Google Test(GTest)使用方法和源码解析——死亡测试技术分析和应用

死亡测试是为了判断一段逻辑是否会导致进程退出而设计的。这种场景并不常见，但是GTest依然为我们设计了这个功能。我们先看下其应用实例。（转载请指明出于breaksoftware的csdn博客）

死亡测试技术应用

我们可以使用TEST声明并注册一个简单的测试特例。其实现内部才是死亡测试相关代码运行的地方。GTest为我们提供了如下的宏用于组织测试逻辑

宏中的statement是测试逻辑的表达式，它可以是个函数，可以是个对象的方法，也可以是几个表达式的组合，比如

EXPECT_DEATH({ int n = 4; n = 5;},"");

regex是一个正则表达式，它用于匹配stderr输出的内容。如果匹配上了，则测试成功，否则测试失败。比如

void Foo() {std::cerr<<"Failed Foo";_exit(0);
}
EXPECT_DEATH(Foo(),".*Foo");
EXPECT_DEATH(Foo(),".*FAAA");

第5行的局部测试匹配上了测试预期，而第6行没有。

注意下正则表达式这个功能只支持linux系统，windows上不支持，所以windows上我们对此参数传空串。我们看个完整的例子

void Foo() {std::cerr<<"Fail Foo";_exit(0);
}TEST(MyDeathTest, Foo) {EXPECT_EXIT(Foo(), ::testing::ExitedWithCode(0), ".*Foo");
}

注意下我们测试用例名——MyDeathTest。GTest强烈建议测试用例名以DeathTest结尾。这是为了让死亡测试在所有其他测试之前运行。

死亡测试技术分析

死亡测试非常依赖于系统的实现。本文并不打算把每个系统都覆盖到，我将以windows系统上的实现详细讲解其过程。在Linux上实现的思路基本和windows上相同，只是在一些系统实现上存在差异导致GTest具有不同的属性。

先概括的讲一下windows上实现的过程

1. 测试实体中准备启动新的进程，进程路径就是本进程可执行文件路径
2. 子进程传入了标准输入输出句柄
3. 启动子进程时传入类型筛选，即指定执行该测试用例
4. 监听子进程的输出
5. 判断子进程退出模式

子进程的执行过程是：

1. 执行父进程指定的测试特例
2. 运行死亡测试宏中的表达式
3. 如果没有crash，则根据情况选择退出模式

我们来看下EXPECT_DEATH的实现，其最终将调用到GTEST_DEATH_TEST_宏中

# define GTEST_DEATH_TEST_(statement, predicate, regex, fail) \GTEST_AMBIGUOUS_ELSE_BLOCKER_ \if (::testing::internal::AlwaysTrue()) { \const ::testing::internal::RE& gtest_regex = (regex); \::testing::internal::DeathTest* gtest_dt; \if (!::testing::internal::DeathTest::Create(#statement, >est_regex, \__FILE__, __LINE__, >est_dt)) { \goto GTEST_CONCAT_TOKEN_(gtest_label_, __LINE__); \} \if (gtest_dt != NULL) { \::testing::internal::scoped_ptr< ::testing::internal::DeathTest> \gtest_dt_ptr(gtest_dt); \switch (gtest_dt->AssumeRole()) { \case ::testing::internal::DeathTest::OVERSEE_TEST: \if (!gtest_dt->Passed(predicate(gtest_dt->Wait()))) { \goto GTEST_CONCAT_TOKEN_(gtest_label_, __LINE__); \} \break; \case ::testing::internal::DeathTest::EXECUTE_TEST: { \::testing::internal::DeathTest::ReturnSentinel \gtest_sentinel(gtest_dt); \GTEST_EXECUTE_DEATH_TEST_STATEMENT_(statement, gtest_dt); \gtest_dt->Abort(::testing::internal::DeathTest::TEST_DID_NOT_DIE); \break; \} \default: \break; \} \} \} else \GTEST_CONCAT_TOKEN_(gtest_label_, __LINE__): \fail(::testing::internal::DeathTest::LastMessage())

第5行我们声明了一个DeathTest*指针，这个类暴露了一个静态方法用于创建对象。可以说它是一个接口类，我们看下它重要的部分定义

  enum TestRole { OVERSEE_TEST, EXECUTE_TEST };// An enumeration of the three reasons that a test might be aborted.enum AbortReason {TEST_ENCOUNTERED_RETURN_STATEMENT,TEST_THREW_EXCEPTION,TEST_DID_NOT_DIE};// Assumes one of the above roles.virtual TestRole AssumeRole() = 0;// Waits for the death test to finish and returns its status.virtual int Wait() = 0;// Returns true if the death test passed; that is, the test process// exited during the test, its exit status matches a user-supplied// predicate, and its stderr output matches a user-supplied regular// expression.// The user-supplied predicate may be a macro expression rather// than a function pointer or functor, or else Wait and Passed could// be combined.virtual bool Passed(bool exit_status_ok) = 0;// Signals that the death test did not die as expected.virtual void Abort(AbortReason reason) = 0;

TestRole就是角色，我们父进程角色是OVERSEE_TEST，子进程的角色是EXECUTE_TEST。因为父子进程都将进入这个测试特例逻辑，所以要通过角色标记来区分执行逻辑。AbortReason枚举中类型表达了测试终止的原因。
        AssumeRole是主要是父进程启动子进程的逻辑。Wait是父进程等待子进程执行完毕，并尝试读取子进程的输出。

DeathTest::Create方法最终会进入DefaultDeathTestFactory::Create方法

bool DefaultDeathTestFactory::Create(const char* statement, const RE* regex,const char* file, int line,DeathTest** test) {UnitTestImpl* const impl = GetUnitTestImpl();const InternalRunDeathTestFlag* const flag =impl->internal_run_death_test_flag();const int death_test_index = impl->current_test_info()->increment_death_test_count();if (flag != NULL) {if (death_test_index > flag->index()) {DeathTest::set_last_death_test_message("Death test count (" + StreamableToString(death_test_index)+ ") somehow exceeded expected maximum ("+ StreamableToString(flag->index()) + ")");return false;}if (!(flag->file() == file && flag->line() == line &&flag->index() == death_test_index)) {*test = NULL;return true;}}

此处通过获取flag变量，得知当前运行的是子进程还是父进程。如果flag不是NULL，则是子进程，它主要做些输出的工作；如果是父进程，则进入下面代码

# if GTEST_OS_WINDOWSif (GTEST_FLAG(death_test_style) == "threadsafe" ||GTEST_FLAG(death_test_style) == "fast") {*test = new WindowsDeathTest(statement, regex, file, line);}# elseif (GTEST_FLAG(death_test_style) == "threadsafe") {*test = new ExecDeathTest(statement, regex, file, line);} else if (GTEST_FLAG(death_test_style) == "fast") {*test = new NoExecDeathTest(statement, regex);}# endif  // GTEST_OS_WINDOWS

可见Windows上死亡测试最终将由WindowsDeathTest代理，而linux系统根据传入参数不同而选择不同的类。它们都是DeathTest的派生类。为什么linux系统上支持参数选择，这要从系统暴露出来的接口和系统实现来说。windows系统上进程创建只要调用CreateProcess之类的函数就可以了，这个函数调用后，子进程就创建出来了。而linux系统上则要调用fork或者clone之类，这两种函数执行机制也不太相同。fork是标准的子进程和父进程分离执行，所以threadsafe对应的ExecDeathTest类在底层调用的是fork，从而可以保证是安全的。但是clone用于创建轻量级进程，即创建的子进程与父进程共用线性地址空间，只是它们的堆栈不同，这样不用执行父子进程分离，执行当然会快些，所以这种方式对应的是fast——NoExecDeathTest。

我们看下WindowsDeathTest::AssumeRole()的实现

// The AssumeRole process for a Windows death test.  It creates a child
// process with the same executable as the current process to run the
// death test.  The child process is given the --gtest_filter and
// --gtest_internal_run_death_test flags such that it knows to run the
// current death test only.
DeathTest::TestRole WindowsDeathTest::AssumeRole() {const UnitTestImpl* const impl = GetUnitTestImpl();const InternalRunDeathTestFlag* const flag =impl->internal_run_death_test_flag();const TestInfo* const info = impl->current_test_info();const int death_test_index = info->result()->death_test_count();if (flag != NULL) {// ParseInternalRunDeathTestFlag() has performed all the necessary// processing.set_write_fd(flag->write_fd());return EXECUTE_TEST;}

这段代码的注释写的很清楚，父进程将向子进程传递什么样的参数。

和之前一样，需要获取flag，如果不是NULL，则是子进程，设置写入句柄，并返回自己角色。如果是父进程则执行下面逻辑

  // WindowsDeathTest uses an anonymous pipe to communicate results of// a death test.SECURITY_ATTRIBUTES handles_are_inheritable = {sizeof(SECURITY_ATTRIBUTES), NULL, TRUE };HANDLE read_handle, write_handle;GTEST_DEATH_TEST_CHECK_(::CreatePipe(&read_handle, &write_handle, &handles_are_inheritable,0)  // Default buffer size.!= FALSE);set_read_fd(::_open_osfhandle(reinterpret_cast<intptr_t>(read_handle),O_RDONLY));write_handle_.Reset(write_handle);event_handle_.Reset(::CreateEvent(&handles_are_inheritable,TRUE,    // The event will automatically reset to non-signaled state.FALSE,   // The initial state is non-signalled.NULL));  // The even is unnamed.GTEST_DEATH_TEST_CHECK_(event_handle_.Get() != NULL);const std::string filter_flag =std::string("--") + GTEST_FLAG_PREFIX_ + kFilterFlag + "=" +info->test_case_name() + "." + info->name();const std::string internal_flag =std::string("--") + GTEST_FLAG_PREFIX_ + kInternalRunDeathTestFlag +"=" + file_ + "|" + StreamableToString(line_) + "|" +StreamableToString(death_test_index) + "|" +StreamableToString(static_cast<unsigned int>(::GetCurrentProcessId())) +// size_t has the same width as pointers on both 32-bit and 64-bit// Windows platforms.// See http://msdn.microsoft.com/en-us/library/tcxf1dw6.aspx."|" + StreamableToString(reinterpret_cast<size_t>(write_handle)) +"|" + StreamableToString(reinterpret_cast<size_t>(event_handle_.Get()));char executable_path[_MAX_PATH + 1];  // NOLINTGTEST_DEATH_TEST_CHECK_(_MAX_PATH + 1 != ::GetModuleFileNameA(NULL,executable_path,_MAX_PATH));std::string command_line =std::string(::GetCommandLineA()) + " " + filter_flag + " \"" +internal_flag + "\"";DeathTest::set_last_death_test_message("");CaptureStderr();// Flush the log buffers since the log streams are shared with the child.FlushInfoLog();// The child process will share the standard handles with the parent.STARTUPINFOA startup_info;memset(&startup_info, 0, sizeof(STARTUPINFO));startup_info.dwFlags = STARTF_USESTDHANDLES;startup_info.hStdInput = ::GetStdHandle(STD_INPUT_HANDLE);startup_info.hStdOutput = ::GetStdHandle(STD_OUTPUT_HANDLE);startup_info.hStdError = ::GetStdHandle(STD_ERROR_HANDLE);PROCESS_INFORMATION process_info;GTEST_DEATH_TEST_CHECK_(::CreateProcessA(executable_path,const_cast<char*>(command_line.c_str()),NULL,   // Retuned process handle is not inheritable.NULL,   // Retuned thread handle is not inheritable.TRUE,   // Child inherits all inheritable handles (for write_handle_).0x0,    // Default creation flags.NULL,   // Inherit the parent's environment.UnitTest::GetInstance()->original_working_dir(),&startup_info,&process_info) != FALSE);child_handle_.Reset(process_info.hProcess);::CloseHandle(process_info.hThread);set_spawned(true);return OVERSEE_TEST;

这段逻辑创建了父进程和子进程通信的匿名管道和事件句柄，这些都通过命令行参数传递给子进程。

我们再看下父进程等待的过程

int WindowsDeathTest::Wait() {if (!spawned())return 0;// Wait until the child either signals that it has acquired the write end// of the pipe or it dies.const HANDLE wait_handles[2] = { child_handle_.Get(), event_handle_.Get() };switch (::WaitForMultipleObjects(2,wait_handles,FALSE,  // Waits for any of the handles.INFINITE)) {case WAIT_OBJECT_0:case WAIT_OBJECT_0 + 1:break;default:GTEST_DEATH_TEST_CHECK_(false);  // Should not get here.}// The child has acquired the write end of the pipe or exited.// We release the handle on our side and continue.write_handle_.Reset();event_handle_.Reset();ReadAndInterpretStatusByte();

它等待子进程句柄或者完成事件。一旦等到，则在ReadAndInterpretStatusByte中读取子进程的输出

void DeathTestImpl::ReadAndInterpretStatusByte() {char flag;int bytes_read;// The read() here blocks until data is available (signifying the// failure of the death test) or until the pipe is closed (signifying// its success), so it's okay to call this in the parent before// the child process has exited.do {bytes_read = posix::Read(read_fd(), &flag, 1);} while (bytes_read == -1 && errno == EINTR);if (bytes_read == 0) {set_outcome(DIED);} else if (bytes_read == 1) {switch (flag) {case kDeathTestReturned:set_outcome(RETURNED);break;case kDeathTestThrew:set_outcome(THREW);break;case kDeathTestLived:set_outcome(LIVED);break;case kDeathTestInternalError:FailFromInternalError(read_fd());  // Does not return.break;default:GTEST_LOG_(FATAL) << "Death test child process reported "<< "unexpected status byte ("<< static_cast<unsigned int>(flag) << ")";}} else {GTEST_LOG_(FATAL) << "Read from death test child process failed: "<< GetLastErrnoDescription();}GTEST_DEATH_TEST_CHECK_SYSCALL_(posix::Close(read_fd()));set_read_fd(-1);
}

这段代码可以用于区分子进程的退出状态。如果子进程crash了，则读取不到数据，进入第14行。
        子进程则是执行完表达式后调用Abort返回相应错误。GTEST_DEATH_TEST_剩下的实现，把这个过程表达的很清楚

    if (gtest_dt != NULL) { \::testing::internal::scoped_ptr< ::testing::internal::DeathTest> \gtest_dt_ptr(gtest_dt); \switch (gtest_dt->AssumeRole()) { \case ::testing::internal::DeathTest::OVERSEE_TEST: \if (!gtest_dt->Passed(predicate(gtest_dt->Wait()))) { \goto GTEST_CONCAT_TOKEN_(gtest_label_, __LINE__); \} \break; \case ::testing::internal::DeathTest::EXECUTE_TEST: { \::testing::internal::DeathTest::ReturnSentinel \gtest_sentinel(gtest_dt); \GTEST_EXECUTE_DEATH_TEST_STATEMENT_(statement, gtest_dt); \gtest_dt->Abort(::testing::internal::DeathTest::TEST_DID_NOT_DIE); \break; \} \default: \break; \} \} \