Fixes to protoc inputs to GenerateProtoTask

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

@@ -324,19 +324,74 @@ public abstract class GenerateProtoTask extends DefaultTask {
     return variant
   }
 
-  @Internal("Input captured by getAlternativePaths()")
+  @Internal("Input captured by getExecutables()")
   abstract Property<ExecutableLocator> getProtocLocator()
 
-  @Internal("Input captured by getAlternativePaths(), this is used to query alternative path by locator name.")
+  @Internal("Input captured by getSnapshotArtifacts(), this is used to query alternative path by locator name.")
   abstract MapProperty<String, FileCollection> getLocatorToAlternativePathsMapping()
 
-  @InputFiles
-  @PathSensitive(PathSensitivity.NONE)
+  @Internal("Input captured by getReleaseDependenciesMapping()")
+  abstract MapProperty<String, String> getLocatorToDependencyMapping()
+
+  @Internal("This property is no longer an input, but kept and marked @Internal for backwards compatibility.")
   ConfigurableFileCollection getAlternativePaths() {
     return objectFactory.fileCollection().from(getLocatorToAlternativePathsMapping().get().values())
   }
 
-  @Internal("Input captured by getAlternativePaths()")
+  /**
+   * For each protoc and code gen plugin defined by an artifact specification, this list will contain a String with the
+   * group, artifact, and version, as long as the version is a stable release version.
+   *
+   * Giving this as an input to the task allows gradle to ignore the OS classifier and use cached outputs generated from
+   * different operating systems since the expectation is that different operating systems will produce the same
+   * generated code.
+   */
+  @Input
+  Provider<List<String>> getReleaseArtifacts() {
+    releaseDependenciesMapping.map { it.values().collect() }
+  }
+
+  /**
+   * This file collection contains the file for each protoc and code gen plugin that is defined by an artifact
+   * specification that specifies a SNAPSHOT version.
+   *
+   * Since snapshots are expected to differ within the same version, this input allows Gradle to consider the file
+   * itself rather than the version number.
+   */
+  @InputFiles
+  @PathSensitive(PathSensitivity.NONE)
+  FileCollection getSnapshotArtifacts() {
+    Provider<Collection<FileCollection>> snapshotArtifacts = locatorToAlternativePathsMapping.map { map ->
+      Set<String> releaseArtifactKeys = releaseDependenciesMapping.get().keySet()
+      map.findAll { entry ->
+        !releaseArtifactKeys.contains(entry.key)
+      }.values()
+    }
+
+    objectFactory.fileCollection().from(snapshotArtifacts)
+  }
+
+  @Internal
+  Provider<Map<String, String>> getReleaseDependenciesMapping() {
+    providerFactory.provider {
+      locatorToDependencyMapping.get()
+          .findAll { entry -> ! entry.value.endsWith ("-SNAPSHOT") }
+    }
+  }
+
+  @InputFiles
+  @PathSensitive(PathSensitivity.NONE)
+  FileCollection getExecutables() {
+    objectFactory.fileCollection().from {
+      protocLocator.getOrNull()?.path
+    }.from {
+      pluginsExecutableLocators.get().values()
+        .collect { it.path }
+        .findAll { it }
+    }
+  }
+
+  @Internal("Input captured by getExecutables()")
   abstract MapProperty<String, ExecutableLocator> getPluginsExecutableLocators()
 
   @Internal("Not an actual input to the task, only used to find tasks belonging to a variant")

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

@@ -112,6 +112,7 @@ class ToolsLocator {
     for (GenerateProtoTask protoTask in protoTasks) {
       if (protoc.is(locator) || protoTask.hasPlugin(locator.name)) {
         protoTask.locatorToAlternativePathsMapping.put(locator.name, artifactFiles)
+        protoTask.locatorToDependencyMapping.put(locator.name, "$groupId:$artifact:$version".toString())
       }
     }
   }

src/test/groovy/com/google/protobuf/gradle/ProtobufJavaPluginTest.groovy

@@ -414,6 +414,131 @@ class ProtobufJavaPluginTest extends Specification {
     gradleVersion << GRADLE_VERSIONS
   }
 
+  @Unroll
+  void "test proto generation is not up-to-date on dependency changes [gradle #gradleVersion]"() {
+    given: "project from testProject"
+    File projectDir = ProtobufPluginTestHelper.projectBuilder('testProject')
+            .copyDirs('testProjectBase', 'testProject')
+            .build()
+
+    when: "build is invoked"
+    BuildResult result = GradleRunner.create()
+            .withProjectDir(projectDir)
+            .withArguments('build', '--stacktrace')
+            .withPluginClasspath()
+            .withGradleVersion(gradleVersion)
+            .forwardStdOutput(new OutputStreamWriter(System.out))
+            .forwardStdError(new OutputStreamWriter(System.err))
+            .withDebug(true)
+            .build()
+
+    then: "it succeeds"
+    result.task(":build").outcome == TaskOutcome.SUCCESS
+
+    when: "protoc artifact is changed and build runs again"
+    new File(projectDir, "build.gradle")
+            .append("""
+              protobuf {
+                protoc {
+                  artifact = 'com.google.protobuf:protoc:3.0.2'
+                }
+              }""")
+    result = GradleRunner.create()
+            .withProjectDir(projectDir)
+            .withArguments('build', '--stacktrace')
+            .withPluginClasspath()
+            .withGradleVersion(gradleVersion)
+            .forwardStdOutput(new OutputStreamWriter(System.out))
+            .forwardStdError(new OutputStreamWriter(System.err))
+            .withDebug(true)
+            .build()
+
+    then: "generateProto is not UP_TO_DATE"
+    result.task(":generateProto").outcome == TaskOutcome.SUCCESS
+
+    when: "plugin artifact is changed and build runs again"
+    new File(projectDir, "build.gradle")
+            .append("""
+              protobuf {
+                plugins {
+                  grpc {
+                    artifact = 'io.grpc:protoc-gen-grpc-java:1.0.3'
+                  }
+                }
+              }""")
+    result = GradleRunner.create()
+            .withProjectDir(projectDir)
+            .withArguments('build', '--stacktrace')
+            .withPluginClasspath()
+            .withGradleVersion(gradleVersion)
+            .forwardStdOutput(new OutputStreamWriter(System.out))
+            .forwardStdError(new OutputStreamWriter(System.err))
+            .withDebug(true)
+            .build()
+
+    then: "generateProto is not UP_TO_DATE"
+    result.task(":generateGrpcProto").outcome == TaskOutcome.SUCCESS
+
+    where:
+    gradleVersion << GRADLE_VERSIONS
+  }
+
+  @Unroll
+  void "test proto generation is not up-to-date on path changes [gradle #gradleVersion]"() {
+    given: "project from testProject"
+    File projectDir = ProtobufPluginTestHelper.projectBuilder('testProject')
+            .copyDirs('testProjectBase', 'testProject')
+            .build()
+
+    when: "protoc path is set and build is invoked"
+    File buildGradleFile = new File(projectDir, "build.gradle")
+    buildGradleFile.append("""
+        configurations {
+          protoc
+        }
+
+        dependencies {
+          protoc "com.google.protobuf:protoc:3.0.0:\$project.osdetector.classifier@exe"
+        }
+
+        protobuf {
+          protoc {
+            path = "\$configurations.protoc.singleFile"
+          }
+        }""")
+    BuildResult result = GradleRunner.create()
+            .withProjectDir(projectDir)
+            .withArguments('build', '--stacktrace')
+            .withPluginClasspath()
+            .withGradleVersion(gradleVersion)
+            .forwardStdOutput(new OutputStreamWriter(System.out))
+            .forwardStdError(new OutputStreamWriter(System.err))
+            .withDebug(true)
+            .build()
+
+    then: "it succeeds"
+    result.task(":generateProto").outcome == TaskOutcome.SUCCESS
+
+    when: "protoc path is changed and build runs again"
+    buildGradleFile.text = buildGradleFile.text.replace("com.google.protobuf:protoc:3.0.0",
+        "com.google.protobuf:protoc:3.0.2")
+    result = GradleRunner.create()
+            .withProjectDir(projectDir)
+            .withArguments('build', '--stacktrace')
+            .withPluginClasspath()
+            .withGradleVersion(gradleVersion)
+            .forwardStdOutput(new OutputStreamWriter(System.out))
+            .forwardStdError(new OutputStreamWriter(System.err))
+            .withDebug(true)
+            .build()
+
+    then: "generateProto is not UP_TO_DATE"
+    result.task(":generateProto").outcome == TaskOutcome.SUCCESS
+
+    where:
+    gradleVersion << GRADLE_VERSIONS
+  }
+
   @Unroll
   void "test proto extraction is up-to-date for testProject when changing java sources [gradle #gradleVersion]"() {
     given: "project from testProject"