src/main/java/com/google/devtools/build/lib/syntax/Mutability.java

// Copyright 2015 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.syntax;

import com.google.common.collect.Iterables;
import com.google.devtools.build.lib.events.Location;
import com.google.devtools.build.lib.util.Preconditions;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.IdentityHashMap;
import java.util.List;

/**
 * Manages the capability to mutate an {@link Environment} and its contained Skylark objects.
 *
 * <p>Once the {@code Environment} is done evaluating, its {@code Mutability} is irreversibly closed
 * ("frozen"). At that point, it is no longer possible to mutate either the {@code Environment} or
 * its objects. This protects each {@code Environment} from unintentional and unsafe modification.
 * Before freezing, only a single thread may use the {@code Environment}, but after freezing, any
 * number of threads may access it.
 *
 * <p>It is illegal for an evaluation in one {@code Environment} to affect another {@code
 * Environment}, even if the second {@code Environment} has not yet been frozen.
 *
 * <p>A {@code Mutability} also tracks which {@link Freezable} objects in its {@code Environment}
 * are temporarily locked from mutation. This is used to prevent modification of iterables during
 * loops. A {@code Freezable} may be locked multiple times (e.g., nested loops over the same
 * iterable). Locking an object does not prohibit mutating its deeply contained values, such as in
 * the case of a list of lists.
 *
 * <p>To ensure safety, {@code Mutability}s must be created using the try-with-resource style:
 * <pre>{@code
 * try (Mutability mutability = Mutability.create(fmt, ...)) { ... }
 * }</pre>
 * The general pattern is to create a {@code Mutability}, build an {@code Environment}, mutate that
 * {@code Environment} and its objects, and possibly return the result from within the {@code try}
 * block, relying on the try-with-resource construct to ensure that everything gets frozen before
 * the result is used. The only code that should create a {@code Mutability} without using
 * try-with-resource is test code that is not part of the Bazel jar.
 */
// TODO(bazel-team): When we start using Java 8, this safe usage pattern can be enforced
// through the use of a higher-order function.
public final class Mutability implements AutoCloseable, Serializable {

  private boolean isFrozen;

  // For each locked Freezable, store all Locations where it is locked.
  // This field is set null once the Mutability is closed. This saves some
  // space, and avoids a concurrency bug from multiple Skylark modules
  // accessing the same Mutability at once.
  private IdentityHashMap<Freezable, List<Location>> lockedItems;

  private final String annotation; // For error reporting.

  /**
   * Creates a Mutability.
   * @param annotation an Object used for error reporting,
   * describing to the user the context in which this Mutability was active.
   */
  private Mutability(String annotation) {
    this.isFrozen = false;
    // Seems unlikely that we'll often lock more than 10 things at once.
    this.lockedItems = new IdentityHashMap<>(10);
    this.annotation = Preconditions.checkNotNull(annotation);
  }

  /**
   * Creates a Mutability.
   * @param pattern is a {@link Printer#format} pattern used to lazily produce a string
   * for error reporting
   * @param arguments are the optional {@link Printer#format} arguments to produce that string
   */
  public static Mutability create(String pattern, Object... arguments) {
    // For efficiency, we could be lazy and use formattable instead of format,
    // but the result is going to be serialized, anyway.
    return new Mutability(Printer.format(pattern, arguments));
  }

  public String getAnnotation() {
    return annotation;
  }

  @Override
  public String toString() {
    return String.format(isFrozen ? "(%s)" : "[%s]", annotation);
  }

  public boolean isFrozen() {
    return isFrozen;
  }

  /**
   * Return whether a {@link Freezable} belonging to this {@code Mutability} is currently locked.
   * Frozen objects are not considered locked, though they are of course immutable nonetheless.
   */
  public boolean isLocked(Freezable object) {
    if (!object.mutability().equals(this)) {
      throw new AssertionError("trying to check the lock of an object from a different context");
    }
    if (isFrozen) {
      return false;
    }
    return lockedItems.containsKey(object);
  }

  /**
   * For a locked {@link Freezable} that belongs to this Mutability, return a List of the
   * {@link Location}s corresponding to its current locks.
   */
  public List<Location> getLockLocations(Freezable object) {
    if (!isLocked(object)) {
      throw new AssertionError("trying to get lock locations for an object that is not locked");
    }
    return lockedItems.get(object);
  }

  /**
   * Add a lock on a {@link Freezable} belonging to this Mutability. The object cannot be
   * mutated until all locks on it are gone. For error reporting purposes each lock is
   * associated with its originating {@link Location}.
   */
  public void lock(Freezable object, Location loc) {
    if (!object.mutability().equals(this)) {
      throw new AssertionError("trying to lock an object from a different context");
    }
    if (isFrozen) {
      return;
    }
    List<Location> locList;
    if (!lockedItems.containsKey(object)) {
      locList = new ArrayList<>();
      lockedItems.put(object, locList);
    } else {
      locList = lockedItems.get(object);
    }
    locList.add(loc);
  }

  /**
   * Remove the lock for a given {@link Freezable} that is associated with the given
   * @{link Location}. It is an error if {@code object} does not belong to this mutability,
   * or has no lock corresponding to {@code loc}.
   */
  public void unlock(Freezable object, Location loc) {
    if (!object.mutability().equals(this)) {
      throw new AssertionError("trying to unlock an object from a different context");
    }
    if (isFrozen) {
      // It's okay if we somehow got frozen while there were still locked objects.
      return;
    }
    if (!lockedItems.containsKey(object)) {
      throw new AssertionError("trying to unlock an object that is not locked");
    }
    List<Location> locList = lockedItems.get(object);
    boolean changed = locList.remove(loc);
    if (!changed) {
      throw new AssertionError(Printer.format("trying to unlock an object for a location at which "
          + "it was not locked (%r)", loc));
    }
    if (locList.isEmpty()) {
      lockedItems.remove(object);
    }
  }

  /**
   * Freezes this Mutability, marking as immutable all {@link Freezable} objects that use it.
   */
  @Override
  public void close() {
    // No need to track per-Freezable info since everything is immutable now.
    lockedItems = null;
    isFrozen = true;
  }

  /**
   * Freezes this Mutability
   * @return it in fluent style.
   */
  public Mutability freeze() {
    close();
    return this;
  }

  /**
   * A MutabilityException will be thrown when the user attempts to mutate an object he shouldn't.
   */
  static class MutabilityException extends Exception {
    MutabilityException(String message) {
      super(message);
    }
  }

  /**
   * Each {@code Freezable} object possesses a {@link Mutability} that determines whether the object
   * is still mutable. All {@code Freezable} objects created in the same {@link Environment} will
   * share the same {@code Mutability}, inherited from this {@code Environment}. Only evaluation in
   * the same {@code Environment} is allowed to mutate these objects, and only until the
   * {@code Mutability} is irreversibly frozen.
   */
  public interface Freezable {
    /**
     * Returns the {@link Mutability} associated with this Freezable object.
     * This should not change over the lifetime of the object.
     */
    Mutability mutability();
  }

  /**
   * Checks that this Freezable object can be mutated from the given {@link Environment}.
   * If the object is mutable, it must be from the environment.
   * @param object a Freezable object that we check is still mutable.
   * @param env the {@link Environment} attempting the mutation.
   * @throws MutabilityException when the object was frozen already, or is locked.
   */
  public static void checkMutable(Freezable object, Environment env)
      throws MutabilityException {
    if (object.mutability().isFrozen()) {
      // Throw MutabilityException, not AssertionError, even if the object was from
      // another context.
      throw new MutabilityException("trying to mutate a frozen object");
    }

    // Consider an {@link Environment} e1, in which is created {@link UserDefinedFunction} f1,
    // that closes over some variable v1 bound to list l1. If somehow, via the magic of callbacks,
    // f1 or l1 is passed as an argument to some function f2 evaluated in {@link Environment} e2
    // while e1 is still mutable, then e2, being a different {@link Environment}, should not be
    // allowed to mutate objects from e1. It's a bug, that shouldn't happen in our current code
    // base, so we throw an AssertionError. If in the future such situations are allowed to happen,
    // then we should throw a MutabilityException instead.
    if (!object.mutability().equals(env.mutability())) {
      throw new AssertionError("trying to mutate an object from a different context");
    }

    if (env.mutability().isLocked(object)) {
      Iterable<String> locs =
          Iterables.transform(env.mutability().getLockLocations(object), Location::print);
      throw new MutabilityException(
          "trying to mutate a locked object (is it currently being iterated over by a for loop "
          + "or comprehension?)\n"
          + "Object locked at the following location(s): "
          + String.join(", ", locs));
    }
  }

  public static final Mutability IMMUTABLE = create("IMMUTABLE").freeze();
}