Reland "[lacros] Support mojo connection in testing environment"

This reverts commit a3e6095c.

Reason for revert: Fix test flakiness

Original change's description:
> Revert "[lacros] Support mojo connection in testing environment"
> 
> This reverts commit 44a2bf3d.
> 
> Reason for revert: New test times out.
> https://ci.chromium.org/p/chromium/builders/ci/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%281%29/38462
> 
> Original change's description:
> > [lacros] Support mojo connection in testing environment
> > 
> > This CL supports setting up mojo connection between lacros and ash in
> > testing and debugging environment, and it is done via a unix domain
> > socket.
> > 
> > Unix domain socket is only used in testing, not production because it
> > allows a random process to connect to ash-chrome as long as it knows
> > the socket path.
> > 
> > Currently, only one lacros-chrome is allowed to connect to ash-chrome,
> > so in the testing environment, test launcher will run tests serially,
> > and will change to run in parallel once ash-chrome supports multiple
> > clients.
> > 
> > Bug: 1120582
> > Change-Id: I9736b03218f8fe937a7d642a35d6ee49b7e081f6
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2384543
> > Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
> > Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
> > Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#805537}
> 
> TBR=jorgelo@chromium.org,hidehiko@chromium.org,liaoyuke@chromium.org
> 
> Change-Id: I911a4d8bd80e8f0a217243503986a1a6d4a16338
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: 1120582
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2403460
> Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
> Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#805696}

TBR=jorgelo@chromium.org,hidehiko@chromium.org,liaoyuke@chromium.org,mstensho@chromium.org

# Not skipping CQ checks because this is a reland.

Bug: 1120582
Change-Id: I3cf4c46e939ee71e57b30d435da58bd23d15c02a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404684Reviewed-by: Yuke Liao <liaoyuke@chromium.org>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805985}

chrome/browser/chromeos/BUILD.gn

@@ -957,6 +957,8 @@ source_set("chromeos") {
     "crosapi/screen_manager_ash.h",
     "crosapi/select_file_ash.cc",
     "crosapi/select_file_ash.h",
+    "crosapi/test_mojo_connection_manager.cc",
+    "crosapi/test_mojo_connection_manager.h",
     "crostini/ansible/ansible_management_service.cc",
     "crostini/ansible/ansible_management_service.h",
     "crostini/ansible/ansible_management_service_factory.cc",
@@ -3156,6 +3158,7 @@ source_set("unit_tests") {
     "concierge_helper_service_unittest.cc",
     "crosapi/browser_util_unittest.cc",
     "crosapi/message_center_ash_unittest.cc",
+    "crosapi/test_mojo_connection_manager_unittest.cc",
     "crostini/ansible/ansible_management_service_unittest.cc",
     "crostini/crostini_disk_unittest.cc",
     "crostini/crostini_export_import_unittest.cc",

chrome/browser/chromeos/crosapi/browser_manager.cc

@@ -30,6 +30,7 @@
 #include "chrome/browser/chromeos/crosapi/ash_chrome_service_impl.h"
 #include "chrome/browser/chromeos/crosapi/browser_loader.h"
 #include "chrome/browser/chromeos/crosapi/browser_util.h"
+#include "chrome/browser/chromeos/crosapi/test_mojo_connection_manager.h"
 #include "chrome/browser/component_updater/cros_component_manager.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chromeos/constants/chromeos_features.h"
@@ -39,7 +40,6 @@
 #include "google_apis/google_api_keys.h"
 #include "mojo/public/cpp/bindings/pending_remote.h"
 #include "mojo/public/cpp/platform/platform_channel.h"
-#include "mojo/public/cpp/system/invitation.h"
 
 // TODO(crbug.com/1101667): Currently, this source has log spamming
 // by LOG(WARNING) for non critical errors to make it easy
@@ -146,6 +146,15 @@ BrowserManager::BrowserManager(
   // devices restart Chrome during login to apply flags. We don't want to run
   // the flag-off cleanup logic until we know we have the final flag state.
   session_manager::SessionManager::Get()->AddObserver(this);
+
+  std::string socket_path =
+      base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
+          chromeos::switches::kLacrosMojoSocketForTesting);
+  if (!socket_path.empty()) {
+    test_mojo_connection_manager_ =
+        std::make_unique<crosapi::TestMojoConnectionManager>(
+            base::FilePath(socket_path));
+  }
 }
 
 BrowserManager::~BrowserManager() {
@@ -282,19 +291,11 @@ void BrowserManager::StartWithLogFile(base::ScopedFD logfd) {
   mojo::PlatformChannel channel;
   channel.PrepareToPassRemoteEndpoint(&options, &command_line);
 
-  // Queue messages to establish the mojo connection,
-  // so that the passed IPC is available already when lacros-chrome accepts
-  // the invitation.
-  // TODO(crbug.com/1115092): Pass the initialization parameter over
-  // mojo connection.
-  mojo::OutgoingInvitation invitation;
-  lacros_chrome_service_.Bind(
-      mojo::PendingRemote<crosapi::mojom::LacrosChromeService>(
-          invitation.AttachMessagePipe(0), /*version=*/0));
-  lacros_chrome_service_.set_disconnect_handler(base::BindOnce(
-      &BrowserManager::OnMojoDisconnected, weak_factory_.GetWeakPtr()));
-  lacros_chrome_service_->Init(crosapi::mojom::LacrosInitParams::New());
-  lacros_chrome_service_->RequestAshChromeServiceReceiver(
+  // TODO(crbug.com/1124490): Support multiple mojo connections from lacros.
+  lacros_chrome_service_ = browser_util::SendMojoInvitationToLacrosChrome(
+      channel.TakeLocalEndpoint(),
+      base::BindOnce(&BrowserManager::OnMojoDisconnected,
+                     weak_factory_.GetWeakPtr()),
       base::BindOnce(&BrowserManager::OnAshChromeServiceReceiverReceived,
                      weak_factory_.GetWeakPtr()));
 
@@ -310,12 +311,7 @@ void BrowserManager::StartWithLogFile(base::ScopedFD logfd) {
   }
   state_ = State::STARTING;
   LOG(WARNING) << "Launched lacros-chrome with pid " << lacros_process_.Pid();
-
-  // Invite the lacros-chrome to the mojo universe.
   channel.RemoteProcessLaunchAttempted();
-  mojo::OutgoingInvitation::Send(std::move(invitation),
-                                 lacros_process_.Handle(),
-                                 channel.TakeLocalEndpoint());
 }
 
 void BrowserManager::OnAshChromeServiceReceiverReceived(

chrome/browser/chromeos/crosapi/browser_manager.h

@@ -24,6 +24,7 @@ namespace crosapi {
 
 class AshChromeServiceImpl;
 class BrowserLoader;
+class TestMojoConnectionManager;
 
 // Manages the lifetime of lacros-chrome, and its loading status. This class is
 // a part of ash-chrome.
@@ -150,6 +151,11 @@ class BrowserManager : public session_manager::SessionManagerObserver {
   // Instantiated on receiving the PendingReceiver from lacros-chrome.
   std::unique_ptr<AshChromeServiceImpl> ash_chrome_service_;
 
+  // Helps set up and manage the mojo connections between lacros-chrome and
+  // ash-chrome in testing environment. Only applicable when
+  // '--lacros-mojo-socket-for-testing' is present in the command line.
+  std::unique_ptr<TestMojoConnectionManager> test_mojo_connection_manager_;
+
   base::WeakPtrFactory<BrowserManager> weak_factory_{this};
 };
 

chrome/browser/chromeos/crosapi/browser_util.cc

@@ -4,8 +4,12 @@
 
 #include "chrome/browser/chromeos/crosapi/browser_util.h"
 
+#include <utility>
+
+#include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/path_service.h"
+#include "base/process/process_handle.h"
 #include "base/strings/string_util.h"
 #include "base/system/sys_info.h"
 #include "chrome/common/channel_info.h"
@@ -15,6 +19,8 @@
 #include "components/user_manager/user_manager.h"
 #include "components/user_manager/user_type.h"
 #include "components/version_info/channel.h"
+#include "mojo/public/cpp/platform/platform_channel.h"
+#include "mojo/public/cpp/system/invitation.h"
 
 using user_manager::User;
 using version_info::Channel;
@@ -89,5 +95,28 @@ bool IsLacrosAllowed(Channel channel) {
   }
 }
 
+mojo::Remote<crosapi::mojom::LacrosChromeService>
+SendMojoInvitationToLacrosChrome(
+    mojo::PlatformChannelEndpoint local_endpoint,
+    base::OnceClosure mojo_disconnected_callback,
+    base::OnceCallback<
+        void(mojo::PendingReceiver<crosapi::mojom::AshChromeService>)>
+        ash_chrome_service_callback) {
+  mojo::OutgoingInvitation invitation;
+  mojo::Remote<crosapi::mojom::LacrosChromeService> lacros_chrome_service;
+  lacros_chrome_service.Bind(
+      mojo::PendingRemote<crosapi::mojom::LacrosChromeService>(
+          invitation.AttachMessagePipe(0 /* token */), /*version=*/0));
+  lacros_chrome_service.set_disconnect_handler(
+      std::move(mojo_disconnected_callback));
+  lacros_chrome_service->Init(crosapi::mojom::LacrosInitParams::New());
+  lacros_chrome_service->RequestAshChromeServiceReceiver(
+      std::move(ash_chrome_service_callback));
+  mojo::OutgoingInvitation::Send(std::move(invitation),
+                                 base::kNullProcessHandle,
+                                 std::move(local_endpoint));
+  return lacros_chrome_service;
+}
+
 }  // namespace browser_util
 }  // namespace crosapi

chrome/browser/chromeos/crosapi/browser_util.h

@@ -5,12 +5,21 @@
 #ifndef CHROME_BROWSER_CHROMEOS_CROSAPI_BROWSER_UTIL_H_
 #define CHROME_BROWSER_CHROMEOS_CROSAPI_BROWSER_UTIL_H_
 
+#include "base/callback_forward.h"
+#include "chromeos/crosapi/mojom/crosapi.mojom.h"
+#include "mojo/public/cpp/bindings/pending_receiver.h"
+#include "mojo/public/cpp/bindings/remote.h"
+
 class PrefRegistrySimple;
 
 namespace base {
 class FilePath;
 }  // namespace base
 
+namespace mojo {
+class PlatformChannelEndpoint;
+}  // namespace mojo
+
 namespace version_info {
 enum class Channel;
 }  // namespace version_info
@@ -35,6 +44,19 @@ bool IsLacrosAllowed();
 // As above, but takes a channel. Exposed for testing.
 bool IsLacrosAllowed(version_info::Channel channel);
 
+// Invite the lacros-chrome to the mojo universe.
+// Queue messages to establish the mojo connection, so that the passed IPC is
+// available already when lacros-chrome accepts the invitation.
+// TODO(crbug.com/1115092): Pass the initialization parameter over mojo
+// connection.
+mojo::Remote<crosapi::mojom::LacrosChromeService>
+SendMojoInvitationToLacrosChrome(
+    mojo::PlatformChannelEndpoint local_endpoint,
+    base::OnceClosure mojo_disconnected_callback,
+    base::OnceCallback<
+        void(mojo::PendingReceiver<crosapi::mojom::AshChromeService>)>
+        ash_chrome_service_callback);
+
 }  // namespace browser_util
 }  // namespace crosapi
 

chrome/browser/chromeos/crosapi/test_mojo_connection_manager.cc

+// Copyright 2020 The Chromium 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 "chrome/browser/chromeos/crosapi/test_mojo_connection_manager.h"
+
+#include <fcntl.h>
+
+#include "base/bind.h"
+#include "base/files/file_util.h"
+#include "base/logging.h"
+#include "base/task/post_task.h"
+#include "base/task/task_traits.h"
+#include "base/task/thread_pool.h"
+#include "chrome/browser/chromeos/crosapi/ash_chrome_service_impl.h"
+#include "chrome/browser/chromeos/crosapi/browser_util.h"
+#include "chromeos/constants/chromeos_switches.h"
+#include "mojo/public/cpp/platform/named_platform_channel.h"
+#include "mojo/public/cpp/platform/platform_channel.h"
+#include "mojo/public/cpp/platform/socket_utils_posix.h"
+#include "mojo/public/cpp/system/invitation.h"
+
+namespace crosapi {
+
+namespace {
+
+// TODO(crbug.com/1124494): Refactor the code to share with ARC.
+base::ScopedFD CreateSocketForTesting(const base::FilePath& socket_path) {
+  auto endpoint = mojo::NamedPlatformChannel({socket_path.value()});
+  base::ScopedFD socket_fd =
+      endpoint.TakeServerEndpoint().TakePlatformHandle().TakeFD();
+  if (!socket_fd.is_valid()) {
+    PLOG(ERROR) << "Failed to create socket file: " << socket_path;
+    return socket_fd;
+  }
+
+  if (!base::SetPosixFilePermissions(socket_path, 0660)) {
+    PLOG(ERROR) << "Could not set permissions on socket file: " << socket_path;
+    return base::ScopedFD();
+  }
+
+  return socket_fd;
+}
+
+}  // namespace
+
+TestMojoConnectionManager::TestMojoConnectionManager(
+    const base::FilePath& socket_path) {
+  base::ThreadPool::PostTaskAndReplyWithResult(
+      FROM_HERE, {base::MayBlock()},
+      base::BindOnce(&CreateSocketForTesting, socket_path),
+      base::BindOnce(&TestMojoConnectionManager::OnTestingSocketCreated,
+                     weak_factory_.GetWeakPtr()));
+}
+
+TestMojoConnectionManager::~TestMojoConnectionManager() = default;
+
+void TestMojoConnectionManager::OnTestingSocketCreated(
+    base::ScopedFD socket_fd) {
+  if (!socket_fd.is_valid())
+    return;
+
+  testing_socket_ = std::move(socket_fd);
+  testing_socket_watcher_ = base::FileDescriptorWatcher::WatchReadable(
+      testing_socket_.get(),
+      base::BindRepeating(&TestMojoConnectionManager::OnTestingSocketAvailable,
+                          weak_factory_.GetWeakPtr()));
+}
+
+void TestMojoConnectionManager::OnTestingSocketAvailable() {
+  base::ScopedFD connection_fd;
+  if (!mojo::AcceptSocketConnection(testing_socket_.get(), &connection_fd,
+                                    /* check_peer_user = */ false) ||
+      !connection_fd.is_valid()) {
+    LOG(ERROR) << "Failed to Accept the socket";
+    return;
+  }
+
+  // TODO(crbug.com/1124490): Support multiple mojo connections from lacros.
+  mojo::PlatformChannel channel;
+  lacros_chrome_service_ = browser_util::SendMojoInvitationToLacrosChrome(
+      channel.TakeLocalEndpoint(),
+      base::BindOnce(&TestMojoConnectionManager::OnMojoDisconnected,
+                     weak_factory_.GetWeakPtr()),
+      base::BindOnce(
+          &TestMojoConnectionManager::OnAshChromeServiceReceiverReceived,
+          weak_factory_.GetWeakPtr()));
+
+  std::vector<base::ScopedFD> fds;
+  fds.emplace_back(channel.TakeRemoteEndpoint().TakePlatformHandle().TakeFD());
+
+  // Version of protocol Chrome is using.
+  uint8_t protocol_version = 0;
+  struct iovec iov[] = {{&protocol_version, sizeof(protocol_version)}};
+  ssize_t result = mojo::SendmsgWithHandles(connection_fd.get(), iov,
+                                            sizeof(iov) / sizeof(iov[0]), fds);
+  if (result == -1) {
+    PLOG(ERROR) << "Failed to send file descriptors to the socket";
+    return;
+  }
+}
+
+void TestMojoConnectionManager::OnAshChromeServiceReceiverReceived(
+    mojo::PendingReceiver<crosapi::mojom::AshChromeService> pending_receiver) {
+  ash_chrome_service_ =
+      std::make_unique<AshChromeServiceImpl>(std::move(pending_receiver));
+  LOG(INFO) << "Connection to lacros-chrome is established.";
+}
+
+void TestMojoConnectionManager::OnMojoDisconnected() {
+  lacros_chrome_service_.reset();
+  ash_chrome_service_ = nullptr;
+  LOG(WARNING) << "Mojo to lacros-chrome is disconnected.";
+}
+
+}  // namespace crosapi

chrome/browser/chromeos/crosapi/test_mojo_connection_manager.h

+// Copyright 2020 The Chromium 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 CHROME_BROWSER_CHROMEOS_CROSAPI_TEST_MOJO_CONNECTION_MANAGER_H_
+#define CHROME_BROWSER_CHROMEOS_CROSAPI_TEST_MOJO_CONNECTION_MANAGER_H_
+
+#include <memory>
+#include <utility>
+#include <vector>
+
+#include "base/files/file_descriptor_watcher_posix.h"
+#include "base/files/file_path.h"
+#include "base/files/scoped_file.h"
+#include "base/memory/weak_ptr.h"
+#include "chromeos/crosapi/mojom/crosapi.mojom.h"
+#include "mojo/public/cpp/bindings/remote.h"
+
+namespace crosapi {
+
+class AshChromeServiceImpl;
+
+// An extension of BrowserManager to help set up and manage the mojo connections
+// between the test executable and ash-chrome in testing environment.
+//
+// In testing environment, the workflow is as following:
+// - Ash-chrome creates a Unix domain socket.
+// - Test executable connects to the Unix domain socket.
+// - When ash-chrome accepts the connection, it creates a |PlatformChannel| and
+//   sends one end of it (as a FD) over the socket.
+// - Test executable reads the FD from the socket and passes it to lacros-chrome
+//   when launching a test.
+//
+// The workflow works for debugging as well, a wapper script can play the role
+// of the test executable above to obtain the FD, and passes it to lacros-chrome
+// when launching it inside gdb.
+class TestMojoConnectionManager {
+ public:
+  explicit TestMojoConnectionManager(const base::FilePath& socket_path);
+
+  TestMojoConnectionManager(const TestMojoConnectionManager&) = delete;
+  TestMojoConnectionManager& operator=(const TestMojoConnectionManager&) =
+      delete;
+
+  ~TestMojoConnectionManager();
+
+ private:
+  // Called when the testing socket is created.
+  void OnTestingSocketCreated(base::ScopedFD socket_fd);
+
+  // Called when a client, such as a test launcher, attempts to connect.
+  void OnTestingSocketAvailable();
+
+  // Called when PendingReceiver of AshChromeService is passed from
+  // lacros-chrome.
+  void OnAshChromeServiceReceiverReceived(
+      mojo::PendingReceiver<crosapi::mojom::AshChromeService> pending_receiver);
+
+  // Called when the Mojo connection to lacros-chrome is disconnected.
+  // It may be "just a Mojo error" or "test is finished".
+  void OnMojoDisconnected();
+
+  // Proxy to LacrosChromeService mojo service in lacros-chrome.
+  // Available during lacros-chrome is running.
+  mojo::Remote<crosapi::mojom::LacrosChromeService> lacros_chrome_service_;
+
+  // Implementation of AshChromeService Mojo APIs.
+  // Instantiated on receiving the PendingReceiver from lacros-chrome.
+  std::unique_ptr<AshChromeServiceImpl> ash_chrome_service_;
+
+  // A socket for a client, such as a test launcher, to connect to.
+  base::ScopedFD testing_socket_;
+
+  // A watcher that watches |testing_scoket_| and invokes
+  // |OnTestingSocketAvailable| when it becomes readable.
+  std::unique_ptr<base::FileDescriptorWatcher::Controller>
+      testing_socket_watcher_;
+
+  base::WeakPtrFactory<TestMojoConnectionManager> weak_factory_{this};
+};
+
+}  // namespace crosapi
+
+#endif  // CHROME_BROWSER_CHROMEOS_CROSAPI_TEST_MOJO_CONNECTION_MANAGER_H_

chrome/browser/chromeos/crosapi/test_mojo_connection_manager_unittest.cc

+// Copyright 2020 The Chromium 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 "chrome/browser/chromeos/crosapi/test_mojo_connection_manager.h"
+
+#include <fcntl.h>
+
+#include "base/bind.h"
+#include "base/command_line.h"
+#include "base/files/file_path.h"
+#include "base/files/file_path_watcher.h"
+#include "base/files/scoped_file.h"
+#include "base/files/scoped_temp_dir.h"
+#include "base/process/launch.h"
+#include "base/run_loop.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/task/thread_pool.h"
+#include "base/test/bind_test_util.h"
+#include "base/test/multiprocess_test.h"
+#include "base/test/task_environment.h"
+#include "base/test/test_timeouts.h"
+#include "chromeos/crosapi/mojom/crosapi.mojom.h"
+#include "mojo/public/cpp/platform/named_platform_channel.h"
+#include "mojo/public/cpp/platform/platform_channel.h"
+#include "mojo/public/cpp/platform/socket_utils_posix.h"
+#include "mojo/public/cpp/system/invitation.h"
+#include "testing/gtest/include/gtest/gtest.h"
+#include "testing/multiprocess_func_list.h"
+
+namespace crosapi {
+
+class TestLacrosChromeService : public crosapi::mojom::LacrosChromeService {
+ public:
+  TestLacrosChromeService(
+      mojo::PendingReceiver<mojom::LacrosChromeService> receiver,
+      base::RunLoop& run_loop)
+      : receiver_(this, std::move(receiver)), run_loop_(run_loop) {}
+
+  ~TestLacrosChromeService() override = default;
+
+  void Init(crosapi::mojom::LacrosInitParamsPtr params) override {
+    init_is_called_ = true;
+  }
+
+  void RequestAshChromeServiceReceiver(
+      RequestAshChromeServiceReceiverCallback callback) override {
+    EXPECT_TRUE(init_is_called_);
+    std::move(callback).Run(ash_chrome_service_.BindNewPipeAndPassReceiver());
+    request_ash_chrome_service_is_called_ = true;
+    run_loop_.Quit();
+  }
+
+  void NewWindow(NewWindowCallback callback) override {}
+
+  bool init_is_called() { return init_is_called_; }
+
+  bool request_ash_chrome_service_is_called() {
+    return request_ash_chrome_service_is_called_;
+  }
+
+ private:
+  mojo::Receiver<mojom::LacrosChromeService> receiver_;
+
+  bool init_is_called_ = false;
+  bool request_ash_chrome_service_is_called_ = false;
+
+  base::RunLoop& run_loop_;
+
+  mojo::Remote<crosapi::mojom::AshChromeService> ash_chrome_service_;
+};
+
+class TestMojoConnectionManagerTest : public testing::Test {
+ public:
+  TestMojoConnectionManagerTest() = default;
+  ~TestMojoConnectionManagerTest() override = default;
+};
+
+TEST_F(TestMojoConnectionManagerTest, ConnectWithLacrosChrome) {
+  // Use IO type to support the FileDescriptorWatcher API on POSIX.
+  base::test::TaskEnvironment task_environment{
+      base::test::TaskEnvironment::MainThreadType::IO};
+
+  // Ash-chrome queues an invitation, drop a socket and wait for connection.
+  base::ScopedTempDir temp_dir;
+  ASSERT_TRUE(temp_dir.CreateUniqueTempDir());
+  std::string socket_path =
+      temp_dir.GetPath().MaybeAsASCII() + "/lacros.socket";
+
+  // Sets up watcher to Wait for the socket to be created by
+  // |TestMojoConnectionManager| before attempting to connect. There is no
+  // garanteen that |OnTestingSocketCreated| has run after the run loop is done,
+  // so this test should NOT depend on the assumption.
+  base::FilePathWatcher watcher;
+  base::RunLoop run_loop1;
+  watcher.Watch(base::FilePath(socket_path), false,
+                base::BindRepeating(base::BindLambdaForTesting(
+                    [&run_loop1](const base::FilePath& path, bool error) {
+                      EXPECT_FALSE(error);
+                      run_loop1.Quit();
+                    })));
+  TestMojoConnectionManager test_mojo_connection_manager{
+      base::FilePath(socket_path)};
+  run_loop1.Run();
+
+  // Test connects with ash-chrome via the socket.
+  auto channel = mojo::NamedPlatformChannel::ConnectToServer(socket_path);
+  ASSERT_TRUE(channel.is_valid());
+  base::ScopedFD socket_fd = channel.TakePlatformHandle().TakeFD();
+
+  uint8_t buf[32];
+  std::vector<base::ScopedFD> descriptors;
+  ssize_t size;
+  base::RunLoop run_loop2;
+  base::ThreadPool::PostTaskAndReply(
+      FROM_HERE, {base::MayBlock()},
+      base::BindOnce(base::BindLambdaForTesting([&]() {
+        // Mark the channel as blocking.
+        int flags = fcntl(socket_fd.get(), F_GETFL);
+        PCHECK(flags != -1);
+        fcntl(socket_fd.get(), F_SETFL, flags & ~O_NONBLOCK);
+        size = mojo::SocketRecvmsg(socket_fd.get(), buf, sizeof(buf),
+                                   &descriptors, true /*block*/);
+      })),
+      run_loop2.QuitClosure());
+  run_loop2.Run();
+  EXPECT_EQ(1, size);
+  EXPECT_EQ(0u, buf[0]);
+  ASSERT_EQ(1u, descriptors.size());
+
+  // Test launches lacros-chrome as child process.
+  base::LaunchOptions options;
+  options.fds_to_remap.emplace_back(descriptors[0].get(), descriptors[0].get());
+  base::CommandLine lacros_cmd(base::GetMultiProcessTestChildBaseCommandLine());
+  lacros_cmd.AppendSwitchASCII(mojo::PlatformChannel::kHandleSwitch,
+                               base::NumberToString(descriptors[0].release()));
+  base::Process lacros_process =
+      base::SpawnMultiProcessTestChild("LacrosMain", lacros_cmd, options);
+
+  // lacros-chrome accepts the invitation to establish mojo connection with
+  // ash-chrome.
+  int rv = -1;
+  ASSERT_TRUE(base::WaitForMultiprocessTestChildExit(
+      lacros_process, TestTimeouts::action_timeout(), &rv));
+  lacros_process.Close();
+  EXPECT_EQ(0, rv);
+}
+
+// Another process that emulates the behavior of lacros-chrome.
+MULTIPROCESS_TEST_MAIN(LacrosMain) {
+  base::test::SingleThreadTaskEnvironment task_environment;
+  mojo::IncomingInvitation invitation = mojo::IncomingInvitation::Accept(
+      mojo::PlatformChannel::RecoverPassedEndpointFromCommandLine(
+          *base::CommandLine::ForCurrentProcess()));
+  base::RunLoop run_loop;
+  TestLacrosChromeService test_lacros_chrome_service(
+      mojo::PendingReceiver<crosapi::mojom::LacrosChromeService>(
+          invitation.ExtractMessagePipe(0)),
+      run_loop);
+  run_loop.Run();
+  DCHECK(test_lacros_chrome_service.init_is_called());
+  DCHECK(test_lacros_chrome_service.request_ash_chrome_service_is_called());
+  return 0;
+}
+
+}  // namespace crosapi

chromeos/constants/chromeos_switches.cc

@@ -431,6 +431,15 @@ const char kLacrosChromeAdditionalArgs[] = "lacros-chrome-additional-args";
 // path should be to a directory that contains a binary named 'chrome'.
 const char kLacrosChromePath[] = "lacros-chrome-path";
 
+// If set, ash-chrome will drop a Unix domain socket to wait for a process to
+// connect to it, and the connection will be used to request file descriptors
+// from ash-chrome, and when the process forks to start a lacros-chrome, the
+// obtained file descriptor will be used by lacros-chrome to set up the mojo
+// connection with ash-chrome. There are mainly two use cases:
+// 1. Test launcher to run browser tests in testing environment.
+// 2. A terminal to start lacros-chrome with a debugger.
+const char kLacrosMojoSocketForTesting[] = "lacros-mojo-socket-for-testing";
+
 // Enables Chrome-as-a-login-manager behavior.
 const char kLoginManager[] = "login-manager";
 

chromeos/constants/chromeos_switches.h

@@ -173,6 +173,8 @@ COMPONENT_EXPORT(CHROMEOS_CONSTANTS) extern const char kKernelnextRestrictVMs[];
 COMPONENT_EXPORT(CHROMEOS_CONSTANTS)
 extern const char kLacrosChromeAdditionalArgs[];
 COMPONENT_EXPORT(CHROMEOS_CONSTANTS) extern const char kLacrosChromePath[];
+COMPONENT_EXPORT(CHROMEOS_CONSTANTS)
+extern const char kLacrosMojoSocketForTesting[];
 COMPONENT_EXPORT(CHROMEOS_CONSTANTS) extern const char kLoginManager[];
 COMPONENT_EXPORT(CHROMEOS_CONSTANTS) extern const char kLoginProfile[];
 COMPONENT_EXPORT(CHROMEOS_CONSTANTS) extern const char kLoginUser[];