src/main/java/com/google/devtools/build/lib/packages/SkylarkProvider.java

// Copyright 2016 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.packages;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;
import com.google.devtools.build.lib.syntax.BaseFunction;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.syntax.FunctionSignature;
import com.google.devtools.build.lib.syntax.Location;
import com.google.devtools.build.lib.syntax.Printer;
import com.google.devtools.build.lib.syntax.Starlark;
import com.google.devtools.build.lib.syntax.StarlarkThread;
import java.util.Collection;
import java.util.Map;
import java.util.Objects;
import javax.annotation.Nullable;

/**
 * A provider defined in Starlark rather than in native code.
 *
 * <p>This is a result of calling the {@code provider()} function from Starlark ({@link
 * com.google.devtools.build.lib.analysis.skylark.SkylarkRuleClassFunctions#provider}).
 *
 * <p>{@code SkylarkProvider}s may be either schemaless or schemaful. Instances of schemaless
 * providers can have any set of fields on them, whereas instances of schemaful providers may have
 * only the fields that are named in the schema.
 *
 * <p>Exporting a {@code SkylarkProvider} creates a key that is used to uniquely identify it.
 * Usually a provider is exported by calling {@link #export}, but a test may wish to just create a
 * pre-exported provider directly. Exported providers use only their key for {@link #equals} and
 * {@link #hashCode}.
 */
public final class SkylarkProvider extends BaseFunction implements SkylarkExportable, Provider {

  /** Default value for {@link #errorMessageFormatForUnknownField}. */
  private static final String DEFAULT_ERROR_MESSAGE_FORMAT = "Object has no '%s' attribute.";

  private final Location location;
  private final FunctionSignature signature;

  /** For schemaful providers, the sorted list of allowed field names. */
  // (The requirement for sortedness comes from SkylarkInfo.fromSortedFieldList,
  // as it permits it to use the same list of names both to interpret the
  // call arguments and to populate the SkylarkInfo.table without temporaries.)
  @Nullable private final ImmutableList<String> fields;

  /** Null iff this provider has not yet been exported. */
  @Nullable
  private SkylarkKey key;

  /** Error message format. Reassigned upon exporting. */
  private String errorMessageFormatForUnknownField;

  /**
   * Creates an unexported {@link SkylarkProvider} with no schema.
   *
   * <p>The resulting object needs to be exported later (via {@link #export}).
   *
   * @param location the location of the Starlark definition for this provider (tests may use {@link
   *     Location#BUILTIN})
   */
  public static SkylarkProvider createUnexportedSchemaless(Location location) {
    return new SkylarkProvider(/*key=*/ null, /*fields=*/ null, location);
  }

  /**
   * Creates an unexported {@link SkylarkProvider} with a schema.
   *
   * <p>The resulting object needs to be exported later (via {@link #export}).
   *
   * @param fields the allowed field names for instances of this provider
   * @param location the location of the Starlark definition for this provider (tests may use {@link
   *     Location#BUILTIN})
   */
  // TODO(adonovan): in what sense is this "schemaful" if fields is null?
  public static SkylarkProvider createUnexportedSchemaful(
      @Nullable Collection<String> fields, Location location) {
    return new SkylarkProvider(
        /*key=*/ null, fields == null ? null : ImmutableList.sortedCopyOf(fields), location);
  }

  /**
   * Creates an exported {@link SkylarkProvider} with no schema.
   *
   * @param key the key that identifies this provider
   * @param location the location of the Starlark definition for this provider (tests may use {@link
   *     Location#BUILTIN})
   */
  public static SkylarkProvider createExportedSchemaless(SkylarkKey key, Location location) {
    return new SkylarkProvider(key, /*fields=*/ null, location);
  }

  /**
   * Creates an exported {@link SkylarkProvider} with no schema.
   *
   * @param key the key that identifies this provider
   * @param fields the allowed field names for instances of this provider
   * @param location the location of the Starlark definition for this provider (tests may use {@link
   *     Location#BUILTIN})
   */
  // TODO(adonovan): in what sense is this "schemaful" if fields is null?
  public static SkylarkProvider createExportedSchemaful(
      SkylarkKey key, @Nullable Collection<String> fields, Location location) {
    return new SkylarkProvider(
        key, fields == null ? null : ImmutableList.sortedCopyOf(fields), location);
  }

  /**
   * Constructs the provider.
   *
   * <p>If {@code key} is null, the provider is unexported. If {@code fields} is null, the provider
   * is schemaless.
   */
  private SkylarkProvider(
      @Nullable SkylarkKey key, @Nullable ImmutableList<String> fields, Location location) {
    this.signature = buildSignature(fields);
    this.location = location;
    this.fields = fields;
    this.key = key;  // possibly null
    this.errorMessageFormatForUnknownField =
        key == null ? DEFAULT_ERROR_MESSAGE_FORMAT
            : makeErrorMessageFormatForUnknownField(key.getExportedName());
  }

  @Override
  public FunctionSignature getSignature() {
    return signature;
  }

  private static FunctionSignature buildSignature(@Nullable ImmutableList<String> fields) {
    return fields == null
        ? FunctionSignature.KWARGS // schemaless
        : FunctionSignature.namedOnly(0, fields.toArray(new String[0]));
  }

  @Override
  public Object fastcall(StarlarkThread thread, Object[] positional, Object[] named)
      throws EvalException, InterruptedException {
    // TODO(adonovan): we can likely come up with a more efficient implementation
    // ...then make matchSignature private again?
    Object[] arguments =
        Starlark.matchSignature(
            signature, this, /*defaults=*/ null, thread.mutability(), positional, named);
    Location loc = thread.getCallerLocation();
    if (fields == null) {
      // provider(**kwargs)
      @SuppressWarnings("unchecked")
      Map<String, Object> kwargs = (Map<String, Object>) arguments[0];
      return SkylarkInfo.create(this, kwargs, loc);
    } else {
      // provider(a=..., b=..., ...)
      // The order of args is determined by the signature: that is, fields, which is sorted.
      return SkylarkInfo.fromSortedFieldList(this, fields, arguments, loc);
    }
  }

  @Override
  public Location getLocation() {
    return location;
  }

  @Override
  public boolean isExported() {
    return key != null;
  }

  @Override
  public SkylarkKey getKey() {
    Preconditions.checkState(isExported());
    return key;
  }

  @Override
  public String getName() {
    return key != null ? key.getExportedName() : "<no name>";
  }

  @Override
  public String getPrintableName() {
    return getName();
  }

  /** Returns the list of fields allowed by this provider, or null if the provider is schemaless. */
  @Nullable
  public ImmutableList<String> getFields() {
    return fields;
  }

  @Override
  public String getErrorMessageFormatForUnknownField() {
    return errorMessageFormatForUnknownField;
  }

  @Override
  public void export(Label extensionLabel, String exportedName) {
    Preconditions.checkState(!isExported());
    this.key = new SkylarkKey(extensionLabel, exportedName);
    this.errorMessageFormatForUnknownField = makeErrorMessageFormatForUnknownField(exportedName);
  }

  private static String makeErrorMessageFormatForUnknownField(String exportedName) {
    return String.format("'%s' value has no field or method '%%s'", exportedName);
  }

  @Override
  public int hashCode() {
    if (isExported()) {
      return getKey().hashCode();
    }
    return System.identityHashCode(this);
  }

  @Override
  public boolean equals(@Nullable Object otherObject) {
    if (!(otherObject instanceof SkylarkProvider)) {
      return false;
    }
    SkylarkProvider other = (SkylarkProvider) otherObject;

    if (this.isExported() && other.isExported()) {
      return this.getKey().equals(other.getKey());
    } else {
      return this == other;
    }
  }

  @Override
  public boolean isImmutable() {
    // Hash code for non exported constructors may be changed
    return isExported();
  }

  @Override
  public void repr(Printer printer) {
    printer.append("<provider>");
  }

  /**
   * A serializable representation of Starlark-defined {@link SkylarkProvider} that uniquely
   * identifies all {@link SkylarkProvider}s that are exposed to SkyFrame.
   */
  @AutoCodec
  public static class SkylarkKey extends Key {
    private final Label extensionLabel;
    private final String exportedName;

    public SkylarkKey(Label extensionLabel, String exportedName) {
      this.extensionLabel = Preconditions.checkNotNull(extensionLabel);
      this.exportedName = Preconditions.checkNotNull(exportedName);
    }

    public Label getExtensionLabel() {
      return extensionLabel;
    }

    public String getExportedName() {
      return exportedName;
    }

    @Override
    public String toString() {
      return exportedName;
    }

    @Override
    public int hashCode() {
      return Objects.hash(extensionLabel, exportedName);
    }

    @Override
    public boolean equals(Object obj) {
      if (this == obj) {
        return true;
      }

      if (!(obj instanceof SkylarkKey)) {
        return false;
      }
      SkylarkKey other = (SkylarkKey) obj;
      return Objects.equals(this.extensionLabel, other.extensionLabel)
          && Objects.equals(this.exportedName, other.exportedName);
    }
  }
}