ProtoConfiguration: Expose some fields to Starlark

This change exposes the values of `--protoc` and `--proto_options` to
Starlark, thus making it possible to implement `proto_toolchain` in
Starlark. See bazelbuild/rules_proto#82 for the
implementation of `proto_toolchain`.

Working towards #10005

src/main/java/com/google/devtools/build/lib/rules/proto/BUILD

@@ -40,6 +40,8 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/analysis:rule_definition_environment",
         "//src/main/java/com/google/devtools/build/lib/analysis:transitive_info_collection",
         "//src/main/java/com/google/devtools/build/lib/analysis:transitive_info_provider",
+        "//src/main/java/com/google/devtools/build/lib/analysis/platform",
+        "//src/main/java/com/google/devtools/build/lib/analysis/starlark/annotations",
         "//src/main/java/com/google/devtools/build/lib/analysis/stringtemplate",
         "//src/main/java/com/google/devtools/build/lib/cmdline",
         "//src/main/java/com/google/devtools/build/lib/collect/nestedset",
@@ -54,6 +56,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/vfs",
         "//src/main/java/com/google/devtools/build/lib/vfs:pathfragment",
         "//src/main/java/com/google/devtools/common/options",
+        "//src/main/java/net/starlark/java/eval",
         "//src/main/java/net/starlark/java/syntax",
         "//third_party:auto_value",
         "//third_party:guava",

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoConfiguration.java

@@ -21,6 +21,7 @@
 import com.google.devtools.build.lib.analysis.config.Fragment;
 import com.google.devtools.build.lib.analysis.config.FragmentOptions;
 import com.google.devtools.build.lib.analysis.config.RequiresOptions;
+import com.google.devtools.build.lib.analysis.starlark.annotations.StarlarkConfigurationField;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
 import com.google.devtools.build.lib.starlarkbuildapi.ProtoConfigurationApi;
@@ -204,6 +205,7 @@ public ProtoConfiguration(BuildOptions buildOptions) {
     this.options = options;
   }
 
+  @Override
   public ImmutableList<String> protocOpts() {
     return protocOpts;
   }
@@ -221,6 +223,11 @@ public boolean runExperimentalProtoExtraActions() {
     return options.experimentalProtoExtraActions;
   }
 
+  // Must match `@rules_proto//proto/private/rules:proto_toolchain.bzl`.
+  @StarlarkConfigurationField(
+      name = "protoc",
+      doc = "Exposes the value of `--proto_compiler`.",
+      documented = false)
   public Label protoCompiler() {
     return options.protoCompiler;
   }

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoToolchainInfo.java

@@ -0,0 +1,62 @@
+// Copyright 2021 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.proto;
+
+import com.google.auto.value.AutoValue;
+import com.google.common.collect.ImmutableList;
+import com.google.devtools.build.lib.analysis.FilesToRunProvider;
+import com.google.devtools.build.lib.analysis.platform.ToolchainInfo;
+import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
+import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.Sequence;
+import net.starlark.java.eval.Starlark;
+
+/** Information about the tools used by the {@code proto_*} and {@code LANG_proto_*} rules. */
+@Immutable
+@AutoValue
+public abstract class ProtoToolchainInfo {
+  private static final String PROTOC_ATTR_NAME = "protoc";
+  private static final String PROTOC_OPTIONS_ATTR_NAME = "protoc_options";
+
+  /** The {@code protoc} binary to use for proto actions. */
+  public abstract FilesToRunProvider getProtoc();
+
+  /** Additional options to pass to {@code protoc}. */
+  public abstract ImmutableList<String> getProtocOptions();
+
+  /**
+   * Constructs a {@link ProtoToolchainInfo} from {@link ToolchainInfo} (i.e. as returned by {@code
+   * proto_toolchain} from {@code @rules_proto}).
+   */
+  public static ProtoToolchainInfo fromToolchainInfo(ToolchainInfo toolchain) throws EvalException {
+    FilesToRunProvider protoc = getValue(toolchain, PROTOC_ATTR_NAME, FilesToRunProvider.class);
+    ImmutableList<String> protocOptions =
+        Sequence.cast(
+                getValue(toolchain, PROTOC_OPTIONS_ATTR_NAME, Object.class),
+                String.class,
+                PROTOC_OPTIONS_ATTR_NAME)
+            .getImmutableList();
+    return new AutoValue_ProtoToolchainInfo(protoc, protocOptions);
+  }
+
+  private static <T> T getValue(ToolchainInfo toolchain, String name, Class<T> clazz)
+      throws EvalException {
+    T value = toolchain.getValue(name, clazz);
+    if (value == null) {
+      throw Starlark.errorf("Proto toolchain does not have mandatory field '%s'", name);
+    }
+    return value;
+  }
+}

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/ProtoConfigurationApi.java

@@ -14,13 +14,22 @@
 
 package com.google.devtools.build.lib.starlarkbuildapi;
 
+import com.google.common.collect.ImmutableList;
 import com.google.devtools.build.docgen.annot.DocCategory;
 import net.starlark.java.annot.StarlarkBuiltin;
+import net.starlark.java.annot.StarlarkMethod;
 import net.starlark.java.eval.StarlarkValue;
 
 /** A configuration fragment representing protocol buffers. */
 @StarlarkBuiltin(
     name = "proto",
     category = DocCategory.CONFIGURATION_FRAGMENT,
     doc = "A configuration fragment representing protocol buffers.")
-public interface ProtoConfigurationApi extends StarlarkValue {}
+public interface ProtoConfigurationApi extends StarlarkValue {
+  @StarlarkMethod(
+      name = "protoc_options",
+      doc = "Exposes the value of `--protocopt`.",
+      documented = false,
+      structField = true)
+  ImmutableList<String> protocOpts();
+}

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/proto/ProtoCommonApi.java

@@ -14,7 +14,9 @@
 
 package com.google.devtools.build.lib.starlarkbuildapi.proto;
 
+import com.google.common.collect.ImmutableList;
 import net.starlark.java.annot.StarlarkBuiltin;
+import net.starlark.java.annot.StarlarkMethod;
 import net.starlark.java.eval.StarlarkValue;
 
 /**
@@ -31,4 +33,16 @@
             + "to load this symbol from <a href=\"https://github.com/bazelbuild/rules_proto\">"
             + "rules_proto</a>"
             + "</p>")
-public interface ProtoCommonApi extends StarlarkValue {}
+public interface ProtoCommonApi extends StarlarkValue {
+  @Deprecated
+  @StarlarkMethod(
+      name = "available_configuration_fields",
+      doc =
+          "Indicates that this version exposes fields on this configuration fragment. Do not use "
+              + "this field, its only purpose is to help with migration.",
+      documented = false,
+      structField = true)
+  default ImmutableList<String> availableConfigurationFields() {
+    return ImmutableList.of("protoc", "protoc_options");
+  }
+}