// 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.github.benmanes.caffeine.cache.Cache;
import com.github.benmanes.caffeine.cache.Caffeine;
import com.google.common.base.Preconditions;
import com.google.common.base.Predicates;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.flogger.GoogleLogger;
import com.google.common.hash.HashFunction;
import com.google.devtools.build.lib.analysis.BlazeDirectories;
import com.google.devtools.build.lib.cmdline.BazelModuleContext;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.cmdline.Label.PackageContext;
import com.google.devtools.build.lib.cmdline.LabelConstants;
import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
import com.google.devtools.build.lib.cmdline.PackageIdentifier;
import com.google.devtools.build.lib.cmdline.RepositoryMapping;
import com.google.devtools.build.lib.cmdline.RepositoryName;
import com.google.devtools.build.lib.concurrent.BlazeInterners;
import com.google.devtools.build.lib.events.Event;
import com.google.devtools.build.lib.events.EventHandler;
import com.google.devtools.build.lib.events.EventKind;
import com.google.devtools.build.lib.events.ExtendedEventHandler;
import com.google.devtools.build.lib.io.InconsistentFilesystemException;
import com.google.devtools.build.lib.packages.BazelStarlarkEnvironment;
import com.google.devtools.build.lib.packages.BuildFileNotFoundException;
import com.google.devtools.build.lib.packages.BzlInitThreadContext;
import com.google.devtools.build.lib.packages.BzlVisibility;
import com.google.devtools.build.lib.packages.RuleClassProvider;
import com.google.devtools.build.lib.packages.StarlarkExportable;
import com.google.devtools.build.lib.packages.SymbolGenerator;
import com.google.devtools.build.lib.packages.WorkspaceFileValue;
import com.google.devtools.build.lib.packages.semantics.BuildLanguageOptions;
import com.google.devtools.build.lib.server.FailureDetails.FailureDetail;
import com.google.devtools.build.lib.server.FailureDetails.StarlarkLoading;
import com.google.devtools.build.lib.server.FailureDetails.StarlarkLoading.Code;
import com.google.devtools.build.lib.skyframe.StarlarkBuiltinsFunction.BuiltinsFailedException;
import com.google.devtools.build.lib.util.DetailedExitCode;
import com.google.devtools.build.lib.util.Fingerprint;
import com.google.devtools.build.lib.util.Pair;
import com.google.devtools.build.lib.vfs.PathFragment;
import com.google.devtools.build.lib.vfs.Root;
import com.google.devtools.build.skyframe.RecordingSkyFunctionEnvironment;
import com.google.devtools.build.skyframe.SkyFunction;
import com.google.devtools.build.skyframe.SkyFunctionException;
import com.google.devtools.build.skyframe.SkyFunctionException.Transience;
import com.google.devtools.build.skyframe.SkyKey;
import com.google.devtools.build.skyframe.SkyValue;
import com.google.devtools.build.skyframe.SkyframeLookupResult;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.function.Consumer;
import javax.annotation.Nullable;
import net.starlark.java.eval.EvalException;
import net.starlark.java.eval.Module;
import net.starlark.java.eval.Mutability;
import net.starlark.java.eval.Starlark;
import net.starlark.java.eval.StarlarkSemantics;
import net.starlark.java.eval.StarlarkThread;
import net.starlark.java.syntax.LoadStatement;
import net.starlark.java.syntax.Location;
import net.starlark.java.syntax.Program;
import net.starlark.java.syntax.StarlarkFile;
import net.starlark.java.syntax.Statement;
import net.starlark.java.syntax.StringLiteral;

/**
 * A Skyframe function to look up and load a single .bzl (or .scl) module.
 *
 * <p>Note: Historically, all modules had the .bzl suffix, but this is no longer true now that Bazel
 * supports the .scl dialect. In identifiers, code comments, and documentation, you should generally
 * assume any "bzl" term could mean a .scl file as well.
 *
 * <p>Given a {@link Label} referencing a .bzl file, attempts to locate the file and load it. The
 * Label must be absolute, and must not reference the special {@code external} package. If loading
 * is successful, returns a {@link BzlLoadValue} that encapsulates the loaded {@link Module} and its
 * transitive digest information. If loading is unsuccessful, throws a {@link
 * BzlLoadFunctionException} that encapsulates the cause of the failure.
 *
 * <p>This Skyframe function supports a special bzl "inlining" mode in which all (indirectly)
 * recursive calls to {@code BzlLoadFunction} are made in the same thread rather than through
 * Skyframe. This inlining mode's entry point is {@link #computeInline}; see that method for more
 * details. Note that it may only be called on an instance of this Skyfunction created by {@link
 * #createForInlining}. Bzl inlining is not to be confused with the separate inlining of {@code
 * BzlCompileFunction}
 */
public class BzlLoadFunction implements SkyFunction {

  // Used for: 1) obtaining info needed to construct the BzlInitThreadContext object and to locate
  // the builtins bzl files; and 2) providing a BazelStarlarkEnvironment to other Skyfunctions
  // (StarlarkBuiltinsFunction, BzlCompileFunction) when they are inlined and called via a static
  // computeInline() entry point.
  private final RuleClassProvider ruleClassProvider;

  // Used for determining paths to builtins bzls that live in the workspace.
  private final BlazeDirectories directories;

  // Handles retrieving BzlCompileValues, either by calling Skyframe or by inlining
  // BzlCompileFunction; the latter is not to be confused with inlining of BzlLoadFunction. See
  // comment in create() for rationale.
  private final ValueGetter getter;

  // Handles inlining of BzlLoadFunction calls.
  @Nullable private final CachedBzlLoadDataManager cachedBzlLoadDataManager;

  private static final GoogleLogger logger = GoogleLogger.forEnclosingClass();

  private BzlLoadFunction(
      RuleClassProvider ruleClassProvider,
      BlazeDirectories directories,
      ValueGetter getter,
      @Nullable CachedBzlLoadDataManager cachedBzlLoadDataManager) {
    this.ruleClassProvider = ruleClassProvider;
    this.directories = directories;
    this.getter = getter;
    this.cachedBzlLoadDataManager = cachedBzlLoadDataManager;
  }

  public static BzlLoadFunction create(
      RuleClassProvider ruleClassProvider,
      BlazeDirectories directories,
      HashFunction hashFunction,
      Cache<BzlCompileValue.Key, BzlCompileValue> bzlCompileCache) {
    return new BzlLoadFunction(
        ruleClassProvider,
        directories,
        // When we are not inlining BzlLoadValue nodes, there is no need to have separate
        // BzlCompileValue nodes for bzl files. Instead we inline BzlCompileFunction for a
        // strict memory win, at a small code complexity cost.
        //
        // Detailed explanation:
        // (1) The BzlCompileValue node for a bzl file is used only for the computation of
        // that file's BzlLoadValue node. So there's no concern about duplicate work that would
        // otherwise get deduped by Skyframe.
        // (2) BzlCompileValue doesn't have an interesting equality relation, so we have no
        // hope of getting any interesting change-pruning of BzlCompileValue nodes. If we
        // had an interesting equality relation that was e.g. able to ignore benign
        // whitespace, then there would be a hypothetical benefit to having separate
        // BzlCompileValue nodes (e.g. on incremental builds we'd be able to not re-execute
        // top-level code in bzl files if the file were reparsed to an equivalent tree).
        // TODO(adonovan): this will change once it truly compiles the code (soon).
        // (3) A BzlCompileValue node lets us avoid redoing work on a BzlLoadFunction Skyframe
        // restart, but we can also achieve that result ourselves with a cache that persists between
        // Skyframe restarts.
        //
        // Therefore, BzlCompileValue nodes are wasteful from two perspectives:
        // (a) BzlCompileValue contains syntax trees, and that business object is really
        // just a temporary thing for bzl execution. Retaining it forever is pure waste.
        // (b) The memory overhead of the extra Skyframe node and edge per bzl file is pure
        // waste.
        new InliningAndCachingGetter(ruleClassProvider, hashFunction, bzlCompileCache),
        /* cachedBzlLoadDataManager= */ null);
  }

  public static BzlLoadFunction createForInlining(
      RuleClassProvider ruleClassProvider,
      BlazeDirectories directories,
      int bzlLoadValueCacheSize) {
    return new BzlLoadFunction(
        ruleClassProvider,
        directories,
        // When we are inlining BzlLoadValue nodes, then we want to have explicit BzlCompileValue
        // nodes, since now (1) in the comment above doesn't hold. This way we read and parse each
        // needed bzl file at most once total globally, rather than once per need (in the worst-case
        // of a BzlLoadValue inlining cache miss). This is important in the situation where a bzl
        // file is loaded by a lot of other bzl files or BUILD files.
        RegularSkyframeGetter.INSTANCE,
        new CachedBzlLoadDataManager(bzlLoadValueCacheSize));
  }

  @Override
  @Nullable
  public SkyValue compute(SkyKey skyKey, Environment env)
      throws SkyFunctionException, InterruptedException {
    BzlLoadValue.Key key = (BzlLoadValue.Key) skyKey.argument();
    try {
      return computeInternal(key, env, /* inliningState= */ null);
    } catch (BzlLoadFailedException e) {
      throw new BzlLoadFunctionException(e);
    }
  }

  /**
   * Entry point for computing "inline", without any direct or indirect Skyframe calls back into
   * {@link BzlLoadFunction}. (Other Skyframe calls are permitted.)
   *
   * <p><b>USAGE NOTES:</b>
   *
   * <ul>
   *   <li>This method is intended to be called from {@link PackageFunction} and {@link
   *       StarlarkBuiltinsFunction} and probably shouldn't be used anywhere else. If you think you
   *       need inline Starlark computation, consult with the Core subteam and check out
   *       cl/305127325 for an example of correcting a misuse.
   *   <li>If this method is used with --keep_going and if Skyframe evaluation will never be
   *       interrupted, then this function ensures that the evaluation graph and any error reported
   *       are deterministic.
   * </ul>
   *
   * <p>Under bzl inlining, there is some calling context that wants to obtain a set of {@link
   * BzlLoadValue}s without Skyframe evaluation. For example, a calling context can be a BUILD file
   * trying to resolve its top-level {@code load} statements. Although this work proceeds in a
   * single thread, multiple calling contexts may evaluate .bzls in parallel. To avoid redundant
   * work, they share a single (global to this Skyfunction instance) cache in lieu of the regular
   * Skyframe cache. Unlike the regular Skyframe cache, this cache stores only successes.
   *
   * <p>If two calling contexts race to compute the same .bzl, each one will see a different copy of
   * it, and only one will end up in the shared cache. This presents a hazard: Suppose A and B both
   * need foo.bzl, and A needs it twice due to a diamond dependency. If A and B race to compute
   * foo.bzl, but B's computation populates the cache, then when A comes back to resolve it the
   * second time it will observe a different {@code BzlLoadValue}. This leads to incorrect Starlark
   * evaluation since Starlark values may rely on Java object identity (see b/138598337). Even if we
   * weren't concerned about racing, A may also reevaluate previously computed items due to cache
   * evictions.
   *
   * <p>To solve this, we keep a second cache, {@link InliningState#successfulLoads}, that is local
   * to the current calling context, and which never evicts entries. Like the global cache discussed
   * above, this cache stores only successes. This cache is always checked in preference to the
   * shared one; it may deviate from the shared one in some of its entries, but the calling context
   * won't know the difference. (Since bzl inlining is only used for the loading phase, we don't
   * need to worry about Starlark values from different packages interacting.) The cache is stored
   * as part of the {@code inliningState} passed in by the caller; the caller can obtain this object
   * using {@link InliningState#create}.
   *
   * <p>As an aside, note that we can't avoid having {@link InliningState#successfulLoads} by simply
   * naively blocking evaluation of .bzls on retrievals from the shared cache. This is because two
   * contexts could deadlock while trying to evaluate an illegal {@code load()} cycle from opposite
   * ends. It would be possible to construct a waits-for graph and perform cycle detection, or to
   * monitor slow threads and do detection lazily, but these do not address the cache eviction
   * issue. Alternatively, we could make Starlark tolerant of reloading, but that would be
   * tantamount to implementing full Starlark serialization.
   *
   * <p>Since our local {@link InliningState#successfulLoads} stores only successes, a separate
   * concern is that we don't want to unsuccessfully visit the same .bzl more than once in the same
   * context. (A visitation is unsuccessful if it fails due to an error or if it cannot complete
   * because of a missing Skyframe dep.) To address this concern we maintain a separate {@link
   * InliningState#unsuccessfulLoads} set, and use this set to return null instead of duplicating an
   * unsuccessful visitation.
   *
   * @return the requested {@code BzlLoadValue}, or null if there was a missing Skyframe dep, an
   *     unspecified exception in a Skyframe dep request, or if this was a duplicate unsuccessful
   *     visitation
   */
  // TODO(brandjon): Pick one of the nouns "load" and "bzl" and use that term consistently.
  @Nullable
  BzlLoadValue computeInline(BzlLoadValue.Key key, InliningState inliningState)
      throws BzlLoadFailedException, InterruptedException {
    Preconditions.checkNotNull(cachedBzlLoadDataManager);
    CachedBzlLoadData cachedData = computeInlineCachedData(key, inliningState);
    return cachedData != null ? cachedData.getValue() : null;
  }

  /**
   * Retrieves or creates the requested {@link CachedBzlLoadData} object for the given bzl, entering
   * it into the local and shared caches. This is the entry point for recursive calls to the inline
   * code path.
   *
   * @return null if there was a missing Skyframe dep, an unspecified exception in a Skyframe dep
   *     request, or if this was a duplicate unsuccessful visitation
   */
  @Nullable
  private CachedBzlLoadData computeInlineCachedData(
      BzlLoadValue.Key key, InliningState inliningState)
      throws BzlLoadFailedException, InterruptedException {
    // Try the caches of successful loads. We must try the thread-local cache before the shared, for
    // consistency purposes (see the javadoc of #computeInline).
    CachedBzlLoadData cachedData = inliningState.successfulLoads.get(key);
    if (cachedData == null) {
      cachedData = cachedBzlLoadDataManager.cache.getIfPresent(key);
      if (cachedData != null) {
        // Found a cache hit from another thread's computation. Register the cache hit's recorded
        // deps as if we had requested them directly in the unwrapped environment. We do this for
        // the unwrapped environment, not the recording environment, because there's no need to
        // embed one CachedBzlLoadData's metadata inside another; the dependency relationship will
        // still be accurately reflected in the cache by the call to addTransitiveDeps() via
        // childCachedDataHandler at the bottom of this function.
        //
        // Also incorporate into successfulLoads any transitive cache hits that it does not already
        // contains.
        cachedData.traverse(
            inliningState.recordingEnv.getDelegate()::registerDependencies,
            inliningState.successfulLoads);
      }
    }

    // See if we've already unsuccessfully visited the bzl. "Unsuccessfully" includes getting null
    // for a missing Skyframe dep; the top-level caller will use a fresh InliningState when it does
    // its Skyframe restart.
    if (inliningState.unsuccessfulLoads.contains(key)) {
      return null;
    }

    // If we're here, the bzl must have never been visited before in this calling context. Compute
    // it ourselves, updating the other data structures as appropriate.
    if (cachedData == null) {
      try {
        cachedData = computeInlineForCacheMiss(key, inliningState);
      } finally {
        if (cachedData != null) {
          inliningState.successfulLoads.put(key, cachedData);
          cachedBzlLoadDataManager.cache.put(key, cachedData);
        } else {
          inliningState.unsuccessfulLoads.add(key);
          // Either propagate an exception or fall through for null return.
        }
      }
    }

    // On success (from cache hit or from scratch), notify the parent CachedBzlLoadData of its new
    // child.
    if (cachedData != null) {
      inliningState.childCachedDataHandler.accept(cachedData);
    }

    return cachedData;
  }

  @Nullable
  private CachedBzlLoadData computeInlineForCacheMiss(
      BzlLoadValue.Key key, InliningState inliningState)
      throws BzlLoadFailedException, InterruptedException {
    // We use an instrumented Skyframe env to capture Skyframe deps in the CachedBzlLoadData (see
    // InliningState#recordingEnv). This generally includes transitive Skyframe deps, but
    // specifically excludes deps underneath recursively loaded .bzls. In this way, the
    // CachedBzlLoadData objects form a DAG that mirrors the bzl load graph: Each node still reaches
    // *all* the transitive skyframe deps needed for its computation, but the bzl-level granularity
    // allows for sharing of cached results for portions of the bzl load graph.
    //
    // Here we are at the boundary between one CachedBzlLoadData and the next. createChildState()
    // unwraps the old recording env and starts a new one for a new node.

    InliningState childState = inliningState.createChildState(cachedBzlLoadDataManager);
    childState.beginLoad(key); // track for cyclic load() detection
    BzlLoadValue value;
    try {
      value = computeInternal(key, childState.recordingEnv, childState);
    } finally {
      childState.finishLoad(key);
    }
    if (value == null) {
      return null;
    }

    return childState.buildCachedData(key, value);
  }

  public void resetInliningCache() {
    cachedBzlLoadDataManager.reset();
  }

  /**
   * An opaque object that holds state for the bzl inlining computation initiated by {@link
   * #computeInline}.
   *
   * <p>An original caller of {@code computeInline} (e.g., {@link PackageFunction}) should obtain
   * one of these objects using {@link InliningState#create}. When the same caller makes several
   * calls to {@code computeInline} (e.g., for multiple top-level loads in the same BUILD file), the
   * same object must be passed to each call.
   *
   * <p>When a Skyfunction that is called by {@code BzlLoadFunction}'s inlining code path in turn
   * calls back into {@code computeInline}, it should forward along the same {@code InliningState}
   * that it received. In particular, {@link StarlarkBuiltinsFunction} forwards the inlining state
   * to ensure that 1) the .bzls that get loaded from the {@code @_builtins} pseudo-repository are
   * properly recorded as dependencies of all .bzl files that use builtins injection, and 2) the
   * builtins .bzls are not reevaluated.
   */
  // TODO(brandjon): Consider making this even more opaque and encapsulating more of the details of
  // inlining. E.g., merge beginLoad/finishLoad with child state tracking, and encapsulate
  // management of [un]successfulLoads.
  static class InliningState {

    /**
     * The Skyframe environment, instrumented to record dependencies inside CachedBzlLoadData
     * objects. A new CachedBzlLoadData, and therefore a new recording environment, is started in
     * each call to computeInlineForCacheMiss(). The initial InliningState's recording environment
     * doesn't instrument anything since it represents the piece of the work that will not be saved
     * in any CachedBzlLoadData.
     */
    private final RecordingSkyFunctionEnvironment recordingEnv;

    /**
     * The builder of the CachedBzlLoadData node that we are currently working on, if any. Null iff
     * we're the initial InliningState, where recordingEnv doesn't instrument anything.
     */
    private final CachedBzlLoadData.Builder cachedDataBuilder;

    /**
     * The set of bzls we're currently in the process of loading but haven't fully visited yet. This
     * is used for cycle detection since we don't have the benefit of Skyframe's internal cycle
     * detection. The set must use insertion order for correct error reporting.
     *
     * <p>This is disjoint with {@link #successfulLoads} and {@link #unsuccessfulLoads}.
     *
     * <p>This is local to current calling context. See {@link #computeInline}.
     */
    // Keyed on the SkyKey, not the label, since label could theoretically be ambiguous, even though
    // in practice keys from BUILD / WORKSPACE / builtins don't call each other. (Not sure if
    // WORKSPACE chunking can cause duplicate labels to appear, but we're robust regardless.)
    private final LinkedHashSet<BzlLoadValue.Key> loadStack;

    /**
     * Cache of bzls that have been fully visited and successfully loaded to a value.
     *
     * <p>This and {@link #unsuccessfulLoads} partition the set of fully visited bzls.
     *
     * <p>This is local to current calling context. See {@link #computeInline}.
     */
    private final Map<BzlLoadValue.Key, CachedBzlLoadData> successfulLoads;

    /**
     * Set of bzls that have been fully visited, but were not successfully loaded to a value.
     *
     * <p>This and {@link #successfulLoads} partition the set of fully visited bzls, and is disjoint
     * with {@link #loadStack}.
     *
     * <p>This is local to current calling context. See {@link #computeInline}.
     */
    private final HashSet<BzlLoadValue.Key> unsuccessfulLoads;

    /** Called when a transitive {@code CachedBzlLoadData} is produced. */
    private final Consumer<CachedBzlLoadData> childCachedDataHandler;

    private InliningState(
        RecordingSkyFunctionEnvironment recordingEnv,
        CachedBzlLoadData.Builder cachedDataBuilder,
        LinkedHashSet<BzlLoadValue.Key> loadStack,
        Map<BzlLoadValue.Key, CachedBzlLoadData> successfulLoads,
        HashSet<BzlLoadValue.Key> unsuccessfulLoads,
        Consumer<CachedBzlLoadData> childCachedDataHandler) {
      this.recordingEnv = recordingEnv;
      this.cachedDataBuilder = cachedDataBuilder;
      this.loadStack = loadStack;
      this.successfulLoads = successfulLoads;
      this.unsuccessfulLoads = unsuccessfulLoads;
      this.childCachedDataHandler = childCachedDataHandler;
    }

    /**
     * Creates an initial {@code InliningState} with no information about previously loaded files
     * (except the shared cache stored in {@link BzlLoadFunction}).
     */
    static InliningState create(Environment env) {
      return new InliningState(
          new RecordingSkyFunctionEnvironment(env, x -> {}, x -> {}, x -> {}),
          /* cachedDataBuilder= */ null,
          /* loadStack= */ new LinkedHashSet<>(),
          /* successfulLoads= */ new HashMap<>(),
          /* unsuccessfulLoads= */ new HashSet<>(),
          // No parent value to mutate
          /* childCachedDataHandler= */ x -> {});
    }

    /**
     * Creates another InliningState from this one, but with the recording Skyframe environment set
     * up to log dependency metadata into a CachedBzlLoadData node that is a child of this
     * InliningState's node.
     */
    private InliningState createChildState(CachedBzlLoadDataManager cachedBzlLoadDataManager) {
      CachedBzlLoadData.Builder newBuilder = cachedBzlLoadDataManager.cachedDataBuilder();
      RecordingSkyFunctionEnvironment newRecordingEnv =
          new RecordingSkyFunctionEnvironment(
              recordingEnv.getDelegate(),
              newBuilder::addDep,
              newBuilder::addDeps,
              newBuilder::noteException);
      return new InliningState(
          newRecordingEnv,
          newBuilder,
          loadStack,
          successfulLoads,
          unsuccessfulLoads,
          newBuilder::addTransitiveDeps);
    }

    /**
     * Finishes construction of the current CachedBzlLoadData node. This InliningState object should
     * not be used after calling this method.
     */
    private CachedBzlLoadData buildCachedData(BzlLoadValue.Key key, BzlLoadValue value) {
      cachedDataBuilder.setValue(value);
      cachedDataBuilder.setKey(key);
      return cachedDataBuilder.build();
    }

    /** Records entry to a {@code load()}, throwing an exception if a cycle is detected. */
    private void beginLoad(BzlLoadValue.Key key) throws BzlLoadFailedException {
      if (!loadStack.add(key)) {
        ImmutableList<BzlLoadValue.Key> cycle =
            CycleUtils.splitIntoPathAndChain(Predicates.equalTo(key), loadStack).second;
        throw new BzlLoadFailedException(
            "Starlark load cycle: " + Lists.transform(cycle, BzlLoadValue.Key::getLabel),
            Code.CYCLE);
      }
    }

    /** Records exit from a {@code load()}. */
    private void finishLoad(BzlLoadValue.Key key) {
      Preconditions.checkState(loadStack.remove(key), key);
    }

    /** Retrieves the Skyframe environment to use to do work under this InliningState. */
    Environment getEnvironment() {
      return recordingEnv;
    }
  }

  /**
   * Entry point for compute logic that's common to both (bzl) inlining and non-inlining code paths.
   */
  // It is vital that we don't return any value if any call to env#getValue(s)OrThrow throws an
  // exception. We are allowed to wrap the thrown exception and rethrow it for any calling functions
  // to handle though.
  @Nullable
  private BzlLoadValue computeInternal(
      BzlLoadValue.Key key, Environment env, @Nullable InliningState inliningState)
      throws BzlLoadFailedException, InterruptedException {
    Label label = key.getLabel();
    PathFragment filePath = label.toPathFragment();

    StarlarkBuiltinsValue builtins = getBuiltins(key, env, inliningState);
    if (builtins == null) {
      return null;
    }

    BzlCompileValue.Key compileKey =
        validatePackageAndGetCompileKey(
            key,
            env,
            builtins.starlarkSemantics.get(BuildLanguageOptions.EXPERIMENTAL_BUILTINS_BZL_PATH));
    if (compileKey == null) {
      return null;
    }
    BzlCompileValue compileValue;
    try {
      compileValue = getter.getBzlCompileValue(compileKey, env);
    } catch (BzlCompileFunction.FailedIOException e) {
      throw BzlLoadFailedException.errorReadingBzl(filePath, e);
    }
    if (compileValue == null) {
      return null;
    }

    BzlLoadValue result;
    // Release the compiled bzl iff the value gets completely evaluated (to either error or non-null
    // result).
    boolean completed = true;
    try {
      result = computeInternalWithCompiledBzl(key, compileValue, builtins, env, inliningState);
      completed = result != null;
    } finally {
      if (completed) { // only false on unexceptional null result
        getter.doneWithBzlCompileValue(compileKey);
      }
    }
    return result;
  }

  /**
   * Obtain a suitable StarlarkBuiltinsValue.
   *
   * <p>For BUILD-loaded, WORKSPACE-loaded and *almost* all bzlmod-loaded .bzl files, this is a real
   * builtins value, obtained using either Skyframe or inlining of StarlarkBuiltinsFunction
   * (depending on whether {@code inliningState} is non-null). The returned value includes the
   * StarlarkSemantics.
   *
   * <p>For other .bzl files, the builtins computation is not needed and would create a Skyframe
   * cycle if requested, so we instead return an empty builtins value that just wraps the
   * StarlarkSemantics.
   */
  @Nullable
  private StarlarkBuiltinsValue getBuiltins(
      BzlLoadValue.Key key, Environment env, @Nullable InliningState inliningState)
      throws BzlLoadFailedException, InterruptedException {
    if (!requiresBuiltinsInjection(key)) {
      StarlarkSemantics starlarkSemantics = PrecomputedValue.STARLARK_SEMANTICS.get(env);
      if (starlarkSemantics == null) {
        return null;
      }
      return StarlarkBuiltinsValue.createEmpty(starlarkSemantics);
    }
    try {
      if (inliningState == null) {
        return (StarlarkBuiltinsValue)
            env.getValueOrThrow(StarlarkBuiltinsValue.key(), BuiltinsFailedException.class);
      } else {
        return StarlarkBuiltinsFunction.computeInline(
            StarlarkBuiltinsValue.key(),
            inliningState,
            ruleClassProvider.getBazelStarlarkEnvironment(),
            /* bzlLoadFunction= */ this);
      }
    } catch (BuiltinsFailedException e) {
      throw BzlLoadFailedException.builtinsFailed(key.getLabel(), e);
    }
  }

  private static boolean requiresBuiltinsInjection(BzlLoadValue.Key key) {
    return key instanceof BzlLoadValue.KeyForBuild
        || key instanceof BzlLoadValue.KeyForWorkspace
        // https://github.com/bazelbuild/bazel/issues/17713
        // `@_builtins` depends on `@bazel_tools` for repo mapping, so we ignore some bzl files
        // to avoid a cyclic dependency
        || (key instanceof BzlLoadValue.KeyForBzlmod
            && !(key instanceof BzlLoadValue.KeyForBzlmodBootstrap));
  }

  /**
   * Given a bzl key, validates that the corresponding package exists (if required) and returns the
   * associated compile key based on the package's root. Returns null for a missing Skyframe dep or
   * unspecified exception.
   *
   * <p>.bzl files are not necessarily targets, because they can be loaded by BUILD and other .bzl
   * files without ever being declared in a BUILD file. However, .bzl files are still identified by
   * a label in the same way that file targets are. In particular, it is illegal to refer to a .bzl
   * file using a label whose package part is not the .bzl file's innermost containing package. For
   * example, if pkg and pkg/subpkg have BUILD files but not pkg/subdir, then {@code
   * pkg/subdir:foo.bzl} and {@code pkg:subpkg/foo.bzl} are disallowed.
   *
   * <p>In the case of builtins .bzl files, all labels are written as if the pseudo-repo constitutes
   * one big package, e.g. {@code @builtins//:some/path/foo.bzl}, but no BUILD file need exist. The
   * compile key's root is determined by {@code --experimental_builtins_bzl_path} (passed as {@code
   * builtinsBzlPath}) instead of by package lookup.
   */
  @Nullable
  private BzlCompileValue.Key validatePackageAndGetCompileKey(
      BzlLoadValue.Key key, Environment env, String builtinsBzlPath)
      throws BzlLoadFailedException, InterruptedException {
    Label label = key.getLabel();

    // Bypass package lookup entirely if builtins.
    if (key.isBuiltins()) {
      if (!label.getPackageName().isEmpty()) {
        throw BzlLoadFailedException.noBuildFile(label, "@_builtins cannot have subpackages");
      }
      return key.getCompileKey(getBuiltinsRoot(builtinsBzlPath));
    }

    // Do package lookup.
    PathFragment dir = Label.getContainingDirectory(label);
    PackageIdentifier dirId = PackageIdentifier.create(label.getRepository(), dir);
    ContainingPackageLookupValue packageLookup;
    try {
      packageLookup =
          (ContainingPackageLookupValue)
              env.getValueOrThrow(
                  ContainingPackageLookupValue.key(dirId),
                  BuildFileNotFoundException.class,
                  InconsistentFilesystemException.class);
    } catch (BuildFileNotFoundException | InconsistentFilesystemException e) {
      throw BzlLoadFailedException.errorFindingContainingPackage(label.toPathFragment(), e);
    }
    if (packageLookup == null) {
      return null;
    }

    // Resolve to compile key or error.
    BzlCompileValue.Key compileKey;
    boolean packageOk =
        packageLookup.hasContainingPackage()
            && packageLookup.getContainingPackageName().equals(label.getPackageIdentifier());
    if (key.isBuildPrelude() && !packageOk) {
      // Ignore the prelude, its package doesn't exist.
      compileKey = BzlCompileValue.EMPTY_PRELUDE_KEY;
    } else {
      if (packageOk) {
        compileKey = key.getCompileKey(packageLookup.getContainingPackageRoot());
      } else {
        if (!packageLookup.hasContainingPackage()) {
          throw BzlLoadFailedException.noBuildFile(
              label, packageLookup.getReasonForNoContainingPackage());
        } else {
          throw BzlLoadFailedException.labelCrossesPackageBoundary(label, packageLookup);
        }
      }
    }
    return compileKey;
  }

  private Root getBuiltinsRoot(String builtinsBzlPath) {
    // TODO(#11437): Remove once injection can't be disabled.
    if (builtinsBzlPath.isEmpty()) {
      throw new IllegalStateException("Requested builtins root, but injection is disabled");
    }

    // TODO(#11437): For the non-bundled case, should we consider interning the Root rather than
    // constructing a new one each time?
    Root root;
    if (builtinsBzlPath.equals("%bundled%")) {
      // May be null in tests, but in that case the builtins path shouldn't be set to %bundled%.
      root =
          Preconditions.checkNotNull(
              ruleClassProvider.getBundledBuiltinsRoot(),
              "rule class provider does not specify a builtins root; either call"
                  + " setBuiltinsBzlZipResource() or else set --experimental_builtins_bzl_path to"
                  + " a root");
    } else if (builtinsBzlPath.equals("%workspace%")) {
      String packagePath =
          Preconditions.checkNotNull(
              ruleClassProvider.getBuiltinsBzlPackagePathInSource(),
              "rule class provider does not specify a canonical package path to a builtins root;"
                  + " either call setBuiltinsBzlPackagePathInSource() or else do not set"
                  + "--experimental_builtins_bzl_path to %workspace%");
      // TODO(brandjon): Here we return a root that is underneath a package root. Since the root is
      // itself not a package root, we don't get the benefit of any special DiffAwareness handling.
      // This case probably isn't important since it doesn't occur in production Bazel, but
      // presumably we might be able to add a special DiffAwareness for it if we wanted.
      root = Root.fromPath(directories.getWorkspace().getRelative(packagePath));
    } else {
      root = Root.fromPath(directories.getWorkspace().getRelative(builtinsBzlPath));
    }
    return root;
  }

  /**
   * Compute logic for once the compiled .bzl has been fetched and confirmed to exist (though it may
   * have Starlark errors).
   */
  @Nullable
  private BzlLoadValue computeInternalWithCompiledBzl(
      BzlLoadValue.Key key,
      BzlCompileValue compileValue,
      StarlarkBuiltinsValue builtins,
      Environment env,
      @Nullable InliningState inliningState)
      throws BzlLoadFailedException, InterruptedException {
    // Ensure the .bzl exists and passes static checks (parsing, resolving).
    // (A missing prelude file still returns a valid but empty BzlCompileValue.)
    if (!compileValue.lookupSuccessful()) {
      throw new BzlLoadFailedException(compileValue.getError(), Code.COMPILE_ERROR);
    }
    Program prog = compileValue.getProgram();
    Label label = key.getLabel();
    PackageIdentifier pkg = label.getPackageIdentifier();

    boolean isSclFlagEnabled =
        builtins.starlarkSemantics.getBool(BuildLanguageOptions.EXPERIMENTAL_ENABLE_SCL_DIALECT);
    if (key.isSclDialect() && !isSclFlagEnabled) {
      throw new BzlLoadFailedException(
          "loading .scl files requires setting --experimental_enable_scl_dialect",
          Code.PARSE_ERROR);
    }

    // Determine dependency BzlLoadValue keys for the load statements in this bzl.
    // Labels are resolved relative to the current repo mapping.
    RepositoryMapping repoMapping = getRepositoryMapping(key, builtins.starlarkSemantics, env);
    if (repoMapping == null) {
      return null;
    }
    ImmutableList<Pair<String, Location>> programLoads = getLoadsFromProgram(prog);
    ImmutableList<Label> loadLabels =
        getLoadLabels(
            env.getListener(),
            programLoads,
            pkg,
            repoMapping,
            key.isSclDialect(),
            isSclFlagEnabled);
    if (loadLabels == null) {
      throw new BzlLoadFailedException(
          String.format(
              "module '%s'%s has invalid load statements",
              label.toPathFragment(),
              StarlarkBuiltinsValue.isBuiltinsRepo(label.getRepository()) ? " (internal)" : ""),
          Code.PARSE_ERROR);
    }
    ImmutableList.Builder<BzlLoadValue.Key> loadKeysBuilder =
        ImmutableList.builderWithExpectedSize(loadLabels.size());
    for (Label loadLabel : loadLabels) {
      loadKeysBuilder.add(key.getKeyForLoad(loadLabel));
    }
    ImmutableList<BzlLoadValue.Key> loadKeys = loadKeysBuilder.build();

    // Load .bzl modules.
    // When not using bzl inlining, this is done in parallel for all loads.
    List<BzlLoadValue> loadValues =
        inliningState == null
            ? computeBzlLoadsWithSkyframe(env, loadKeys, programLoads)
            : computeBzlLoadsWithInlining(env, loadKeys, programLoads, inliningState);
    if (loadValues == null) {
      return null; // Skyframe deps unavailable
    }

    // Validate that the current .bzl file satisfies each loaded dependency's load visibility.
    // Violations are reported as error events (since there can be more than one in a single file)
    // and also trigger a BzlLoadFailedException.
    checkLoadVisibilities(
        pkg,
        "module " + label.getCanonicalForm(),
        loadValues,
        loadKeys,
        programLoads,
        /* demoteErrorsToWarnings= */ !builtins.starlarkSemantics.getBool(
            BuildLanguageOptions.CHECK_BZL_VISIBILITY),
        env.getListener());

    // Accumulate a transitive digest of the bzl file, the digests of its direct loads, and the
    // digest of the @_builtins pseudo-repository (if applicable).
    Fingerprint fp = new Fingerprint();
    fp.addBytes(compileValue.getDigest());

    // Populate the load map and add transitive digests to the fingerprint.
    Map<String, Module> loadMap = Maps.newLinkedHashMapWithExpectedSize(programLoads.size());
    int i = 0;
    for (Pair<String, Location> load : programLoads) {
      BzlLoadValue v = loadValues.get(i++);
      loadMap.put(load.first, v.getModule()); // dups ok
      fp.addBytes(v.getTransitiveDigest());
    }

    // Retrieve predeclared symbols and complete the digest computation.
    ImmutableMap<String, Object> predeclared =
        getAndDigestPredeclaredEnvironment(key, builtins, fp);
    if (predeclared == null) {
      return null;
    }
    byte[] transitiveDigest = fp.digestAndReset();

    // The BazelModuleContext holds additional contextual info to be associated with the Module,
    // including the label and a reified copy of the load DAG.
    BazelModuleContext bazelModuleContext =
        BazelModuleContext.create(
            label,
            repoMapping,
            prog.getFilename(),
            ImmutableList.copyOf(loadMap.values()),
            transitiveDigest);

    // Construct the initial Starlark module used for executing the program.
    // The set of keys in the predeclared environment matches the set of predeclareds used to
    // compile the .bzl file into a Program.
    Module module =
        Module.withPredeclaredAndData(builtins.starlarkSemantics, predeclared, bazelModuleContext);

    // The BzlInitThreadContext holds Starlark thread-local state to be read and updated during
    // evaluation.
    BzlInitThreadContext context =
        new BzlInitThreadContext(
            label,
            transitiveDigest,
            ruleClassProvider.getToolsRepository(),
            ruleClassProvider.getNetworkAllowlistForTests(),
            ruleClassProvider.getConfigurationFragmentMap(),
            new SymbolGenerator<>(label));

    // executeBzlFile may post events to the Environment's handler, but events do not matter when
    // caching BzlLoadValues. Note that executing the code mutates the Module and
    // BzlInitThreadContext.
    executeBzlFile(
        prog, label, module, loadMap, context, builtins.starlarkSemantics, env.getListener());

    BzlVisibility bzlVisibility = context.getBzlVisibility();
    if (bzlVisibility == null) {
      bzlVisibility = BzlVisibility.PUBLIC;
    }
    // We save load visibility in the BzlLoadValue rather than the BazelModuleContext because
    // visibility doesn't need to be introspected by any Starlark builtin methods, and because the
    // alternative would mean mutating or overwriting the BazelModuleContext after evaluation.
    return new BzlLoadValue(module, transitiveDigest, bzlVisibility);
  }

  @Nullable
  private static RepositoryMapping getRepositoryMapping(
      BzlLoadValue.Key key, StarlarkSemantics semantics, Environment env)
      throws InterruptedException {
    boolean bzlmod = semantics.getBool(BuildLanguageOptions.ENABLE_BZLMOD);
    if (key.isBuiltins() && !bzlmod) {
      // Without Bzlmod, builtins .bzls never have a repo mapping defined for them, so return
      // without requesting a RepositoryMappingValue. (NB: In addition to being a slight
      // optimization, this avoids adding a reverse dependency on the special //external package,
      // which helps avoid tickling some peculiarities of the Google-internal Skyframe
      // implementation; see b/182293526 for details.)
      // Otherwise, builtins .bzls should use the repo mapping of @bazel_tools, and *do* request a
      // normal RepositoryMappingValue (see logic in RepositoryMappingFunction).
      return RepositoryMapping.ALWAYS_FALLBACK;
    }

    Label enclosingFileLabel = key.getLabel();
    RepositoryName repoName = enclosingFileLabel.getRepository();

    if (key instanceof BzlLoadValue.KeyForWorkspace) {
      // Still during workspace file evaluation
      BzlLoadValue.KeyForWorkspace keyForWorkspace = (BzlLoadValue.KeyForWorkspace) key;
      RepositoryMapping pureWorkspaceMapping;
      if (keyForWorkspace.getWorkspaceChunk() == 0) {
        // There is no previous workspace chunk
        pureWorkspaceMapping = RepositoryMapping.ALWAYS_FALLBACK;
      } else {
        SkyKey workspaceFileKey =
            WorkspaceFileValue.key(
                keyForWorkspace.getWorkspacePath(), keyForWorkspace.getWorkspaceChunk() - 1);
        WorkspaceFileValue workspaceFileValue = (WorkspaceFileValue) env.getValue(workspaceFileKey);
        // Note: we know for sure that the requested WorkspaceFileValue is fully computed so we do
        // not need to check if it is null
        pureWorkspaceMapping =
            RepositoryMapping.createAllowingFallback(
                workspaceFileValue
                    .getRepositoryMapping()
                    .getOrDefault(repoName, ImmutableMap.of()));
      }
      if (!bzlmod) {
        // Without Bzlmod, we just return the mapping purely computed from WORKSPACE stuff.
        return pureWorkspaceMapping;
      }
      // If Bzlmod is in play, we need to make sure that pure WORKSPACE mapping is composed with the
      // root module's mapping (just like how all WORKSPACE repos can see what the root module sees
      // _after_ WORKSPACE evaluation).
      RepositoryMappingValue rootModuleMappingValue =
          (RepositoryMappingValue)
              env.getValue(RepositoryMappingValue.KEY_FOR_ROOT_MODULE_WITHOUT_WORKSPACE_REPOS);
      if (rootModuleMappingValue == null) {
        return null;
      }
      return pureWorkspaceMapping.composeWith(rootModuleMappingValue.getRepositoryMapping());
    }

    if (key instanceof BzlLoadValue.KeyForBzlmod) {
      if (key instanceof BzlLoadValue.KeyForBzlmodBootstrap) {
        // Special case: we're only here to get one of the rules in the @bazel_tools repo that
        // load Bazel modules. At this point we can't load from any other modules and thus use a
        // repository mapping that contains only @bazel_tools itself.
        return RepositoryMapping.create(
            ImmutableMap.of("bazel_tools", RepositoryName.BAZEL_TOOLS), RepositoryName.BAZEL_TOOLS);
      }
      if (repoName.isMain()) {
        // Special case: when we try to run an extension in the main repo, we need to grab the repo
        // mapping for the main repo, which normally would include all WORKSPACE repos. This is
        // problematic if the reason we're running an extension at all is that we're trying to do a
        // `load` in WORKSPACE. So we specifically say that, to run an extension in the main repo,
        // we ask for a repo mapping *without* WORKSPACE repos.
        RepositoryMappingValue repositoryMappingValue =
            (RepositoryMappingValue)
                env.getValue(RepositoryMappingValue.KEY_FOR_ROOT_MODULE_WITHOUT_WORKSPACE_REPOS);
        if (repositoryMappingValue == null) {
          return null;
        }
        return repositoryMappingValue.getRepositoryMapping();
      }
    }

    // This is either a .bzl loaded from BUILD files, or a .bzl loaded for bzlmod, so we can just
    // use the full repo mapping from RepositoryMappingFunction.
    RepositoryMappingValue repositoryMappingValue =
        (RepositoryMappingValue) env.getValue(RepositoryMappingValue.key(repoName));
    if (repositoryMappingValue == null) {
      return null;
    }
    return repositoryMappingValue.getRepositoryMapping();
  }

  /**
   * Validates a label appearing in a {@code load()} statement, throwing {@link
   * LabelSyntaxException} on failure.
   *
   * <p>Different restrictions apply depending on what type of source file the load appears in. For
   * all kinds of files, {@code label}:
   *
   * <ul>
   *   <li>may not be within {@code @//external}.
   *   <li>must end with either {@code .bzl} or {@code .scl}.
   * </ul>
   *
   * <p>For source files appearing within {@code @_builtins}, {@code label} must also be within
   * {@code @_builtins}. (The reverse, that those files may not be loaded by user-defined files, is
   * enforced by the fact that the {@code @_builtins} pseudorepo cannot be resolved as an ordinary
   * repo.)
   *
   * <p>For .scl files only, {@code label} must end with {@code .scl} (not {@code .bzl}). (Loads in
   * .scl also should always begin with {@code //}, but that's syntactic and can't be enforced in
   * this method.)
   *
   * @param label the label to validate
   * @param fromBuiltinsRepo true if the file containing the load is within {@code @_builtins}
   * @param withinSclDialect true if the file containing the load is a .scl file
   * @param mentionSclInErrorMessage true if ".scl" should be advertised as a possible extension in
   *     error messaging
   */
  private static void checkValidLoadLabel(
      Label label,
      boolean fromBuiltinsRepo,
      boolean withinSclDialect,
      boolean mentionSclInErrorMessage)
      throws LabelSyntaxException {
    // Check file extension.
    String baseName = label.getName();
    if (withinSclDialect) {
      if (!baseName.endsWith(".scl")) {
        String msg = "The label must reference a file with extension \".scl\"";
        if (baseName.endsWith(".bzl")) {
          msg += " (.scl files cannot load .bzl files)";
        }
        throw new LabelSyntaxException(msg);
      }
    } else {
      if (!(baseName.endsWith(".scl") || baseName.endsWith(".bzl"))) {
        String msg = "The label must reference a file with extension \".bzl\"";
        if (mentionSclInErrorMessage) {
          msg += " or \".scl\"";
        }
        throw new LabelSyntaxException(msg);
      }
    }

    if (label.getPackageIdentifier().equals(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER)) {
      throw new LabelSyntaxException(
          "Starlark files may not be loaded from the //external package");
    }
    if (fromBuiltinsRepo && !StarlarkBuiltinsValue.isBuiltinsRepo(label.getRepository())) {
      throw new LabelSyntaxException(
          ".bzl files in @_builtins cannot load from outside of @_builtins");
    }
  }

  /**
   * Validates a label appearing in a {@code load()} statement, throwing {@link
   * LabelSyntaxException} on failure.
   */
  public static void checkValidLoadLabel(Label label, StarlarkSemantics starlarkSemantics)
      throws LabelSyntaxException {
    checkValidLoadLabel(
        label,
        /* fromBuiltinsRepo= */ false,
        /* withinSclDialect= */ false,
        /* mentionSclInErrorMessage= */ starlarkSemantics.getBool(
            BuildLanguageOptions.EXPERIMENTAL_ENABLE_SCL_DIALECT));
  }

  /**
   * Given a list of {@code load("module")} strings and their locations, in source order, returns a
   * corresponding list of Labels they each resolve to. Labels are resolved relative to {@code
   * base}, the file's package. If any label is malformed, the function reports one or more errors
   * to the handler and returns null.
   *
   * <p>If {@code withinSclDialect} is true, the labels are validated according to the rules of the
   * .scl dialect: Only strings beginning with {@code //} are allowed (no repo syntax, no relative
   * labels), and only .scl files may be loaded (not .bzl). If {@code isSclFlagEnabled} is true,
   * then ".scl" is mentioned as a possible file extension in error messages.
   */
  @Nullable
  private static ImmutableList<Label> getLoadLabels(
      EventHandler handler,
      ImmutableList<Pair<String, Location>> loads,
      PackageIdentifier base,
      RepositoryMapping repoMapping,
      boolean withinSclDialect,
      boolean isSclFlagEnabled) {
    boolean ok = true;

    ImmutableList.Builder<Label> loadLabels = ImmutableList.builderWithExpectedSize(loads.size());
    for (Pair<String, Location> load : loads) {
      // Parse the load statement's module string as a label. Validate the unparsed string for
      // syntax and the parsed label for structure.
      String unparsedLabel = load.first;
      try {
        if (withinSclDialect && !unparsedLabel.startsWith("//")) {
          throw new LabelSyntaxException("in .scl files, load labels must begin with \"//\"");
        }
        Label label =
            Label.parseWithPackageContext(unparsedLabel, PackageContext.of(base, repoMapping));
        checkValidLoadLabel(
            label,
            /* fromBuiltinsRepo= */ StarlarkBuiltinsValue.isBuiltinsRepo(base.getRepository()),
            /* withinSclDialect= */ withinSclDialect,
            /* mentionSclInErrorMessage= */ isSclFlagEnabled);
        loadLabels.add(label);
      } catch (LabelSyntaxException ex) {
        handler.handle(Event.error(load.second, "in load statement: " + ex.getMessage()));
        ok = false;
      }
    }
    return ok ? loadLabels.build() : null;
  }

  /**
   * Given a list of {@code load("module")} strings and their locations, in source order, returns a
   * corresponding list of Labels they each resolve to. Labels are resolved relative to {@code
   * base}, the file's package. If any label is malformed, the function reports one or more errors
   * to the handler and returns null.
   */
  @Nullable
  static ImmutableList<Label> getLoadLabels(
      EventHandler handler,
      ImmutableList<Pair<String, Location>> loads,
      PackageIdentifier base,
      RepositoryMapping repoMapping,
      StarlarkSemantics starlarkSemantics) {
    return getLoadLabels(
        handler,
        loads,
        base,
        repoMapping,
        /* withinSclDialect= */ false,
        /* isSclFlagEnabled= */ starlarkSemantics.getBool(
            BuildLanguageOptions.EXPERIMENTAL_ENABLE_SCL_DIALECT));
  }

  /** Extracts load statements from compiled program (see {@link #getLoadLabels}). */
  static ImmutableList<Pair<String, Location>> getLoadsFromProgram(Program prog) {
    int n = prog.getLoads().size();
    ImmutableList.Builder<Pair<String, Location>> loads = ImmutableList.builderWithExpectedSize(n);
    for (int i = 0; i < n; i++) {
      loads.add(Pair.of(prog.getLoads().get(i), prog.getLoadLocation(i)));
    }
    return loads.build();
  }

  /** Extracts load statements from file syntax (see {@link #getLoadLabels}). */
  static ImmutableList<Pair<String, Location>> getLoadsFromStarlarkFiles(List<StarlarkFile> files) {
    ImmutableList.Builder<Pair<String, Location>> loads = ImmutableList.builder();
    for (StarlarkFile file : files) {
      for (Statement stmt : file.getStatements()) {
        if (stmt instanceof LoadStatement) {
          StringLiteral module = ((LoadStatement) stmt).getImport();
          loads.add(Pair.of(module.getValue(), module.getStartLocation()));
        }
      }
    }
    return loads.build();
  }

  /**
   * Computes the BzlLoadValue for all given .bzl load keys using ordinary Skyframe evaluation,
   * returning {@code null} if Skyframe deps were missing and have been requested. {@code
   * programLoads} provides the locations of the load statements in source order, for error
   * reporting.
   */
  @Nullable
  private static List<BzlLoadValue> computeBzlLoadsWithSkyframe(
      Environment env, List<BzlLoadValue.Key> keys, List<Pair<String, Location>> programLoads)
      throws BzlLoadFailedException, InterruptedException {
    List<BzlLoadValue> bzlLoads = Lists.newArrayListWithExpectedSize(keys.size());
    SkyframeLookupResult values = env.getValuesAndExceptions(keys);
    // Process loads (and report first error) in source order.
    for (int i = 0; i < keys.size(); i++) {
      try {
        bzlLoads.add((BzlLoadValue) values.getOrThrow(keys.get(i), BzlLoadFailedException.class));
      } catch (BzlLoadFailedException ex) {
        throw BzlLoadFailedException.whileLoadingDep(programLoads.get(i).second, ex);
      }
    }
    return env.valuesMissing() ? null : bzlLoads;
  }

  /**
   * Computes the BzlLoadValue for all given keys by reusing this instance of the BzlLoadFunction,
   * bypassing traditional Skyframe evaluation. {@code programLoads} provides the locations of the
   * load statements in source order, for error reporting.
   *
   * @return null if there was a missing Skyframe dep, an unspecified exception in a Skyframe dep
   *     request, or if this was a duplicate unsuccessful visitation
   */
  @Nullable
  private List<BzlLoadValue> computeBzlLoadsWithInlining(
      Environment env,
      List<BzlLoadValue.Key> keys,
      List<Pair<String, Location>> programLoads,
      InliningState inliningState)
      throws BzlLoadFailedException, InterruptedException {
    Preconditions.checkState(env == inliningState.recordingEnv);

    List<BzlLoadValue> bzlLoads = Lists.newArrayListWithExpectedSize(keys.size());
    // For the sake of ensuring the graph structure is deterministic, we need to request all of our
    // deps, even if some of them yield errors. The first exception that is seen gets deferred, to
    // be raised after the loop. All other exceptions are swallowed.
    //
    // To see how immediately returning the first error leads to non-determinism, consider the case
    // of two dependencies A and B, where A is in error and appears in a load statement above B.
    // If A has completed at the time we request it, and if we were to immediately propagate that
    // error, we never request B. On the other hand, if A is missing (null return), we do request B
    // in the meantime for the sake of parallelism.
    //
    // This approach assumes --keep_going; determinism is not guaranteed otherwise. It also assumes
    // InterruptedException does not occur, since we don't catch and defer it.
    BzlLoadFailedException deferredException = null;
    boolean valuesMissing = false;
    for (int i = 0; i < keys.size(); i++) {
      CachedBzlLoadData cachedData;
      try {
        cachedData = computeInlineCachedData(keys.get(i), inliningState);
      } catch (BzlLoadFailedException e) {
        if (deferredException == null) {
          deferredException = BzlLoadFailedException.whileLoadingDep(programLoads.get(i).second, e);
        }
        continue;
      }
      if (cachedData == null) {
        // A null value for `cachedData` can occur when it (or its transitive loads) has a Skyframe
        // dep that is missing or in error. It can also occur if there's a transitive load on a bzl
        // that was already seen by inliningState and which returned null (note: in this case, it's
        // not necessarily true that there are missing Skyframe deps because this bzl could have
        // already been visited unsuccessfully). In both these cases, we want to continue making our
        // inline calls, so as to maximize the number of dependent (non-inlined) SkyFunctions that
        // are requested and avoid a quadratic number of restarts.
        valuesMissing = true;
      } else {
        bzlLoads.add(cachedData.getValue());
      }
    }
    if (deferredException != null) {
      throw deferredException;
    }
    return valuesMissing ? null : bzlLoads;
  }

  /**
   * Checks that all (directly) requested loads are visible to the requesting file's package.
   *
   * <p>Each load that is not visible is reported as an error on the event handler. If there is at
   * least one error, {@link BzlLoadFailedException} is thrown.
   *
   * <p>The requesting file may be a .bzl file or another Starlark file (BUILD, WORKSPACE, etc.).
   * {@code requestingPackage} is its logical containing package used for visibility validation,
   * while {@code requestingFileDescription} is a piece of text for error messages, e.g. "module
   * foo.bzl".
   *
   * <p>{@code loadValues}, {@code loadKeys}, and {@code programLoads} are all ordered corresponding
   * to the load statements of the requesting bzl.
   */
  // TODO(brandjon): It'd be nice to pass in a single Label argument that unifies requestingPackage
  // and requestingFileDescription. But some callers of PackageFunction#loadBzlModules don't have
  // such a label handy. Ex: Workspace logic has multiple possible sources of workspace file
  // content.
  static void checkLoadVisibilities(
      PackageIdentifier requestingPackage,
      String requestingFileDescription,
      List<BzlLoadValue> loadValues,
      List<BzlLoadValue.Key> loadKeys,
      List<Pair<String, Location>> programLoads,
      boolean demoteErrorsToWarnings,
      EventHandler handler)
      throws BzlLoadFailedException {
    boolean foundViolation = false;
    for (int i = 0; i < loadValues.size(); i++) {
      BzlVisibility loadVisibility = loadValues.get(i).getBzlVisibility();
      Label loadLabel = loadKeys.get(i).getLabel();
      PackageIdentifier loadPackage = loadLabel.getPackageIdentifier();
      if (!(requestingPackage.equals(loadPackage)
          || loadVisibility.allowsPackage(requestingPackage))) {
        Location loc = programLoads.get(i).second;
        String msg =
            String.format(
                // TODO(brandjon): Consider whether we should try to report error messages (here
                // and elsewhere) using the literal text of the load() rather than the (already
                // repo-remapped) label.
                "Starlark file %s is not visible for loading from package %s. Check the"
                    + " file's `visibility()` declaration.",
                loadLabel, requestingPackage.getCanonicalForm());
        if (demoteErrorsToWarnings) {
          msg += " Continuing because --nocheck_bzl_visibility is active";
          handler.handle(Event.warn(loc, msg));
        } else {
          handler.handle(Event.error(loc, msg));
        }
        foundViolation = true;
      }
    }
    if (foundViolation && !demoteErrorsToWarnings) {
      throw BzlLoadFailedException.visibilityViolation(requestingFileDescription);
    }
  }

  /**
   * Obtains the predeclared environment for a .bzl (or .scl) file, based on the type of .bzl and
   * (if applicable) the injected builtins.
   *
   * <p>Returns null if there was a missing Skyframe dep or unspecified exception.
   *
   * <p>In the case that injected builtins are used, updates the given fingerprint with the digest
   * of the {@code @_builtins} pseudo-repository.
   */
  @Nullable
  private ImmutableMap<String, Object> getAndDigestPredeclaredEnvironment(
      BzlLoadValue.Key key, StarlarkBuiltinsValue builtins, Fingerprint fp) {
    BazelStarlarkEnvironment starlarkEnv = ruleClassProvider.getBazelStarlarkEnvironment();
    if (key.isSclDialect()) {
      // .scl doesn't use injection and doesn't care what kind of key it is.
      return starlarkEnv.getStarlarkGlobals().getSclToplevels();
    } else {
      // TODO(#11437): Remove ability to disable injection by setting flag to empty string.
      boolean injectionDisabled =
          builtins
              .starlarkSemantics
              .get(BuildLanguageOptions.EXPERIMENTAL_BUILTINS_BZL_PATH)
              .isEmpty();
      if (key instanceof BzlLoadValue.KeyForBuild) {
        if (injectionDisabled) {
          return starlarkEnv.getUninjectedBuildBzlEnv();
        }
        fp.addBytes(builtins.transitiveDigest);
        return builtins.predeclaredForBuildBzl;
      } else if (key instanceof BzlLoadValue.KeyForWorkspace
          || key instanceof BzlLoadValue.KeyForBzlmod) {
        // TODO(#11954): We should converge all .bzl dialects regardless of whether they're loaded
        //  by BUILD, WORKSPACE, or MODULE. At the moment, WORKSPACE-loaded and MODULE-loaded .bzl
        //  files are already converged, so we use the same environment for both.
        if (injectionDisabled || key instanceof BzlLoadValue.KeyForBzlmodBootstrap) {
          return starlarkEnv.getUninjectedWorkspaceBzlEnv();
        }
        // Note that we don't actually fingerprint the injected builtins here. The actual builtins
        // values should not be used in WORKSPACE-loaded or MODULE-loaded .bzl files; they're only
        // injected to avoid certain type errors at loading time (e.g. #17713). If we included their
        // digest, we'd be causing widespread repo refetches when _any_ builtin bzl file changes
        // (when Bazel upgrades, for example), and potentially even thrashing if the user is using
        // Bazelisk. Thus we make the explicit choice to not fingerprint the injected builtins, and
        // thereby prohibit any meaningful use of injected builtins in WORKSPACE/MODULE-loaded .bzl
        // files. This additionally means that native repo rules should not be migrated to
        // @_builtins; they should just live in @bazel_tools instead.
        return builtins.predeclaredForWorkspaceBzl;
      } else if (key instanceof BzlLoadValue.KeyForBuiltins) {
        return starlarkEnv.getBuiltinsBzlEnv();
      } else {
        throw new AssertionError("Unknown key type: " + key.getClass());
      }
    }
  }

  /** Executes the compiled .bzl file defining the module to be loaded. */
  private static void executeBzlFile(
      Program prog,
      Label label,
      Module module,
      Map<String, Module> loadedModules,
      BzlInitThreadContext context,
      StarlarkSemantics starlarkSemantics,
      ExtendedEventHandler skyframeEventHandler)
      throws BzlLoadFailedException, InterruptedException {
    try (Mutability mu = Mutability.create("loading", label)) {
      StarlarkThread thread = new StarlarkThread(mu, starlarkSemantics);
      thread.setLoader(loadedModules::get);

      // Wrap the skyframe event handler to listen for starlark errors.
      AtomicBoolean sawStarlarkError = new AtomicBoolean(false);
      EventHandler starlarkEventHandler =
          event -> {
            if (event.getKind() == EventKind.ERROR) {
              sawStarlarkError.set(true);
            }
            skyframeEventHandler.handle(event);
          };
      thread.setPrintHandler(Event.makeDebugPrintHandler(starlarkEventHandler));
      context.storeInThread(thread);

      execAndExport(prog, label, starlarkEventHandler, module, thread);
      if (sawStarlarkError.get()) {
        throw BzlLoadFailedException.executionFailed(label);
      }
    }
  }

  // Precondition: thread has a valid transitiveDigest.
  // TODO(adonovan): executeBzlFile would make a better public API than this function.
  public static void execAndExport(
      Program prog, Label label, EventHandler handler, Module module, StarlarkThread thread)
      throws InterruptedException {

    // Intercept execution after every assignment at top level
    // and "export" any newly assigned exportable globals.
    // TODO(adonovan): change the semantics; see b/65374671.
    thread.setPostAssignHook(
        (name, value) -> {
          if (value instanceof StarlarkExportable) {
            StarlarkExportable exp = (StarlarkExportable) value;
            if (!exp.isExported()) {
              exp.export(handler, label, name);
            }
          }
        });

    try {
      Starlark.execFileProgram(prog, module, thread);
    } catch (EvalException ex) {
      handler.handle(Event.error(null, ex.getMessageWithStack()));
    }
  }

  /**
   * A manager abstracting over the method for obtaining {@code BzlCompileValue}s. See comment in
   * {@link #create}.
   */
  private interface ValueGetter {
    @Nullable
    BzlCompileValue getBzlCompileValue(BzlCompileValue.Key key, Environment env)
        throws BzlCompileFunction.FailedIOException, InterruptedException;

    void doneWithBzlCompileValue(BzlCompileValue.Key key);
  }

  /** A manager that obtains compiled .bzl files from Skyframe calls. */
  private static class RegularSkyframeGetter implements ValueGetter {
    private static final RegularSkyframeGetter INSTANCE = new RegularSkyframeGetter();

    @Nullable
    @Override
    public BzlCompileValue getBzlCompileValue(BzlCompileValue.Key key, Environment env)
        throws BzlCompileFunction.FailedIOException, InterruptedException {
      return (BzlCompileValue) env.getValueOrThrow(key, BzlCompileFunction.FailedIOException.class);
    }

    @Override
    public void doneWithBzlCompileValue(BzlCompileValue.Key key) {}
  }

  /**
   * A manager that obtains compiled .bzls by inlining {@link BzlCompileFunction} (not to be
   * confused with inlining of {@code BzlLoadFunction}). Values are cached within the manager and
   * released explicitly by calling {@link #doneWithBzlCompileValue}.
   */
  private static class InliningAndCachingGetter implements ValueGetter {
    private final RuleClassProvider ruleClassProvider;
    private final HashFunction hashFunction;
    // We keep a cache of BzlCompileValues that have been computed but whose corresponding
    // BzlLoadValue has not yet completed. This avoids repeating the BzlCompileValue work in case
    // of Skyframe restarts. (If we weren't inlining, Skyframe would cache this for us.)
    private final Cache<BzlCompileValue.Key, BzlCompileValue> bzlCompileCache;

    private InliningAndCachingGetter(
        RuleClassProvider ruleClassProvider,
        HashFunction hashFunction,
        Cache<BzlCompileValue.Key, BzlCompileValue> bzlCompileCache) {
      this.ruleClassProvider = ruleClassProvider;
      this.hashFunction = hashFunction;
      this.bzlCompileCache = bzlCompileCache;
    }

    @Nullable
    @Override
    public BzlCompileValue getBzlCompileValue(BzlCompileValue.Key key, Environment env)
        throws BzlCompileFunction.FailedIOException, InterruptedException {
      BzlCompileValue value = bzlCompileCache.getIfPresent(key);
      if (value == null) {
        value =
            BzlCompileFunction.computeInline(
                key, env, ruleClassProvider.getBazelStarlarkEnvironment(), hashFunction);
        if (value != null) {
          bzlCompileCache.put(key, value);
        }
      }
      return value;
    }

    @Override
    public void doneWithBzlCompileValue(BzlCompileValue.Key key) {
      bzlCompileCache.invalidate(key);
    }
  }

  /**
   * Per-instance manager for {@link CachedBzlLoadData}, used when {@code BzlLoadFunction} calls are
   * inlined.
   */
  private static class CachedBzlLoadDataManager {
    private final int cacheSize;
    private Cache<BzlLoadValue.Key, CachedBzlLoadData> cache;
    private CachedBzlLoadDataBuilderFactory cachedDataBuilderFactory =
        new CachedBzlLoadDataBuilderFactory();

    private CachedBzlLoadDataManager(int cacheSize) {
      this.cacheSize = cacheSize;
    }

    private CachedBzlLoadData.Builder cachedDataBuilder() {
      return cachedDataBuilderFactory.newCachedBzlLoadDataBuilder();
    }

    private void reset() {
      if (cache != null) {
        logger.atInfo().log("Starlark inlining cache stats from earlier build: %s", cache.stats());
      }
      cachedDataBuilderFactory = new CachedBzlLoadDataBuilderFactory();
      Preconditions.checkState(
          cacheSize >= 0, "Expected positive Starlark cache size if caching. %s", cacheSize);
      cache =
          Caffeine.newBuilder()
              .initialCapacity(BlazeInterners.concurrencyLevel())
              .maximumSize(cacheSize)
              .recordStats()
              .build();
    }
  }

  /** Indicates a failure to load a .bzl file. */
  public static final class BzlLoadFailedException extends Exception
      implements SaneAnalysisException {
    private final Transience transience;
    private final DetailedExitCode detailedExitCode;

    private BzlLoadFailedException(
        String errorMessage, DetailedExitCode detailedExitCode, Transience transience) {
      super(errorMessage);
      this.transience = transience;
      this.detailedExitCode = detailedExitCode;
    }

    private BzlLoadFailedException(String errorMessage, DetailedExitCode detailedExitCode) {
      this(errorMessage, detailedExitCode, Transience.PERSISTENT);
    }

    private BzlLoadFailedException(
        String errorMessage,
        DetailedExitCode detailedExitCode,
        Exception cause,
        Transience transience) {
      super(errorMessage, cause);
      this.transience = transience;
      this.detailedExitCode = detailedExitCode;
    }

    private BzlLoadFailedException(String errorMessage, Code code) {
      this(errorMessage, createDetailedExitCode(errorMessage, code), Transience.PERSISTENT);
    }

    private BzlLoadFailedException(
        String errorMessage, Code code, Exception cause, Transience transience) {
      this(errorMessage, createDetailedExitCode(errorMessage, code), cause, transience);
    }

    Transience getTransience() {
      return transience;
    }

    @Override
    public DetailedExitCode getDetailedExitCode() {
      return detailedExitCode;
    }

    private static DetailedExitCode createDetailedExitCode(String message, Code code) {
      return DetailedExitCode.of(
          FailureDetail.newBuilder()
              .setMessage(message)
              .setStarlarkLoading(StarlarkLoading.newBuilder().setCode(code))
              .build());
    }

    private static BzlLoadFailedException whileLoadingDep(
        Location loc, BzlLoadFailedException cause) {
      // Don't chain exception cause, just incorporate the message with a prefix.
      // TODO(bazel-team): This exception should hold a Location of the requesting file's load
      // statement, and code that catches it should use the location in the Event they create.
      return new BzlLoadFailedException(
          "at " + loc + ": " + cause.getMessage(), cause.getDetailedExitCode());
    }

    static BzlLoadFailedException executionFailed(Label label) {
      return new BzlLoadFailedException(
          String.format(
              "initialization of module '%s'%s failed",
              // TODO(brandjon): This error message drops the repo part of the label.
              label.toPathFragment(),
              StarlarkBuiltinsValue.isBuiltinsRepo(label.getRepository()) ? " (internal)" : ""),
          Code.EVAL_ERROR);
    }

    static BzlLoadFailedException errorFindingContainingPackage(
        PathFragment file, Exception cause) {
      String errorMessage =
          String.format(
              "Encountered error while reading extension file '%s': %s", file, cause.getMessage());
      DetailedExitCode detailedExitCode =
          cause instanceof DetailedException
              ? ((DetailedException) cause).getDetailedExitCode()
              : createDetailedExitCode(errorMessage, Code.CONTAINING_PACKAGE_NOT_FOUND);
      return new BzlLoadFailedException(
          errorMessage, detailedExitCode, cause, Transience.PERSISTENT);
    }

    static BzlLoadFailedException errorReadingBzl(
        PathFragment file, BzlCompileFunction.FailedIOException cause) {
      String errorMessage =
          String.format(
              "Encountered error while reading extension file '%s': %s", file, cause.getMessage());
      return new BzlLoadFailedException(errorMessage, Code.IO_ERROR, cause, cause.getTransience());
    }

    static BzlLoadFailedException noBuildFile(Label file, @Nullable String reason) {
      if (reason != null) {
        return new BzlLoadFailedException(
            String.format("Unable to find package for %s: %s.", file, reason),
            Code.PACKAGE_NOT_FOUND);
      }
      return new BzlLoadFailedException(
          String.format(
              "Every .bzl file must have a corresponding package, but '%s' does not have one."
                  + " Please create a BUILD file in the same or any parent directory. Note that"
                  + " this BUILD file does not need to do anything except exist.",
              file),
          Code.PACKAGE_NOT_FOUND);
    }

    static BzlLoadFailedException labelCrossesPackageBoundary(
        Label label, ContainingPackageLookupValue containingPackageLookupValue) {
      return new BzlLoadFailedException(
          ContainingPackageLookupValue.getErrorMessageForLabelCrossingPackageBoundary(
              // We don't actually know the proper Root to pass in here (since we don't e.g. know
              // the root of the bzl/BUILD file that is trying to load 'label'). Therefore we just
              // pass in the Root of the containing package in order to still get a useful error
              // message for the user.
              containingPackageLookupValue.getContainingPackageRoot(),
              label,
              containingPackageLookupValue),
          Code.LABEL_CROSSES_PACKAGE_BOUNDARY);
    }

    static BzlLoadFailedException builtinsFailed(Label file, BuiltinsFailedException cause) {
      return new BzlLoadFailedException(
          String.format(
              "Internal error while loading Starlark builtins for %s: %s",
              file, cause.getMessage()),
          Code.BUILTINS_ERROR,
          cause,
          cause.getTransience());
    }

    /**
     * Returns an exception for load visibility violations.
     *
     * <p>{@code fileDescription} is a string like {@code "module //pkg:foo.bzl"} or {@code "file
     * //pkg:BUILD"}.
     */
    static BzlLoadFailedException visibilityViolation(String fileDescription) {
      return new BzlLoadFailedException(
          String.format("%s contains .bzl load visibility violations", fileDescription),
          Code.VISIBILITY_ERROR);
    }
  }

  private static final class BzlLoadFunctionException extends SkyFunctionException {
    private BzlLoadFunctionException(BzlLoadFailedException cause) {
      super(cause, cause.transience);
    }
  }
}