Move .cxx directory out of android/app (#166277)

This is a build artifact, so move to `build/app/intermediates/flutter`.

See/ fixes https://github.com/flutter/flutter/issues/160372

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Features we expect every widget to implement]:
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[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
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[Data Driven Fixes]:
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---------

Co-authored-by: Gray Mackall <mackall@google.com>

dev/devicelab/bin/tasks/gradle_plugin_fat_apk_test.dart

@@ -152,6 +152,27 @@ Future<void> main() async {
             throw TaskResult.failure("Shared library doesn't exist");
           }
         }
+
+        section('AGP cxx build artifacts');
+
+        final String defaultPath = path.join(project.rootPath, 'android', 'app', '.cxx');
+
+        final String modifiedPath = path.join(
+          project.rootPath,
+          'build',
+          'app',
+          'intermediates',
+          'flutter',
+          '.cxx',
+        );
+        if (Directory(defaultPath).existsSync()) {
+          throw TaskResult.failure('Producing unexpected build artifacts in $defaultPath');
+        }
+        if (!Directory(modifiedPath).existsSync()) {
+          throw TaskResult.failure(
+            'Not producing external native build output directory in $modifiedPath',
+          );
+        }
       });
 
       return TaskResult.success(null);

packages/flutter_tools/gradle/src/main/kotlin/FlutterPluginUtils.kt

@@ -610,6 +610,23 @@ object FlutterPluginUtils {
             "$flutterSdkRootPath/packages/flutter_tools/gradle/src/main/groovy/CMakeLists.txt"
         )
 
+        // AGP defaults to outputting build artifacts in `android/app/.cxx`. This directory is a
+        // build artifact, so we move it from that directory to within Flutter's build directory
+        // to avoid polluting source directories with build artifacts.
+        //
+        // AGP explicitely recommends not setting the buildStagingDirectory to be within a build
+        // directory in
+        // https://developer.android.com/reference/tools/gradle-api/8.3/null/com/android/build/api/dsl/Cmake#buildStagingDirectory(kotlin.Any),
+        // but as we are not actually building anything (and are instead only tricking AGP into
+        // downloading the NDK), it is acceptable for the buildStagingDirectory to be removed
+        // and rebuilt when running clean builds.
+        gradleProjectAndroidExtension.externalNativeBuild.cmake.buildStagingDirectory(
+            gradleProject.layout.buildDirectory
+                .dir("${FlutterPluginConstants.INTERMEDIATES_DIR}/flutter/.cxx")
+                .get()
+                .asFile.path
+        )
+
         // CMake will print warnings when you try to build an empty project.
         // These arguments silence the warnings - our project is intentionally
         // empty.

packages/flutter_tools/gradle/src/test/kotlin/FlutterPluginUtilsTest.kt

@@ -22,6 +22,8 @@ import org.gradle.api.Project
 import org.gradle.api.Task
 import org.gradle.api.UnknownTaskException
 import org.gradle.api.artifacts.dsl.DependencyHandler
+import org.gradle.api.file.Directory
+import org.gradle.api.file.DirectoryProperty
 import org.gradle.api.logging.Logger
 import org.gradle.api.tasks.TaskContainer
 import org.gradle.api.tasks.TaskProvider
@@ -845,6 +847,8 @@ class FlutterPluginUtilsTest {
         val project = mockk<Project>()
         val mockCmakeOptions = mockk<CmakeOptions>()
         val mockDefaultConfig = mockk<DefaultConfig>()
+        val mockDirectoryProperty = mockk<DirectoryProperty>()
+        val mockDirectory = mockk<Directory>()
         every {
             project.extensions
                 .findByType(BaseExtension::class.java)!!
@@ -852,17 +856,24 @@ class FlutterPluginUtilsTest {
         } returns mockCmakeOptions
         every { project.extensions.findByType(BaseExtension::class.java)!!.defaultConfig } returns mockDefaultConfig
 
+        val basePath = "/base/path"
+        val fakeBuildPath = "/randomapp/build/app/"
         every { mockCmakeOptions.path } returns null
         every { mockCmakeOptions.path(any()) } returns Unit
         every { mockDefaultConfig.externalNativeBuild.cmake.arguments(any(), any()) } returns Unit
+        every { mockCmakeOptions.buildStagingDirectory(any()) } returns Unit
+        every { project.layout.buildDirectory } returns mockDirectoryProperty
+        every { mockDirectoryProperty.dir(any<String>()) } returns mockDirectoryProperty
+        every { mockDirectoryProperty.get() } returns mockDirectory
+        every { mockDirectory.asFile.path } returns fakeBuildPath
 
-        val basePath = "/base/path"
         FlutterPluginUtils.forceNdkDownload(project, basePath)
 
         verify(exactly = 1) {
             mockCmakeOptions.path
         }
         verify(exactly = 1) { mockCmakeOptions.path("$basePath/packages/flutter_tools/gradle/src/main/groovy/CMakeLists.txt") }
+        verify(exactly = 1) { mockCmakeOptions.buildStagingDirectory(any()) }
         verify(exactly = 1) {
             mockDefaultConfig.externalNativeBuild.cmake.arguments(
                 "-Wno-dev",
@@ -1276,11 +1287,18 @@ class FlutterPluginUtilsTest {
                 every { tasks } returns
                     mockk<TaskContainer> {
                         val registerTaskNameSlot = slot<String>()
-                        every { register(capture(registerTaskNameSlot), capture(registerTaskSlot)) } answers registerAnswer@{
+                        every {
+                            register(
+                                capture(registerTaskNameSlot),
+                                capture(registerTaskSlot)
+                            )
+                        } answers registerAnswer@{
                             val mockRegisterTask =
                                 mockk<Task> {
                                     every { name } returns registerTaskNameSlot.captured
-                                    every { description = capture(descriptionSlot) } returns Unit
+                                    every {
+                                        description = capture(descriptionSlot)
+                                    } returns Unit
                                     every { dependsOn(any<ProcessAndroidResources>()) } returns mockk()
                                     val doLastActionSlot = slot<Action<Task>>()
                                     every { doLast(capture(doLastActionSlot)) } answers doLastAnswer@{
@@ -1302,7 +1320,8 @@ class FlutterPluginUtilsTest {
             }
 
         variants.forEach { variant ->
-            val testOutputs: DomainObjectCollection<BaseVariantOutput> = mockk<DomainObjectCollection<BaseVariantOutput>>()
+            val testOutputs: DomainObjectCollection<BaseVariantOutput> =
+                mockk<DomainObjectCollection<BaseVariantOutput>>()
             val baseVariantSlot = slot<Action<BaseVariantOutput>>()
             val baseVariantOutput = mockk<BaseVariantOutput>()
             // Create a real file in a temp directory.
@@ -1313,7 +1332,9 @@ class FlutterPluginUtilsTest {
             manifest.writeText(manifestText)
             val mockProcessResourcesProvider = mockk<TaskProvider<ProcessAndroidResources>>()
             val mockProcessResources = mockk<ProcessAndroidResources>()
-            every { mockProcessResourcesProvider.hint(ProcessAndroidResources::class).get() } returns mockProcessResources
+            every {
+                mockProcessResourcesProvider.hint(ProcessAndroidResources::class).get()
+            } returns mockProcessResources
             every { baseVariantOutput.processResourcesProvider } returns mockProcessResourcesProvider
             // Fallback processing.
             every { mockProcessResources.manifestFile } returns manifest
@@ -1336,7 +1357,10 @@ class FlutterPluginUtilsTest {
         assert(descriptionSlot.captured.contains("stores app links settings for the given build variant"))
         assertEquals(variants.size, registerTaskList.size)
         for (i in 0 until variants.size) {
-            assertEquals("output${FlutterPluginUtils.capitalize(variants[i].name)}AppLinkSettings", registerTaskList[i].name)
+            assertEquals(
+                "output${FlutterPluginUtils.capitalize(variants[i].name)}AppLinkSettings",
+                registerTaskList[i].name
+            )
             verify(exactly = 1) { registerTaskList[i].dependsOn(any<ProcessAndroidResources>()) }
         }
         // Output assertions are minimal which ensures code is running but is not exhaustive testing.