src/main/java/net/starlark/java/eval/FlagGuardedValue.java

// Copyright 2018 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 net.starlark.java.eval;

/**
 * Wrapper on a value in the predeclared lexical block that controls its accessibility to Starlark
 * based on the value of a semantic flag.
 *
 * <p>For example, this could control whether symbol "Foo" exists in the Starlark global frame: such
 * a symbol might only be accessible if --experimental_foo is set to true. In order to create this
 * control, an instance of this class should be added to the global frame under "Foo". This flag
 * guard will throw a descriptive {@link EvalException} when "Foo" would be accessed without the
 * proper flag.
 */
public final class FlagGuardedValue {
  private final Object obj;
  private final String flag;
  private final FlagType flagType;

  private enum FlagType {
    DEPRECATION,
    EXPERIMENTAL;
  }

  private FlagGuardedValue(Object obj, String flag, FlagType flagType) {
    this.obj = obj;
    if (flag.charAt(0) != '-' && flag.charAt(0) != '+') {
      throw new IllegalArgumentException(String.format("flag needs [+-] prefix: %s", flag));
    }
    this.flag = flag;
    this.flagType = flagType;
  }

  /**
   * Creates a flag guard which only permits access of the given object when the given boolean flag
   * is true. If the given flag is false and the object would be accessed, an error is thrown
   * describing the feature as experimental, and describing that the flag must be set to true.
   *
   * <p>The flag identifier must have a + or - prefix; see StarlarkSemantics.
   */
  public static FlagGuardedValue onlyWhenExperimentalFlagIsTrue(String flag, Object obj) {
    return new FlagGuardedValue(obj, flag, FlagType.EXPERIMENTAL);
  }

  /**
   * Creates a flag guard which only permits access of the given object when the given boolean flag
   * is false. If the given flag is true and the object would be accessed, an error is thrown
   * describing the feature as deprecated, and describing that the flag must be set to false.
   *
   * <p>The flag identifier must have a + or - prefix; see StarlarkSemantics.
   */
  public static FlagGuardedValue onlyWhenIncompatibleFlagIsFalse(String flag, Object obj) {
    return new FlagGuardedValue(obj, flag, FlagType.DEPRECATION);
  }

  /**
   * Returns an error describing an attempt to access this guard's protected object when it should
   * be inaccessible in the (contextually implied) semantics.
   */
  String getErrorFromAttemptingAccess(String name) {
    return flagType == FlagType.EXPERIMENTAL
        ? name
            + " is experimental and thus unavailable with the current flags. It may be enabled by"
            + " setting --"
            + flag.substring(1)
        : name
            + " is deprecated and will be removed soon. It may be temporarily re-enabled by"
            + " setting --"
            + flag.substring(1)
            + "=false";
  }

  /**
   * Returns this guard's underlying object. This should be called when appropriate validation has
   * occurred to ensure that the object is accessible with the (implied) semantics.
   */
  public Object getObject() {
    return obj;
  }

  /** Returns true if this guard's underlying object is accessible under the given semantics. */
  boolean isObjectAccessibleUsingSemantics(StarlarkSemantics semantics) {
    if (flagType == FlagType.EXPERIMENTAL) {
      return semantics.isFeatureEnabledBasedOnTogglingFlags(flag, "");
    } else {
      return semantics.isFeatureEnabledBasedOnTogglingFlags("", flag);
    }
  }
}