Support gtest-parallel when running Impeller unit tests

ci/licenses_golden/licenses_flutter

@@ -34705,6 +34705,7 @@ ORIGIN: ../../../flutter/fml/platform/posix/mapping_posix.cc + ../../../flutter/
 ORIGIN: ../../../flutter/fml/platform/posix/native_library_posix.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/posix/paths_posix.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/posix/posix_wrappers_posix.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/fml/platform/posix/process_posix.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/posix/shared_mutex_posix.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/posix/shared_mutex_posix.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/win/command_line_win.cc + ../../../flutter/LICENSE
@@ -34717,9 +34718,11 @@ ORIGIN: ../../../flutter/fml/platform/win/message_loop_win.h + ../../../flutter/
 ORIGIN: ../../../flutter/fml/platform/win/native_library_win.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/win/paths_win.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/win/posix_wrappers_win.cc + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/fml/platform/win/process_win.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/win/wstring_conversion.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/platform/win/wstring_conversion.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/posix_wrappers.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/fml/process.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/raster_thread_merger.cc + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/raster_thread_merger.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/fml/shared_thread_merger.cc + ../../../flutter/LICENSE
@@ -37546,6 +37549,7 @@ FILE: ../../../flutter/fml/platform/posix/mapping_posix.cc
 FILE: ../../../flutter/fml/platform/posix/native_library_posix.cc
 FILE: ../../../flutter/fml/platform/posix/paths_posix.cc
 FILE: ../../../flutter/fml/platform/posix/posix_wrappers_posix.cc
+FILE: ../../../flutter/fml/platform/posix/process_posix.cc
 FILE: ../../../flutter/fml/platform/posix/shared_mutex_posix.cc
 FILE: ../../../flutter/fml/platform/posix/shared_mutex_posix.h
 FILE: ../../../flutter/fml/platform/win/command_line_win.cc
@@ -37558,9 +37562,11 @@ FILE: ../../../flutter/fml/platform/win/message_loop_win.h
 FILE: ../../../flutter/fml/platform/win/native_library_win.cc
 FILE: ../../../flutter/fml/platform/win/paths_win.cc
 FILE: ../../../flutter/fml/platform/win/posix_wrappers_win.cc
+FILE: ../../../flutter/fml/platform/win/process_win.cc
 FILE: ../../../flutter/fml/platform/win/wstring_conversion.cc
 FILE: ../../../flutter/fml/platform/win/wstring_conversion.h
 FILE: ../../../flutter/fml/posix_wrappers.h
+FILE: ../../../flutter/fml/process.h
 FILE: ../../../flutter/fml/raster_thread_merger.cc
 FILE: ../../../flutter/fml/raster_thread_merger.h
 FILE: ../../../flutter/fml/shared_thread_merger.cc

fml/BUILD.gn

@@ -64,6 +64,7 @@ source_set("fml") {
     "paths.cc",
     "paths.h",
     "posix_wrappers.h",
+    "process.h",
     "raster_thread_merger.cc",
     "raster_thread_merger.h",
     "shared_thread_merger.cc",
@@ -251,6 +252,7 @@ source_set("fml") {
       "platform/win/native_library_win.cc",
       "platform/win/paths_win.cc",
       "platform/win/posix_wrappers_win.cc",
+      "platform/win/process_win.cc",
     ]
   } else {
     sources += [
@@ -260,6 +262,7 @@ source_set("fml") {
       "platform/posix/native_library_posix.cc",
       "platform/posix/paths_posix.cc",
       "platform/posix/posix_wrappers_posix.cc",
+      "platform/posix/process_posix.cc",
     ]
   }
 }

fml/platform/posix/process_posix.cc

@@ -0,0 +1,13 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <unistd.h>
+
+namespace fml {
+
+int GetCurrentProcId() {
+  return static_cast<int>(getpid());
+}
+
+}  // namespace fml

fml/platform/win/process_win.cc

@@ -0,0 +1,13 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <windows.h>
+
+namespace fml {
+
+int GetCurrentProcId() {
+  return static_cast<int>(::GetCurrentProcessId());
+}
+
+}  // namespace fml

fml/process.h

@@ -0,0 +1,14 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_FML_PROCESS_H_
+#define FLUTTER_FML_PROCESS_H_
+
+namespace fml {
+
+int GetCurrentProcId();
+
+}  // namespace fml
+
+#endif  // FLUTTER_FML_PROCESS_H_

impeller/compiler/compiler_test.cc

@@ -3,29 +3,39 @@
 // found in the LICENSE file.
 
 #include "impeller/compiler/compiler_test.h"
+#include "flutter/fml/paths.h"
+#include "flutter/fml/process.h"
 
 #include <algorithm>
+#include <filesystem>
 
 namespace impeller {
 namespace compiler {
 namespace testing {
 
-static fml::UniqueFD CreateIntermediatesDirectory() {
+static std::string GetIntermediatesPath() {
   auto test_name = flutter::testing::GetCurrentTestName();
   std::replace(test_name.begin(), test_name.end(), '/', '_');
   std::replace(test_name.begin(), test_name.end(), '.', '_');
-  return fml::OpenDirectory(flutter::testing::OpenFixturesDirectory(),
-                            test_name.c_str(),
-                            true,  // create if necessary
-                            fml::FilePermission::kReadWrite);
+  std::stringstream dir_name;
+  dir_name << test_name << "_" << std::to_string(fml::GetCurrentProcId());
+  return fml::paths::JoinPaths(
+      {flutter::testing::GetFixturesPath(), dir_name.str()});
 }
 
-CompilerTest::CompilerTest()
-    : intermediates_directory_(CreateIntermediatesDirectory()) {
+CompilerTest::CompilerTest() : intermediates_path_(GetIntermediatesPath()) {
+  intermediates_directory_ =
+      fml::OpenDirectory(intermediates_path_.c_str(),
+                         true,  // create if necessary
+                         fml::FilePermission::kReadWrite);
   FML_CHECK(intermediates_directory_.is_valid());
 }
 
-CompilerTest::~CompilerTest() = default;
+CompilerTest::~CompilerTest() {
+  intermediates_directory_.reset();
+
+  std::filesystem::remove_all(std::filesystem::path(intermediates_path_));
+}
 
 static std::string ReflectionHeaderName(const char* fixture_name) {
   std::stringstream stream;

impeller/compiler/compiler_test.h

@@ -36,6 +36,7 @@ class CompilerTest : public ::testing::TestWithParam<TargetPlatform> {
       const char* entry_point_name = "main") const;
 
  private:
+  std::string intermediates_path_;
   fml::UniqueFD intermediates_directory_;
 
   CompilerTest(const CompilerTest&) = delete;