Make compileProtoPath variant selection criteria tighter (#489)

Add usage attribute to compileProtoPath, which is used to resolve dependencies inherited from compileOnly and implementation, for extracting included proto files. Setting attributes too loosely can lead to ambiguous variant selection errors as more than one variants of the dependency can be matched. Adding JAVA_RUNTIME usage attribute, which would select the same variant as by compileClasspath configuration.

src/main/groovy/com/google/protobuf/gradle/ProtobufPlugin.groovy

@@ -40,6 +40,7 @@ import org.gradle.api.Task
 import org.gradle.api.artifacts.Configuration
 import org.gradle.api.attributes.Attribute
 import org.gradle.api.attributes.LibraryElements
+import org.gradle.api.attributes.Usage
 import org.gradle.api.file.FileCollection
 import org.gradle.api.file.SourceDirectorySet
 import org.gradle.api.internal.file.FileResolver
@@ -179,7 +180,7 @@ class ProtobufPlugin implements Plugin<Project> {
      * Creates an internal 'compileProtoPath' configuration for the given source set that extends
      * compilation configurations as a bucket of dependencies with resources attribute.
      * The extract-include-protos task of each source set will extract protobuf files from
-     * dependencies in this configuration.
+     * resolved dependencies in this configuration.
      *
      * <p> For Java projects only.
      * <p> This works around 'java-library' plugin not exposing resources to consumers for compilation.
@@ -196,8 +197,22 @@ class ProtobufPlugin implements Plugin<Project> {
             transitive = true
             extendsFrom = [compileConfig, implementationConfig]
             canBeConsumed = false
-        }.getAttributes().attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE,
-                project.getObjects().named(LibraryElements, LibraryElements.RESOURCES))
+        }.getAttributes()
+                // Variant attributes are not inherited. Setting it too loosely can
+                // result in ambiguous variant selection errors.
+                // CompileProtoPath only need proto files from dependency's resources.
+                // LibraryElement "resources" is compatible with "jar" (if a "resources" variant is
+                // not found, the "jar" variant will be used).
+                .attribute(
+                        LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE,
+                        project.getObjects().named(LibraryElements, LibraryElements.RESOURCES))
+                // Although variants with any usage has proto files, not setting usage attribute
+                // can result in ambiguous variant selection if the producer provides multiple
+                // variants with different usage attribute.
+                // Preserve the usage attribute from CompileOnly and Implementation.
+                .attribute(
+                        Usage.USAGE_ATTRIBUTE,
+                        project.getObjects().named(Usage, Usage.JAVA_RUNTIME))
       }
     }