the graph. Wait for it to be present.
      // We must check this and return here before attempting to establish any Skyframe dependencies
      // of the action; see establishSkyframeDependencies why.
      return null;
    }

    Object skyframeDepsResult;
    try {
      skyframeDepsResult = establishSkyframeDependencies(env, action);
    } catch (ActionExecutionException e) {
      // Remove action from state map in case it's there (won't be unless it discovers inputs).
      stateMap.remove(action);
      throw new ActionExecutionFunctionException(
          skyframeActionExecutor.processAndGetExceptionToThrow(
              env.getListener(), null, action, e, new FileOutErr(), ErrorTiming.BEFORE_EXECUTION));
    }
    if (env.valuesMissing()) {
      return null;
    }

    if (checkedInputs != null) {
      Preconditions.checkState(!state.hasArtifactData(), "%s %s", state, action);
      state.inputArtifactData = checkedInputs.actionInputMap;
      state.expandedArtifacts = checkedInputs.expandedArtifacts;
      state.expandedFilesets = checkedInputs.expandedFilesets;
      if (skyframeActionExecutor.actionFileSystemType().isEnabled()) {
        state.actionFileSystem =
            skyframeActionExecutor.createActionFileSystem(
                directories.getRelativeOutputPath(),
                checkedInputs.actionInputMap,
                action.getOutputs());
      }
    }

    long actionStartTime = BlazeClock.nanoTime();
    ActionExecutionValue result;
    try {
      result =
          checkCacheAndExecuteIfNeeded(
              action,
              state,
              env,
              clientEnv,
              actionLookupData,
              previousExecution,
              skyframeDepsResult,
              actionStartTime);
    } catch (LostInputsActionExecutionException e) {
      return handleLostInputs(
          e, actionLookupData, action, actionStartTime, env, inputDeps, allInputs, state);
    } catch (ActionExecutionException e) {
      // Remove action from state map in case it's there (won't be unless it discovers inputs).
      stateMap.remove(action);
      // In this case we do not report the error to the action reporter because we have already
      // done it in SkyframeActionExecutor.reportErrorIfNotAbortingMode() method. That method
      // prints the error in the top-level reporter and also dumps the recorded StdErr for the
      // action. Label can be null in the case of, e.g., the SystemActionOwner (for build-info.txt).
      throw new ActionExecutionFunctionException(new AlreadyReportedActionExecutionException(e));
    }

    if (env.valuesMissing()) {
      // Only input-discovering actions are present in the stateMap. Other actions may have
      // valuesMissing() here in rare circumstances related to Fileset inputs being unavailable.
      // See comments in ActionInputMapHelper#getFilesets().
      Preconditions.checkState(!action.discoversInputs() || stateMap.containsKey(action), action);
      return null;
    }

    // Remove action from state map in case it's there (won't be unless it discovers inputs).
    stateMap.remove(action);
    if (sketch != null && result.dataIsShareable()) {
      topDownActionCache.put(sketch, result);
    }
    return result;
  }

  /**
   * Evaluate the supplied input deps. Declare deps on known inputs to action. We do this
   * unconditionally to maintain our invariant of asking for the same deps each build.
   *
   * <p>TODO(b/142300168): Address potential dependency inconsistency if the threshold is changed
   * between runs.
   */
  private static Map<SkyKey, ValueOrException2<IOException, ActionExecutionException>> getInputDeps(
      Environment env, int nestedSetSizeThreshold, Iterable<Artifact> allInputs)
      throws InterruptedException {
    if (evalInputsAsNestedSet(nestedSetSizeThreshold, allInputs)) {
      SkyValue artifactNestedSetValue =
          env.getValue(
              new ArtifactNestedSetKey(
                  NestedSetView.getRawChildren((NestedSet<Artifact>) allInputs)));
      if (env.valuesMissing()) {
        return null;
      }
      return ((ArtifactNestedSetValue) artifactNestedSetValue).getArtifactToSkyValueMap();
    }

    return env.getValuesOrThrow(
        Artifact.keys(allInputs), IOException.class, ActionExecutionException.class);
  }

  /**
   * Do one traversal of the set to get the size. The traversal costs CPU time so only do it when
   * necessary. The default case (without --experimental_nestedset_as_skykey_threshold) will ignore
   * this path.
   */
  private static boolean evalInputsAsNestedSet(
      int nestedSetSizeThreshold, Iterable<Artifact> inputs) {
    return inputs instanceof NestedSet
        && nestedSetSizeThreshold > 0
        && (((NestedSet<Artifact>) inputs).memoizedFlattenAndGetSize() >= nestedSetSizeThreshold);
  }

  private Environment getProgressEventSuppressingEnvironmentIfPreviouslyCompleted(
      Action action, Environment env) {
    if (skyframeActionExecutor.probeCompletedAndReset(action)) {
      return new ProgressEventSuppressingEnvironment(env);
    }
    return env;
  }

  private boolean declareDepsOnLostDiscoveredInputsIfAny(Environment env, Action action)
      throws InterruptedException, ActionExecutionFunctionException {
    ImmutableList<SkyKey> previouslyLostDiscoveredInputs =
        skyframeActionExecutor.getLostDiscoveredInputs(action);
    if (previouslyLostDiscoveredInputs != null) {
      Map<SkyKey, ValueOrException2<MissingInputFileException, ActionExecutionException>>
          lostInputValues =
              env.getValuesOrThrow(
                  previouslyLostDiscoveredInputs,
                  MissingInputFileException.class,
                  ActionExecutionException.class);
      if (env.valuesMissing()) {
        return true;
      }
      for (Map.Entry<SkyKey, ValueOrException2<MissingInputFileException, ActionExecutionException>>
          lostInput : lostInputValues.entrySet()) {
        try {
          lostInput.getValue().get();
        } catch (MissingInputFileException e) {
          // MissingInputFileException comes from problems with source artifact construction.
          // Rewinding never invalidates source artifacts.
          throw new IllegalStateException(
              "MissingInputFileException unexpected from rewound generated discovered input. key="
                  + lostInput.getKey(),
              e);
        } catch (ActionExecutionException e) {
          throw new ActionExecutionFunctionException(e);
        }
      }
    }
    return false;
  }

  /**
   * Clean up state associated with the current action execution attempt and return a {@link
   * Restart} value which rewinds the actions that generate the lost inputs.
   */
  private SkyFunction.Restart handleLostInputs(
      LostInputsActionExecutionException e,
      ActionLookupData actionLookupData,
      Action action,
      long actionStartTime,
      Environment env,
      Map<SkyKey, ValueOrException2<IOException, ActionExecutionException>> inputDeps,
      Iterable<Artifact> allInputs,
      ContinuationState state)
      throws InterruptedException, ActionExecutionFunctionException {
    // Remove action from state map in case it's there (won't be unless it discovers inputs).
    stateMap.remove(action);

    RewindPlan rewindPlan = null;
    try {
      // Reconstruct the relationship between lost inputs and this action's direct deps if any of
      // the lost inputs came from runfiles:
      ActionInputDepOwners runfilesDepOwners;
      Set<ActionInput> lostRunfiles = e.getInputOwners().getRunfilesInputsAndOwners();
      if (!lostRunfiles.isEmpty()) {
        try {
          runfilesDepOwners =
              getInputDepOwners(
                  env,
                  action,
                  inputDeps,
                  allInputs,
                  action.discoversInputs()
                      ? ImmutableSet.copyOf(action.getMandatoryInputs())
                      : null,
                  lostRunfiles);
        } catch (ActionExecutionException unexpected) {
          // getInputDepOwners should not be able to throw, because it does the same work as
          // checkInputs, so if getInputDepOwners throws then checkInputs should have thrown, and if
          // checkInputs threw then we shouldn't have reached this point in action execution.
          throw new IllegalStateException(unexpected);
        }
      } else {
        runfilesDepOwners = ActionInputDepOwners.EMPTY_INSTANCE;
      }

      // Collect the set of direct deps of this action which may be responsible for the lost inputs,
      // some of which may be discovered.
      ImmutableList<SkyKey> lostDiscoveredInputs = ImmutableList.of();
      Iterable<? extends SkyKey> failedActionDeps;
      if (e.isFromInputDiscovery()) {
        // Lost inputs found during input discovery are necessarily ordinary derived artifacts.
        // Their keys may not be direct deps yet, but the next time this Skyframe node is evaluated
        // they will be. See SkyframeActionExecutor's lostDiscoveredInputsMap.
        lostDiscoveredInputs =
            e.getLostInputs().values().stream()
                .map(i -> (Artifact) i)
                .map(Artifact::key)
                .collect(ImmutableList.toImmutableList());
        failedActionDeps = lostDiscoveredInputs;
      } else if (state.discoveredInputs != null) {
        failedActionDeps =
            Iterables.concat(
                inputDeps.keySet(), Iterables.transform(state.discoveredInputs, Artifact::key));
      } else {
        failedActionDeps = inputDeps.keySet();
      }

      try {
        rewindPlan =
            actionRewindStrategy.getRewindPlan(
                action, actionLookupData, failedActionDeps, e, runfilesDepOwners, env);
      } catch (ActionExecutionException rewindingFailedException) {
        // This call to processAndGetExceptionToThrow will emit an ActionExecutedEvent and report
        // the error. The previous call to processAndGetExceptionToThrow didn't.
        throw new ActionExecutionFunctionException(
            new AlreadyReportedActionExecutionException(
                skyframeActionExecutor.processAndGetExceptionToThrow(
                    env.getListener(),
                    e.getPrimaryOutputPath(),
                    action,
                    rewindingFailedException,
                    e.getFileOutErr(),
                    ActionExecutedEvent.ErrorTiming.AFTER_EXECUTION)));
      }

      if (e.isActionStartedEventAlreadyEmitted()) {
        env.getListener().post(new ActionRewoundEvent(actionStartTime, action));
      }
      skyframeActionExecutor.resetFailedActionExecution(action, lostDiscoveredInputs);
      for (Action actionToRestart : rewindPlan.getAdditionalActionsToRestart()) {
        skyframeActionExecutor.resetPreviouslyCompletedActionExecution(actionToRestart);
      }
      return rewindPlan.getNodesToRestart();
    } finally {
      if (rewindPlan == null && e.isActionStartedEventAlreadyEmitted()) {
        // Rewinding was unsuccessful. SkyframeActionExecutor's ActionRunner didn't emit an
        // ActionCompletionEvent because it hoped rewinding would fix things. Because it won't, this
        // must emit one to compensate.
        env.getListener()
            .post(new ActionCompletionEvent(actionStartTime, action, actionLookupData));
      }
    }
  }

  @Nullable
  static Action getActionForLookupData(Environment env, ActionLookupData actionLookupData)
      throws InterruptedException {
    ActionLookupValue actionLookupValue =
        ArtifactFunction.getActionLookupValue(actionLookupData.getActionLookupKey(), env);
    return actionLookupValue != null
        ? actionLookupValue.getAction(actionLookupData.getActionIndex())
        : null;
  }

  /**
   * An action's inputs needed for execution. May not just be the result of Action#getInputs(). If
   * the action cache's view of this action contains additional inputs, it will request metadata for
   * them, so we consider those inputs as dependencies of this action as well. Returns null if some
   * dependencies were missing and this ActionExecutionFunction needs to restart.
   */
  @Nullable
  private AllInputs collectInputs(Action action, Environment env) throws InterruptedException {
    Iterable<Artifact> allKnownInputs = action.getInputs();
    if (action.inputsDiscovered()) {
      return new AllInputs(allKnownInputs);
    }

    Preconditions.checkState(action.discoversInputs(), action);
    PackageRootResolverWithEnvironment resolver = new PackageRootResolverWithEnvironment(env);
    Iterable<Artifact> actionCacheInputs =
        skyframeActionExecutor.getActionCachedInputs(action, resolver);
    if (actionCacheInputs == null) {
      Preconditions.checkState(env.valuesMissing(), action);
      return null;
    }
    return new AllInputs(allKnownInputs, actionCacheInputs, resolver.keysRequested);
  }

  private static class AllInputs {
    final Iterable<Artifact> defaultInputs;
    @Nullable final Iterable<Artifact> actionCacheInputs;
    @Nullable final List<SkyKey> keysRequested;

    AllInputs(Iterable<Artifact> defaultInputs) {
      this.defaultInputs = Preconditions.checkNotNull(defaultInputs);
      this.actionCacheInputs = null;
      this.keysRequested = null;
    }

    AllInputs(
        Iterable<Artifact> defaultInputs,
        Iterable<Artifact> actionCacheInputs,
        List<SkyKey> keysRequested) {
      this.defaultInputs = Preconditions.checkNotNull(defaultInputs);
      this.actionCacheInputs = Preconditions.checkNotNull(actionCacheInputs);
      this.keysRequested = keysRequested;
    }

    Iterable<Artifact> getAllInputs(boolean maybeAsNestedSet) {
      if (maybeAsNestedSet && defaultInputs instanceof NestedSet) {
        return getAllInputsAsNestedSet();
      }
      return actionCacheInputs == null
          ? defaultInputs
          : Iterables.concat(defaultInputs, actionCacheInputs);
    }

    private NestedSet<Artifact> getAllInputsAsNestedSet() {
      Preconditions.checkState(defaultInputs instanceof NestedSet);
      if (actionCacheInputs == null) {
        return (NestedSet<Artifact>) defaultInputs;
      }

      NestedSetBuilder<Artifact> builder = new NestedSetBuilder<>(Order.STABLE_ORDER);
      // actionCacheInputs is never a NestedSet.
      builder.addAll(actionCacheInputs);
      builder.addTransitive((NestedSet<Artifact>) defaultInputs);

      return builder.build();
    }
  }

  /**
   * Skyframe implementation of {@link PackageRootResolver}. Should be used only from SkyFunctions,
   * because it uses SkyFunction.Environment for evaluation of ContainingPackageLookupValue.
   */
  private static class PackageRootResolverWithEnvironment implements PackageRootResolver {
    final List<SkyKey> keysRequested = new ArrayList<>();
    private final Environment env;

    private PackageRootResolverWithEnvironment(Environment env) {
      this.env = env;
    }

    @Override
    public Map<PathFragment, Root> findPackageRootsForFiles(Iterable<PathFragment> execPaths)
        throws InterruptedException {
      Preconditions.checkState(
          keysRequested.isEmpty(),
          "resolver should only be called once: %s %s",
          keysRequested,
          execPaths);
      // Create SkyKeys list based on execPaths.
      Map<PathFragment, SkyKey> depKeys = new HashMap<>();
      for (PathFragment path : execPaths) {
        PathFragment parent =
            Preconditions.checkNotNull(
                path.getParentDirectory(), "Must pass in files, not root directory");
        Preconditions.checkArgument(!parent.isAbsolute(), path);
        try {
          SkyKey depKey =
              ContainingPackageLookupValue.key(PackageIdentifier.discoverFromExecPath(path, true));
          depKeys.put(path, depKey);
          keysRequested.add(depKey);
        } catch (LabelSyntaxException e) {
          // This code is only used to do action cache checks. If one of the file names we got from
          // the action cache is corrupted, or if the action cache is from a different Bazel
          // binary, then the path may not be valid for this Bazel binary, and trigger this
          // exception. In that case, it's acceptable for us to ignore the exception - we'll get an
          // action cache miss and re-execute the action, which is what we should do.
          continue;
        }
      }

      Map<SkyKey, SkyValue> values = env.getValues(depKeys.values());
      if (env.valuesMissing()) {
        return null;
      }

      Map<PathFragment, Root> result = new HashMap<>();
      for (PathFragment path : execPaths) {
        if (!depKeys.containsKey(path)) {
          continue;
        }
        ContainingPackageLookupValue value =
            (ContainingPackageLookupValue) values.get(depKeys.get(path));
        if (value.hasContainingPackage()) {
          // We have found corresponding root for current execPath.
          result.put(
              path,
              SkyframeExecutor.maybeTransformRootForRepository(
                  value.getContainingPackageRoot(),
                  value.getContainingPackageName().getRepository()));
        } else {
          // We haven't found corresponding root for current execPath.
          result.put(path, null);
        }
      }
      return result;
    }
  }

  private ActionExecutionValue checkCacheAndExecuteIfNeeded(
      Action action,
      ContinuationState state,
      Environment env,
      Map<String, String> clientEnv,
      ActionLookupData actionLookupData,
      @Nullable ActionExecutionState previousAction,
      Object skyframeDepsResult,
      long actionStartTime)
      throws ActionExecutionException, InterruptedException {
    if (previousAction != null) {
      // There are two cases where we can already have an executing action for a specific output:
      // 1. Another instance of a shared action won the race and got executed first.
      // 2. The action was already started earlier, and this SkyFunction got restarted since
      //    there's progress to be made.
      // In either case, we must use this continuation to continue. Note that in the first case,
      // we don't have any input metadata available, so we couldn't re-execute the action even if we
      // wanted to.
      return previousAction.getResultOrDependOnFuture(
          env,
          actionLookupData,
          action,
          skyframeActionExecutor.getSharedActionCallback(
              env.getListener(), state.discoveredInputs != null, action, actionLookupData));
    }
    // The metadataHandler may be recreated if we discover inputs.
    ArtifactPathResolver pathResolver =
        ArtifactPathResolver.createPathResolver(
            state.actionFileSystem, skyframeActionExecutor.getExecRoot());
    ActionMetadataHandler metadataHandler =
        new ActionMetadataHandler(
            state.inputArtifactData,
            /* missingArtifactsAllowed= */ action.discoversInputs(),
            action.getOutputs(),
            tsgm.get(),
            pathResolver,
            newOutputStore(state));
    // We only need to check the action cache if we haven't done it on a previous run.
    if (!state.hasCheckedActionCache()) {
      state.token =
          skyframeActionExecutor.checkActionCache(
              env.getListener(),
              action,
              metadataHandler,
              actionStartTime,
              state.allInputs.actionCacheInputs,
              clientEnv);
    }

    if (state.token == null) {
      // We got a hit from the action cache -- no need to execute.
      Preconditions.checkState(
          !(action instanceof SkyframeAwareAction),
          "Error, we're not re-executing a "
              + "SkyframeAwareAction which should be re-executed unconditionally. Action: %s",
          action);
      return ActionExecutionValue.createFromOutputStore(
          metadataHandler.getOutputStore(),
          /*outputSymlinks=*/ null,
          (action instanceof IncludeScannable)
              ? ((IncludeScannable) action).getDiscoveredModules()
              : null,
          actionDependsOnBuildId(action));
    }

    // Delete the metadataHandler's cache of the action's outputs, since they are being deleted.
    metadataHandler.discardOutputMetadata();

    if (action.discoversInputs()) {
      if (state.discoveredInputs == null) {
        try (SilentCloseable c = Profiler.instance().profile(ProfilerTask.INFO, "discoverInputs")) {
          try {
            state.updateFileSystemContext(
                skyframeActionExecutor, env, metadataHandler, ImmutableMap.of());
          } catch (IOException e) {
            throw new ActionExecutionException(
                "Failed to update filesystem context: " + e.getMessage(),
                e,
                action,
                /*catastrophe=*/ false);
          }
          try {
            state.discoveredInputs =
                skyframeActionExecutor.discoverInputs(
                    action,
                    metadataHandler,
                    metadataHandler,
                    skyframeActionExecutor.probeCompletedAndReset(action)
                        ? SkyframeActionExecutor.ProgressEventBehavior.SUPPRESS
                        : SkyframeActionExecutor.ProgressEventBehavior.EMIT,
                    env,
                    state.actionFileSystem);
          } catch (IOException e) {
            throw new ActionExecutionException(
                "Failed during input discovery: " + e.getMessage(),
                e,
                action,
                /*catastrophe=*/ false);
          }
          Preconditions.checkState(
              env.valuesMissing() == (state.discoveredInputs == null),
              "discoverInputs() must return null iff requesting more dependencies.");
          if (state.discoveredInputs == null) {
            return null;
          }
        } catch (MissingDepException e) {
          Preconditions.checkState(env.valuesMissing(), action);
          return null;
        }
      }
      switch (addDiscoveredInputs(
          state.inputArtifactData,
          state.expandedArtifacts,
          filterKnownInputs(state.discoveredInputs, state.inputArtifactData),
          env)) {
        case VALUES_MISSING:
          return null;
        case NO_DISCOVERED_DATA:
          break;
        case DISCOVERED_DATA:
          metadataHandler =
              new ActionMetadataHandler(
                  state.inputArtifactData,
                  /*missingArtifactsAllowed=*/ false,
                  action.getOutputs(),
                  tsgm.get(),
                  pathResolver,
                  newOutputStore(state));
          // Set the MetadataHandler to accept output information.
          metadataHandler.discardOutputMetadata();
      }
    }

    // Make sure this is a regular HashMap rather than ImmutableMapBuilder so that we are safe
    // in case of collisions.
    Map<Artifact, ImmutableList<FilesetOutputSymlink>> filesetMappings = new HashMap<>();
    for (Artifact actionInput : action.getInputs()) {
      if (!actionInput.isFileset()) {
        continue;
      }

      ImmutableList<FilesetOutputSymlink> mapping =
          ActionInputMapHelper.getFilesets(env, (Artifact.SpecialArtifact) actionInput);
      if (mapping == null) {
        return null;
      }
      filesetMappings.put(actionInput, mapping);
    }

    ImmutableMap<Artifact, ImmutableList<FilesetOutputSymlink>> topLevelFilesets =
        ImmutableMap.copyOf(filesetMappings);

    // Aggregate top-level Filesets with Filesets nested in Runfiles. Both should be used to update
    // the FileSystem context.
    state.expandedFilesets.forEach(filesetMappings::put);
    ImmutableMap<Artifact, ImmutableList<FilesetOutputSymlink>> expandedFilesets =
        ImmutableMap.copyOf(filesetMappings);
    try {
      state.updateFileSystemContext(skyframeActionExecutor, env, metadataHandler, expandedFilesets);
    } catch (IOException e) {
      throw new ActionExecutionException(
          "Failed to update filesystem context: " + e.getMessage(),
          e,
          action,
          /*catastrophe=*/ false);
    }

    try (ActionExecutionContext actionExecutionContext =
        skyframeActionExecutor.getContext(
            metadataHandler,
            metadataHandler,
            skyframeActionExecutor.probeCompletedAndReset(action)
                ? SkyframeActionExecutor.ProgressEventBehavior.SUPPRESS
                : SkyframeActionExecutor.ProgressEventBehavior.EMIT,
            Collections.unmodifiableMap(state.expandedArtifacts),
            expandedFilesets,
            topLevelFilesets,
            state.actionFileSystem,
            skyframeDepsResult)) {
      return skyframeActionExecutor.executeAction(
          env,
          action,
          metadataHandler,
          actionStartTime,
          actionExecutionContext,
          actionLookupData,
          new ActionPostprocessingImpl(state),
          state.discoveredInputs != null);
    } catch (IOException e) {
      throw new ActionExecutionException(
          "Failed to close action output: " + e.getMessage(), e, action, /*catastrophe=*/ false);
    }
  }

  private OutputStore newOutputStore(ContinuationState state) {
    Preconditions.checkState(
        !skyframeActionExecutor.actionFileSystemType().isEnabled()
            || state.actionFileSystem != null,
        "actionFileSystem must not be null");

    if (skyframeActionExecutor.actionFileSystemType().inMemoryFileSystem()) {
      return new MinimalOutputStore();
    }
    return new OutputStore();
  }

  /** Implementation of {@link ActionPostprocessing}. */
  private final class ActionPostprocessingImpl implements ActionPostprocessing {
    private final ContinuationState state;

    ActionPostprocessingImpl(ContinuationState state) {
      this.state = state;
    }

    public void run(
        Environment env,
        Action action,
        ActionMetadataHandler metadataHandler,
        Map<String, String> clientEnv)
        throws InterruptedException, ActionExecutionException {
      if (action.discoversInputs()) {
        Iterable<Artifact> newInputs =
            filterKnownInputs(action.getInputs(), state.inputArtifactData);
        state.discoveredInputs = newInputs;
        switch (addDiscoveredInputs(
            state.inputArtifactData, state.expandedArtifacts, newInputs, env)) {
          case VALUES_MISSING:
            return;
          case NO_DISCOVERED_DATA:
            break;
          case DISCOVERED_DATA:
            // We are in the interesting case of an action that discovered its inputs during
            // execution, and found some new ones, but the new ones were already present in the
            // graph. We must therefore cache the metadata for those new ones.
            metadataHandler =
                new ActionMetadataHandler(
                    state.inputArtifactData,
                    /*missingArtifactsAllowed=*/ false,
                    action.getOutputs(),
                    tsgm.get(),
                    metadataHandler.getArtifactPathResolver(),
                    metadataHandler.getOutputStore());
        }
      }
      Preconditions.checkState(!env.valuesMissing(), action);
      skyframeActionExecutor.updateActionCache(action, metadataHandler, state.token, clientEnv);
    }
  }

  private enum DiscoveredState {
    VALUES_MISSING,
    NO_DISCOVERED_DATA,
    DISCOVERED_DATA
  }

  private static DiscoveredState addDiscoveredInputs(
      ActionInputMap inputData,
      Map<Artifact, Collection<Artifact>> expandedArtifacts,
      Iterable<Artifact> discoveredInputs,
      Environment env)
      throws InterruptedException {
    // We do not do a getValuesOrThrow() call for the following reasons:
    // 1. No exceptions can be thrown for non-mandatory inputs;
    // 2. Any derived inputs must be in the transitive closure of this action's inputs. Therefore,
    // if there was an error building one of them, then that exception would have percolated up to
    // this action already, through one of its declared inputs, and we would not have reached input
    // discovery.
    // Therefore there is no need to catch and rethrow exceptions as there is with #checkInputs.
    Map<SkyKey, SkyValue> nonMandatoryDiscovered =
        env.getValues(Iterables.transform(discoveredInputs, Artifact::key));
    if (env.valuesMissing()) {
      return DiscoveredState.VALUES_MISSING;
    }
    if (nonMandatoryDiscovered.isEmpty()) {
      return DiscoveredState.NO_DISCOVERED_DATA;
    }
    for (Artifact input : discoveredInputs) {
      SkyValue retrievedMetadata = nonMandatoryDiscovered.get(Artifact.key(input));
      if (retrievedMetadata instanceof TreeArtifactValue) {
        TreeArtifactValue treeValue = (TreeArtifactValue) retrievedMetadata;
        expandedArtifacts.put(input, ImmutableSet.copyOf(treeValue.getChildren()));
        for (Map.Entry<Artifact.TreeFileArtifact, FileArtifactValue> child :
            treeValue.getChildValues().entrySet()) {
          inputData.putWithNoDepOwner(child.getKey(), child.getValue());
        }
        inputData.putWithNoDepOwner(input, treeValue.getSelfData());
      } else if (retrievedMetadata instanceof ActionExecutionValue) {
        inputData.putWithNoDepOwner(
            input,
            ArtifactFunction.createSimpleFileArtifactValue(
                (Artifact.DerivedArtifact) input, (ActionExecutionValue) retrievedMetadata));
      } else if (retrievedMetadata instanceof MissingFileArtifactValue) {
        inputData.putWithNoDepOwner(input, FileArtifactValue.MISSING_FILE_MARKER);
      } else if (retrievedMetadata instanceof FileArtifactValue) {
        inputData.putWithNoDepOwner(input, (FileArtifactValue) retrievedMetadata);
      } else {
        throw new IllegalStateException(
            "unknown metadata for " + input.getExecPathString() + ": " + retrievedMetadata);
      }
    }
    return DiscoveredState.DISCOVERED_DATA;
  }

  private static <E extends Exception> Object establishSkyframeDependencies(
      Environment env, Action action) throws ActionExecutionException, InterruptedException {
    // Before we may safely establish Skyframe dependencies, we must build all action inputs by
    // requesting their ArtifactValues.
    // This is very important to do, because the establishSkyframeDependencies method may request
    // FileValues for input files of this action (directly requesting them, or requesting some other
    // SkyValue whose builder requests FileValues), which may not yet exist if their generating
    // actions have not yet run.
    // See SkyframeAwareActionTest.testRaceConditionBetweenInputAcquisitionAndSkyframeDeps
    Preconditions.checkState(!env.valuesMissing(), action);

    if (action instanceof SkyframeAwareAction) {
      // Skyframe-aware actions should be executed unconditionally, i.e. bypass action cache
      // checking. See documentation of SkyframeAwareAction.
      Preconditions.checkState(action.executeUnconditionally(), action);

      @SuppressWarnings("unchecked")
      SkyframeAwareAction<E> skyframeAwareAction = (SkyframeAwareAction<E>) action;
      ImmutableList<? extends SkyKey> keys = skyframeAwareAction.getDirectSkyframeDependencies();
      Map<SkyKey, ValueOrException<E>> values =
          env.getValuesOrThrow(keys, skyframeAwareAction.getExceptionType());

      try {
        return skyframeAwareAction.processSkyframeValues(keys, values, env.valuesMissing());
      } catch (SkyframeAwareAction.ExceptionBase e) {
        throw new ActionExecutionException(e, action, false);
      }
    }
    return null;
  }

  private static class CheckInputResults {
    /** Metadata about Artifacts consumed by this Action. */
    private final ActionInputMap actionInputMap;
    /** Artifact expansion mapping for Runfiles tree and tree artifacts. */
    private final Map<Artifact, Collection<Artifact>> expandedArtifacts;
    /** Artifact expansion mapping for Filesets embedded in Runfiles. */
    private final Map<Artifact, ImmutableList<FilesetOutputSymlink>> expandedFilesets;

    public CheckInputResults(
        ActionInputMap actionInputMap,
        Map<Artifact, Collection<Artifact>> expandedArtifacts,
        Map<Artifact, ImmutableList<FilesetOutputSymlink>> expandedFilesets) {
      this.actionInputMap = actionInputMap;
      this.expandedArtifacts = expandedArtifacts;
      this.expandedFilesets = expandedFilesets;
    }
  }

  private interface AccumulateInputResultsFactory<S extends ActionInputMapSink, R> {
    R create(
        S actionInputMapSink,
        Map<Artifact, Collection<Artifact>> expandedArtifacts,
        Map<Artifact, ImmutableList<FilesetOutputSymlink>> expandedFilesets);
  }

  /**
   * Declare dependency on all known inputs of action. Throws exception if any are known to be
   * missing. Some inputs may not yet be in the graph, in which case the builder should abort.
   */
  private static CheckInputResults checkInputs(
      Environment env,
      Action action,
      Map<SkyKey, ValueOrException2<IOException, ActionExecutionException>> inputDeps,
      Iterable<Artifact> allInputs,
      ImmutableSet<Artifact> mandatoryInputs)
      throws ActionExecutionException, InterruptedException {
    return accumulateInputs(
        env,
        action,
        inputDeps,
        allInputs,
        mandatoryInputs,
        ActionInputMap::new,
        CheckInputResults::new);
  }

  /**
   * Reconstructs the relationships between lost inputs and the direct deps responsible for them.
   */
  private static ActionInputDepOwners getInputDepOwners(
      Environment env,
      Action action,
      Map<SkyKey, ValueOrException2<IOException, ActionExecutionException>> inputDeps,
      Iterable<Artifact> allInputs,
      ImmutableSet<Artifact> mandatoryInputs,
      Collection<ActionInput> lostInputs)
      throws ActionExecutionException, InterruptedException {
    return accumulateInputs(
        env,
        action,
        inputDeps,
        allInputs,
        mandatoryInputs,
        ignoredInputDepsSize -> new ActionInputDepOwnerMap(lostInputs),
        (actionInputMapSink, expandedArtifacts, expandedFilesets) -> actionInputMapSink);
  }

  private static <S extends ActionInputMapSink, R> R accumulateInputs(
      Environment env,
      Action action,
      Map<SkyKey, ValueOrException2<IOException, ActionExecutionException>> inputDeps,
      Iterable<Artifact> allInputs,
      ImmutableSet<Artifact> mandatoryInputs,
      IntFunction<S> actionInputMapSinkFactory,
      AccumulateInputResultsFactory<S, R> accumulateInputResultsFactory)
      throws ActionExecutionException, InterruptedException {
    int missingCount = 0;
    int actionFailures = 0;
    // Only populate input data if we have the input values, otherwise they'll just go unused.
    // We still want to loop through the inputs to collect missing deps errors. During the
    // evaluator "error bubbling", we may get one last chance at reporting errors even though
    // some deps are still missing.
    boolean populateInputData = !env.valuesMissing();
    NestedSetBuilder<Cause> rootCauses = NestedSetBuilder.stableOrder();
    S inputArtifactData = actionInputMapSinkFactory.apply(populateInputData ? inputDeps.size() : 0);
    Map<Artifact, Collection<Artifact>> expandedArtifacts =
        new HashMap<>(populateInputData ? 128 : 0);
    Map<Artifact, ImmutableList<FilesetOutputSymlink>> expandedFilesets = new HashMap<>();

    ActionExecutionException firstActionExecutionException = null;
    for (Artifact input : allInputs) {
      ValueOrException2<IOException, ActionExecutionException> valueOrException =
          inputDeps.get(Artifact.key(input));
      if (valueOrException == null) {
        continue;
      }

      // Some inputs do not need to exist: we depend on the inputs of the action as registered in
      // the action cache so that we can verify the validity of the cache entry, but if the
      // reference to the file went away together with the file itself (e.g. when deleting a file
      // and removing the #include statement referencing it), we re-execute the action anyway so it
      // does not matter if the file is missing.
      //
      // This mechanism fails, though, if we remove a #include statement referencing a header and
      // then introduce a symlink cycle in its place: then there will be an IOException which will
      // be propagated even though we shouldn't have read the file in the first place. This is not
      // really avoidable (at least not without redesigning the action cache), because once the
      // ArtifactFunction throws an exception, Skyframe evaluation must stop, so all we can do is
      // signal the error in a more meaningful way.
      //
      // In particular, making it possible to check only the up-to-dateness of mandatory inputs in
      // the action cache is not enough: it can be that the reference to the symlink cycle arose
      // from a discovered input, so even though no mandatory inputs change, it can still be that
      // the need to read the newly introduced symlink cycle went away.
      boolean mandatory =
          !input.isSourceArtifact() || mandatoryInputs == null || mandatoryInputs.contains(input);
      SkyValue value = FileArtifactValue.MISSING_FILE_MARKER;
      try {
        value = valueOrException.get();
      } catch (IOException e) {
        if (mandatory) {
          missingCount++;
          if (input.getOwner() != null) {
            rootCauses.add(new LabelCause(input.getOwner(), e.getMessage()));
          }
          continue;
        }
      } catch (ActionExecutionException e) {
        if (mandatory) {
          actionFailures++;
          // Prefer a catastrophic exception as the one we propagate.
          if (firstActionExecutionException == null
              || (!firstActionExecutionException.isCatastrophe() && e.isCatastrophe())) {
            firstActionExecutionException = e;
          }
          rootCauses.addTransitive(e.getRootCauses());
          continue;
        }
      }

      if (value instanceof MissingFileArtifactValue) {
        if (mandatory) {
          MissingInputFileException e = ((MissingFileArtifactValue) value).getException();
          env.getListener().handle(Event.error(e.getLocation(), e.getMessage()));
          missingCount++;
          if (input.getOwner() != null) {
            rootCauses.add(new LabelCause(input.getOwner(), e.getMessage()));
          }
          continue;
        } else {
          value = FileArtifactValue.MISSING_FILE_MARKER;
        }
      }

      if (populateInputData) {
        ActionInputMapHelper.addToMap(
            inputArtifactData, expandedArtifacts, expandedFilesets, input, value, env);
      }
    }

    // We need to rethrow first exception because it can contain useful error message
    if (firstActionExecutionException != null) {
      if (missingCount == 0 && actionFailures == 1) {
        // In the case a single action failed, just propagate the exception upward. This avoids
        // having to copy the root causes to the upwards transitive closure.
        throw firstActionExecutionException;
      }
      throw new ActionExecutionException(
          firstActionExecutionException.getMessage(),
          firstActionExecutionException.getCause(),
          action,
          rootCauses.build(),
          firstActionExecutionException.isCatastrophe(),
          firstActionExecutionException.getExitCode());
    }

    if (missingCount > 0) {
      for (Cause missingInput : rootCauses.build()) {
        env.getListener()
            .handle(
                Event.error(
                    action.getOwner().getLocation(),
                    String.format(
                        "%s: missing input file '%s'",
                        action.getOwner().getLabel(), missingInput.getLabel())));
      }
      throw new ActionExecutionException(
          missingCount + " input file(s) do not exist",
          action,
          rootCauses.build(),
          /*catastrophe=*/ false);
    }
    return accumulateInputResultsFactory.create(
        inputArtifactData, expandedArtifacts, expandedFilesets);
  }

  private static Iterable<Artifact> filterKnownInputs(
      Iterable<Artifact> newInputs, ActionInputMap inputArtifactData) {
    return Iterables.filter(newInputs, input -> inputArtifactData.getMetadata(input) == null);
  }

  static boolean actionDependsOnBuildId(Action action) {
    // Volatile build actions may need to execute even if none of their known inputs have changed.
    // Depending on the build id ensures that these actions have a chance to execute.
    // SkyframeAwareActions do not need to depend on the build id because their volatility is due to
    // their dependence on Skyframe nodes that are not captured in the action cache. Any changes to
    // those nodes will cause this action to be rerun, so a build id dependency is unnecessary.
    return (action.isVolatile() && !(action instanceof SkyframeAwareAction))
        || action instanceof NotifyOnActionCacheHit;
  }

  /** All info/warning messages associated with actions should be always displayed. */
  @Override
  public String extractTag(SkyKey skyKey) {
    return null;
  }

  /**
   * Should be called once execution is over, and the intra-build cache of in-progress computations
   * should be discarded. If the cache is non-empty (due to an interrupted/failed build), failure to
   * call complete() can both cause a memory leak and incorrect results on the subsequent build.
   */
  public void complete(ExtendedEventHandler eventHandler) {
    // Discard all remaining state (there should be none after a successful execution).
    stateMap = Maps.newConcurrentMap();
    actionRewindStrategy.reset(eventHandler);
  }

  private ContinuationState getState(Action action) {
    ContinuationState state = stateMap.get(action);
    if (state == null) {
      state = new ContinuationState();
      Preconditions.checkState(stateMap.put(action, state) == null, action);
    }
    return state;
  }

  /**
   * State to save work across restarts of ActionExecutionFunction due to missing values in the
   * graph for actions that discover inputs. There are three places where we save work, all for
   * actions that discover inputs:
   *
   * <ol>
   *   <li>If not all known input metadata (coming from Action#getInputs) is available yet, then the
   *       calculated set of inputs (including the inputs resolved from the action cache) is saved.
   *   <li>If not all discovered inputs' metadata is available yet, then the known input metadata
   *       together with the set of discovered inputs is saved, as well as the Token used to
   *       identify this action to the action cache.
   *   <li>If, after execution, new inputs are discovered whose metadata is not yet available, then
   *       the same data as in the previous case is saved, along with the actual result of
   *       execution.
   * </ol>
   */
  private static class ContinuationState {
    AllInputs allInputs;
    /** Mutable map containing metadata for known artifacts. */
    ActionInputMap inputArtifactData = null;

    Map<Artifact, Collection<Artifact>> expandedArtifacts = null;
    Map<Artifact, ImmutableList<FilesetOutputSymlink>> expandedFilesets = null;
    Token token = null;
    Iterable<Artifact> discoveredInputs = null;
    FileSystem actionFileSystem = null;

    boolean hasCollectedInputs() {
      return allInputs != null;
    }

    boolean hasArtifactData() {
      boolean result = inputArtifactData != null;
      Preconditions.checkState(result == (expandedArtifacts != null), this);
      return result;
    }

    boolean hasCheckedActionCache() {
      // If token is null because there was an action cache hit, this method is never called again
      // because we return immediately.
      return token != null;
    }

    /** Must be called to assign values to the given variables as they change. */
    void updateFileSystemContext(
        SkyframeActionExecutor executor,
        Environment env,
        ActionMetadataHandler metadataHandler,
        ImmutableMap<Artifact, ImmutableList<FilesetOutputSymlink>> filesets)
        throws IOException {
      if (actionFileSystem != null) {
        executor.updateActionFileSystemContext(
            actionFileSystem, env, metadataHandler.getOutputStore()::injectOutputData, filesets);
      }
    }

    @Override
    public String toString() {
      return MoreObjects.toStringHelper(this)
          .add("token", token)
          .add("allInputs", allInputs)
          .add("inputArtifactData", inputArtifactData)
          .add("discoveredInputs", discoveredInputs)
          .toString();
    }
  }

  /**
   * Used to declare all the exception types that can be wrapped in the exception thrown by {@link
   * ActionExecutionFunction#compute}.
   */
  static final class ActionExecutionFunctionException extends SkyFunctionException {

    private final ActionExecutionException actionException;

    ActionExecutionFunctionException(ActionExecutionException e) {
      // We conservatively assume that the error is transient. We don't have enough information to
      // distinguish non-transient errors (e.g. compilation error from a deterministic compiler)
      // from transient ones (e.g. IO error).
      // TODO(bazel-team): Have ActionExecutionExceptions declare their transience.
      super(e, Transience.TRANSIENT);
      this.actionException = e;
    }

    @Override
    public boolean isCatastrophic() {
      return actionException.isCatastrophe();
    }
  }
}