src/main/java/com/google/devtools/build/lib/rules/config/ConfigRuleClasses.java

// 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.rules.config;

import static com.google.devtools.build.lib.packages.Attribute.attr;
import static com.google.devtools.build.lib.packages.BuildType.LABEL_KEYED_STRING_DICT;
import static com.google.devtools.build.lib.packages.BuildType.LABEL_LIST;
import static com.google.devtools.build.lib.syntax.Type.STRING;
import static com.google.devtools.build.lib.syntax.Type.STRING_DICT;
import static com.google.devtools.build.lib.syntax.Type.STRING_LIST;

import com.google.common.collect.ImmutableList;
import com.google.devtools.build.lib.analysis.BaseRuleClasses;
import com.google.devtools.build.lib.analysis.PlatformConfiguration;
import com.google.devtools.build.lib.analysis.RuleContext;
import com.google.devtools.build.lib.analysis.RuleDefinition;
import com.google.devtools.build.lib.analysis.RuleDefinitionEnvironment;
import com.google.devtools.build.lib.packages.Attribute.ComputedDefault;
import com.google.devtools.build.lib.packages.AttributeMap;
import com.google.devtools.build.lib.packages.NonconfigurableAttributeMapper;
import com.google.devtools.build.lib.packages.RuleClass;
import com.google.devtools.build.lib.syntax.Type;

/**
 * Definitions for rule classes that specify or manipulate configuration settings.
 *
 * <p>These are not "traditional" rule classes in that they can't be requested as top-level
 * targets and don't translate input artifacts into output artifacts. Instead, they affect
 * how *other* rules work. See individual class comments for details.
 */
public class ConfigRuleClasses {

  private static final String NONCONFIGURABLE_ATTRIBUTE_REASON =
      "part of a rule class that *triggers* configurable behavior";

  /**
   * Common settings for all configurability rules.
   */
  public static final class ConfigBaseRule implements RuleDefinition {
    @Override
    public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env) {
      return builder
          .override(attr("tags", Type.STRING_LIST)
               // No need to show up in ":all", etc. target patterns.
              .value(ImmutableList.of("manual"))
              .nonconfigurable(NONCONFIGURABLE_ATTRIBUTE_REASON))
          .exemptFromConstraintChecking(
              "these rules don't include content that gets built into their dependers")
          .build();
    }

    @Override
    public Metadata getMetadata() {
      return RuleDefinition.Metadata.builder()
          .name("$config_base_rule")
          .type(RuleClass.Builder.RuleClassType.ABSTRACT)
          .ancestors(BaseRuleClasses.BaseRule.class)
          .build();
    }
  }

  /**
   * A named "partial configuration setting" that specifies a set of command-line "flag=value"
   * bindings.
   *
   * <p>For example:
   *
   * <pre>
   *   config_setting(
   *       name = 'foo',
   *       values = {
   *           'flag1': 'aValue'
   *           'flag2': 'bValue'
   *       })
   * </pre>
   *
   * <p>declares a setting that binds command-line flag
   *
   * <pre>flag1</pre>
   *
   * to value
   *
   * <pre>aValue</pre>
   *
   * and
   *
   * <pre>flag2</pre>
   *
   * to
   *
   * <pre>bValue</pre>
   *
   * .
   *
   * <p>This is used by configurable attributes to determine which branch to follow based on which
   *
   * <pre>config_setting</pre>
   *
   * instance matches all its flag values in the configurable attribute owner's configuration.
   *
   * <p>This rule isn't accessed through the standard {@link RuleContext#getPrerequisites}
   * interface. This is because Bazel constructs a rule's configured attribute map *before* its
   * {@link RuleContext} is created (in fact, the map is an input to the context's constructor). And
   * the config_settings referenced by the rule's configurable attributes are themselves inputs to
   * that map. So Bazel has special logic to read and properly apply config_setting instances. See
   * {@link com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction#getConfigConditions} for
   * details.
   */
  public static final class ConfigSettingRule implements RuleDefinition {
    /**
     * The name of this rule.
     */
    public static final String RULE_NAME = "config_setting";

    /** The name of the attribute that declares flag bindings. */
    public static final String SETTINGS_ATTRIBUTE = "values";
    /** The name of the attribute that declares "--define foo=bar" flag bindings.*/
    public static final String DEFINE_SETTINGS_ATTRIBUTE = "define_values";
    /** The name of the attribute that declares user-defined flag bindings. */
    public static final String FLAG_SETTINGS_ATTRIBUTE = "flag_values";
    /** The name of the attribute that declares constraint_values. */
    public static final String CONSTRAINT_VALUES_ATTRIBUTE = "constraint_values";

    /** The name of the tools repository. */
    public static final String TOOLS_REPOSITORY_ATTRIBUTE = "$tools_repository";

    @Override
    public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env) {
      return builder
          .setIgnoreLicenses()
          .requiresConfigurationFragments(PlatformConfiguration.class)
          .add(
              attr(TOOLS_REPOSITORY_ATTRIBUTE, STRING)
                  .value(
                      new ComputedDefault() {
                        @Override
                        public Object getDefault(AttributeMap rule) {
                          return env.getToolsRepository();
                        }
                      }))

          /* <!-- #BLAZE_RULE(config_setting).ATTRIBUTE(values) -->
          The set of configuration values that match this rule (expressed as Bazel flags)

          <p>This rule inherits the configuration of the configured target that
            references it in a <code>select</code> statement. It is considered to
            "match" a Bazel invocation if, for every entry in the dictionary, its
            configuration matches the entry's expected value. For example
            <code>values = {"compilation_mode": "opt"}</code> matches the invocations
            <code>bazel build --compilation_mode=opt ...</code> and
            <code>bazel build -c opt ...</code> on target-configured rules.
          </p>

          <p>For convenience's sake, configuration values are specified as Bazel flags (without
            the preceding <code>"--"</code>). But keep in mind that the two are not the same. This
            is because targets can be built in multiple configurations within the same
            build. For example, a host configuration's "cpu" matches the value of
            <code>--host_cpu</code>, not <code>--cpu</code>. So different instances of the
            same <code>config_setting</code> may match the same invocation differently
            depending on the configuration of the rule using them.
          </p>

          <p>If a flag is not explicitly set at the command line, its default value is used.
             If a key appears multiple times in the dictionary, only the last instance is used.
             If a key references a flag that can be set multiple times on the command line (e.g.
             <code>bazel build --copt=foo --copt=bar --copt=baz ...</code>), a match occurs if
             <i>any</i> of those settings match.
          <p>

          <p>This and <a href="${link config_setting.define_values}"><code>define_values</code></a>
             cannot both be empty.
          </p>
          <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
          .add(
              attr(SETTINGS_ATTRIBUTE, STRING_DICT)
                  .nonconfigurable(NONCONFIGURABLE_ATTRIBUTE_REASON))
          /* <!-- #BLAZE_RULE(config_setting).ATTRIBUTE(define_values) -->
          The same as <a href="${link config_setting.values}"><code>values</code></a> but
          specifically for the <code>--define</code> flag.

          <p><code>--define</code> is special because its syntax (<code>--define KEY=VAL</code>)
            means <code>KEY=VAL</code> is a <i>value</i> from a Bazel flag perspective.
          </p>

          <p>That means:

          <pre class="code">
            config_setting(
                name = "a_and_b",
                values = {
                    "define": "a=1",
                    "define": "b=2",
                })
          </pre>

          <p>doesn't work because the same key (<code>define</code>) appears twice in the
          dictionary. This attribute solves that problem:

          <pre class="code">
            config_setting(
                name = "a_and_b",
                define_values = {
                    "a": "1",
                    "b": "2",
                })
          </pre>

          <p>correctly matches <code>bazel build //foo --define a=1 --define b=2</code>.

          <p><code>--define</code> can still appear in
          <a href="${link config_setting.values}"><code>values</code></a> with normal flag syntax,
          and can be mixed freely with this attribute as long as dictionary keys remain distinct.
          <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
          .add(
              attr(DEFINE_SETTINGS_ATTRIBUTE, STRING_DICT)
                  .nonconfigurable(NONCONFIGURABLE_ATTRIBUTE_REASON))

          /* <!-- #BLAZE_RULE(config_setting).ATTRIBUTE(flag_values) -->
          The same as <a href="${link config_setting.values}"><code>values</code></a> but
          for <a href="https://docs.bazel.build/versions/master/skylark/config.html#user-defined-build-settings">
          Starlark-defined flags</a>.
          <!-- #END_BLAZE_RULE.ATTRIBUTE --> */

          // Originally this attribute was a map of feature flags targets -> feature flag values,
          // the latter of which are always strings. Now it also includes starlark build setting
          // targets -> starlark build setting values. In other places in the starlark configuration
          // API, starlark setting values are passed as their actual object instead of a string
          // representation. It would be more consistent to be able to pass starlark setting values
          // as objects to this attribute as well. But attributes are strongly-typed so
          // label->object dict is not an option for attribute types right now.
          .add(
              attr(FLAG_SETTINGS_ATTRIBUTE, LABEL_KEYED_STRING_DICT)
                  .allowedFileTypes()
                  .nonconfigurable(NONCONFIGURABLE_ATTRIBUTE_REASON))
          /* <!-- #BLAZE_RULE(config_setting).ATTRIBUTE(constraint_values) -->
          The minimum set of <code>constraint_values</code> that the target platform must specify
          in order to match this <code>config_setting</code>. (The execution platform is not
          considered here.) Any additional constraint values that the platform has are ignored. See
          <a href="https://docs.bazel.build/versions/master/configurable-attributes.html#platforms">
          Configurable Build Attributes</a> for details.

          <p>In the case where two <code>config_setting</code>s both match in the same
          <code>select</code>, this attribute is not considered for the purpose of determining
          whether one of the <code>config_setting</code>s is a specialization of the other. In other
          words, one <code>config_setting</code> cannot match a platform more strongly than another.
          <!-- #END_BLAZE_RULE.ATTRIBUTE --> */
          .add(
              attr(CONSTRAINT_VALUES_ATTRIBUTE, LABEL_LIST)
                  .nonconfigurable(NONCONFIGURABLE_ATTRIBUTE_REASON)
                  .allowedFileTypes())
          .setOptionReferenceFunctionForConfigSettingOnly(
              rule ->
                  NonconfigurableAttributeMapper.of(rule)
                      .get(SETTINGS_ATTRIBUTE, Type.STRING_DICT)
                      .keySet())
          .build();
    }

    @Override
    public Metadata getMetadata() {
      return RuleDefinition.Metadata.builder()
          .name(RULE_NAME)
          .type(RuleClass.Builder.RuleClassType.NORMAL)
          .ancestors(ConfigBaseRule.class)
          .factoryClass(ConfigSetting.class)
          .build();
    }
  }
  /*<!-- #BLAZE_RULE (NAME = config_setting, TYPE = OTHER, FAMILY = General)[GENERIC_RULE] -->

  <p>
    Matches an expected configuration state (expressed as Bazel flags or platform constraints) for
    the purpose of triggering configurable attributes. See <a href="${link select}">select</a> for
    how to consume this rule and <a href="${link common-definitions#configurable-attributes}">
    Configurable attributes</a> for an overview of the general feature.

  <h4 id="config_setting_examples">Examples</h4>

  <p>The following matches any Bazel invocation that specifies <code>--compilation_mode=opt</code>
     or <code>-c opt</code> (either explicitly at the command line or implicitly from .blazerc
     files):
  </p>

  <pre class="code">
  config_setting(
      name = "simple",
      values = {"compilation_mode": "opt"}
  )
  </pre>

  <p>The following matches any Bazel invocation that builds for ARM and that applies the custom
     define <code>FOO=bar</code> (for instance, <code>bazel build --cpu=arm --define FOO=bar ...
     </code>):
  </p>

  <pre class="code">
  config_setting(
      name = "two_conditions",
      values = {
          "cpu": "arm",
          "define": "FOO=bar"
      }
  )
  </pre>

  <p>The following matches any Bazel invocation that builds for a platform that has an x86_64
     architecture and glibc version 2.25, assuming the existence of a <code>constraint_value</code>
     with label <code>//example:glibc_2_25</code>. Note that a platform still matches if it defines
     additional constraint values beyond these two.
  </p>

  <pre class=""code">
  config_setting(
      name = "64bit_glibc_2_25",
      constraint_values = [
          "@platforms//cpu:x86_64",
          "//example:glibc_2_25",
      ]
  )
  </pre>

  In all these cases, it is possible for the configuration state to change during the build, for
  instance if a dependency of a target needs to be built for a different platform than the target
  itself. This means that even when a <code>config_setting</code> does not match the top-level
  command-line flags, it may still match a deeper part of the build, and vice versa.

  <h4 id="config_setting_notes">Notes</h4>

  <p>See <a href="${link select}">select</a> for what happens when multiple
     <code>config_setting</code>s match the current configuration state.
  </p>

  <p>For flags that support shorthand forms (e.g. <code>--compilation_mode</code> vs.
    <code>-c</code>), <code>values</code> definitions must use the full form. These automatically
    match invocations using either form.
  </p>

  <p>
    If a flag takes multiple values (like <code>--define a=b --define c=d</code> or a list-typed
    <a href="https://docs.bazel.build/versions/master/skylark/config.html#user-defined-build-settings">
    Starlark flag</a>), a <code>config_setting</code> that expects <code>"foo"</code> matches if
    <code>"foo"</code> is present <i>anywhere</i> in the actual list.
  </p>

  <p>If you need to define conditions that aren't modeled by built-in Bazel flags, use
  <a href="https://docs.bazel.build/versions/master/skylark/config.html#user-defined-build-settings">
  Starlark-defined flags</a>. You can also use <code>--define</code>, but this offers weaker
  support and is not recommended. See <a href="${link common-definitions#configurable-attributes}">
  for more discussion.
  </p>

  <p>Avoid repeating identical <code>config_setting</code> definitions in different packages.
    Instead, prefer to reference a common <code>config_setting</code> target that is defined in a
    single package.
  </p>

  <p><a href="general.html#config_setting.values"><code>values</code></a>,
     <a href="general.html#config_setting.define_values"><code>define_values</code></a>, and
     <a href="general.html#config_setting.constraint_values"><code>constraint_values</code></a>
     can be used in any combination in the same <code>config_setting</code> but at least one must be
     set for any given <code>config_setting</code>.
  </p>
  <!-- #END_BLAZE_RULE -->*/

  /** Rule definition for Android's config_feature_flag rule. */
  public static final class ConfigFeatureFlagRule implements RuleDefinition {
    public static final String RULE_NAME = "config_feature_flag";

    @Override
    public RuleClass build(RuleClass.Builder builder, RuleDefinitionEnvironment env) {
      return builder
          .setUndocumented(/* the feature flag feature has not yet been launched */)
          .requiresConfigurationFragments(ConfigFeatureFlagConfiguration.class)
          .add(
              attr("allowed_values", STRING_LIST)
                  .mandatory()
                  .nonEmpty()
                  .orderIndependent()
                  .nonconfigurable(NONCONFIGURABLE_ATTRIBUTE_REASON))
          .add(
              attr("default_value", STRING)
                  .nonconfigurable(NONCONFIGURABLE_ATTRIBUTE_REASON))
          .add(ConfigFeatureFlag.getWhitelistAttribute(env))
          .removeAttribute(BaseRuleClasses.TAGGED_TRIMMING_ATTR)
          .build();
    }

    @Override
    public RuleDefinition.Metadata getMetadata() {
      return RuleDefinition.Metadata.builder()
          .name(RULE_NAME)
          .ancestors(ConfigBaseRule.class)
          .factoryClass(ConfigFeatureFlag.class)
          .build();
    }
  }
}