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init进程会读取rc文件，然后孵化很多其他系统服务进程，为防止子进程死亡后称为僵尸进程，init需要监测子进程是否死亡，如果死亡，则清除子进程资源，并重新拉起进程。
system/core/init/init.cpp

InstallSignalFdHandler(&epoll);

init进程第二阶段启动的时候，创建了epoll，并将子进程相关处理注册到epoll中

static void InstallSignalFdHandler(Epoll* epoll) {// Applying SA_NOCLDSTOP to a defaulted SIGCHLD handler prevents the signalfd from receiving// SIGCHLD when a child process stops or continues (b/77867680#comment9).const struct sigaction act { .sa_handler = SIG_DFL, .sa_flags = SA_NOCLDSTOP };sigaction(SIGCHLD, &act, nullptr);sigset_t mask;sigemptyset(&mask);sigaddset(&mask, SIGCHLD);if (!IsRebootCapable()) {// If init does not have the CAP_SYS_BOOT capability, it is running in a container.// In that case, receiving SIGTERM will cause the system to shut down.sigaddset(&mask, SIGTERM);}if (sigprocmask(SIG_BLOCK, &mask, nullptr) == -1) {PLOG(FATAL) << "failed to block signals";}// Register a handler to unblock signals in the child processes.const int result = pthread_atfork(nullptr, nullptr, &UnblockSignals);if (result != 0) {LOG(FATAL) << "Failed to register a fork handler: " << strerror(result);}signal_fd = signalfd(-1, &mask, SFD_CLOEXEC | SFD_NONBLOCK);if (signal_fd == -1) {PLOG(FATAL) << "failed to create signalfd";}constexpr int flags = EPOLLIN | EPOLLPRI;auto handler = std::bind(HandleSignalFd, false);if (auto result = epoll->RegisterHandler(signal_fd, handler, flags); !result.ok()) {LOG(FATAL) << result.error();}
}

使用epoll注册监听信号量fd，子进程死亡会发送信号给init进程，这里就会调用epoll中注册的回调函数HandleSignalFd

static void HandleSignalFd(bool one_off) {signalfd_siginfo siginfo;auto started = std::chrono::steady_clock::now();do {ssize_t bytes_read = TEMP_FAILURE_RETRY(read(signal_fd, &siginfo, sizeof(siginfo)));if (bytes_read < 0 && errno == EAGAIN) {auto now = std::chrono::steady_clock::now();std::chrono::duration<double> waited = now - started;if (waited >= kDiagnosticTimeout) {LOG(ERROR) << "epoll() woke us up, but we waited with no SIGCHLD!";started = now;}std::this_thread::sleep_for(100ms);continue;}if (bytes_read != sizeof(siginfo)) {PLOG(ERROR) << "Failed to read siginfo from signal_fd";return;}break;} while (!one_off);switch (siginfo.ssi_signo) {case SIGCHLD:ReapAnyOutstandingChildren();break;case SIGTERM:HandleSigtermSignal(siginfo);break;default:PLOG(ERROR) << "signal_fd: received unexpected signal " << siginfo.ssi_signo;break;}
}

对于SIGCHLD信号，调用ReapAnyOutstandingChildren
system/core/init/sigchld_handler.cpp

void ReapAnyOutstandingChildren() {while (ReapOneProcess() != 0) {}}

static pid_t ReapOneProcess() {siginfo_t siginfo = {};// This returns a zombie pid or informs us that there are no zombies left to be reaped.// It does NOT reap the pid; that is done below.if (TEMP_FAILURE_RETRY(waitid(P_ALL, 0, &siginfo, WEXITED | WNOHANG | WNOWAIT)) != 0) {PLOG(ERROR) << "waitid failed";return 0;}auto pid = siginfo.si_pid;if (pid == 0) return 0;// At this point we know we have a zombie pid, so we use this scopeguard to reap the pid// whenever the function returns from this point forward.// We do NOT want to reap the zombie earlier as in Service::Reap(), we kill(-pid, ...) and we// want the pid to remain valid throughout that (and potentially future) usages.auto reaper = make_scope_guard([pid] { TEMP_FAILURE_RETRY(waitpid(pid, nullptr, WNOHANG)); });std::string name;std::string wait_string;Service* service = nullptr;if (SubcontextChildReap(pid)) {name = "Subcontext";} else {//查找死亡进程pidservice = ServiceList::GetInstance().FindService(pid, &Service::pid);if (service) {name = StringPrintf("Service '%s' (pid %d)", service->name().c_str(), pid);if (service->flags() & SVC_EXEC) {auto exec_duration = boot_clock::now() - service->time_started();auto exec_duration_ms =std::chrono::duration_cast<std::chrono::milliseconds>(exec_duration).count();wait_string = StringPrintf(" waiting took %f seconds", exec_duration_ms / 1000.0f);} else if (service->flags() & SVC_ONESHOT) {auto exec_duration = boot_clock::now() - service->time_started();auto exec_duration_ms =std::chrono::duration_cast<std::chrono::milliseconds>(exec_duration).count();wait_string = StringPrintf(" oneshot service took %f seconds in background",exec_duration_ms / 1000.0f);}} else {name = StringPrintf("Untracked pid %d", pid);}}if (siginfo.si_code == CLD_EXITED) {LOG(INFO) << name << " exited with status " << siginfo.si_status << wait_string;} else {LOG(INFO) << name << " received signal " << siginfo.si_status << wait_string;}if (!service) {LOG(INFO) << name << " did not have an associated service entry and will not be reaped";return pid;}//杀死进程，清除资源，并设置重启标志service->Reap(siginfo);if (service->flags() & SVC_TEMPORARY) {ServiceList::GetInstance().RemoveService(*service);}return pid;
}

调用Reap函数清理子进程
system/core/init/service.cpp

void Service::Reap(const siginfo_t& siginfo) {//杀死进程if (!(flags_ & SVC_ONESHOT) || (flags_ & SVC_RESTART)) {KillProcessGroup(SIGKILL, false);} else {// Legacy behavior from ~2007 until Android R: this else branch did not exist and we did not// kill the process group in this case.if (SelinuxGetVendorAndroidVersion() >= __ANDROID_API_R__) {// The new behavior in Android R is to kill these process groups in all cases.  The// 'true' parameter instructions KillProcessGroup() to report a warning message where it// detects a difference in behavior has occurred.KillProcessGroup(SIGKILL, true);}}// Remove any socket resources we may have created.//清除socket节点for (const auto& socket : sockets_) {if (socket.persist) {continue;}auto path = ANDROID_SOCKET_DIR "/" + socket.name;unlink(path.c_str());}for (const auto& f : reap_callbacks_) {f(siginfo);}if ((siginfo.si_code != CLD_EXITED || siginfo.si_status != 0) && on_failure_reboot_target_) {LOG(ERROR) << "Service with 'reboot_on_failure' option failed, shutting down system.";trigger_shutdown(*on_failure_reboot_target_);}if (flags_ & SVC_EXEC) UnSetExec();if (name_ == "zygote" || name_ == "zygote64") {//如果是zygote，则清除所有服务进程removeAllEmptyProcessGroups();}if (flags_ & SVC_TEMPORARY) return;pid_ = 0;flags_ &= (~SVC_RUNNING);start_order_ = 0;// Oneshot processes go into the disabled state on exit,// except when manually restarted.if ((flags_ & SVC_ONESHOT) && !(flags_ & SVC_RESTART) && !(flags_ & SVC_RESET)) {flags_ |= SVC_DISABLED;}// Disabled and reset processes do not get restarted automatically.if (flags_ & (SVC_DISABLED | SVC_RESET))  {NotifyStateChange("stopped");return;}#if INIT_FULL_SOURCESstatic bool is_apex_updatable = android::sysprop::ApexProperties::updatable().value_or(false);
#elsestatic bool is_apex_updatable = false;
#endifconst bool is_process_updatable = !use_bootstrap_ns_ && is_apex_updatable;// If we crash > 4 times in 'fatal_crash_window_' minutes or before boot_completed,// reboot into bootloader or set crashing propertyboot_clock::time_point now = boot_clock::now();if (((flags_ & SVC_CRITICAL) || is_process_updatable) && !(flags_ & SVC_RESTART)) {bool boot_completed = GetBoolProperty("sys.boot_completed", false);if (now < time_crashed_ + fatal_crash_window_ || !boot_completed) {if (++crash_count_ > 4) {auto exit_reason = boot_completed ?"in " + std::to_string(fatal_crash_window_.count()) + " minutes" :"before boot completed";if (flags_ & SVC_CRITICAL) {if (!GetBoolProperty("init.svc_debug.no_fatal." + name_, false)) {// Aborts into `fatal_reboot_target_'.SetFatalRebootTarget(fatal_reboot_target_);LOG(FATAL) << "critical process '" << name_ << "' exited 4 times "<< exit_reason;}} else {LOG(ERROR) << "process with updatable components '" << name_<< "' exited 4 times " << exit_reason;// Notifies update_verifier and apexdSetProperty("sys.init.updatable_crashing_process_name", name_);SetProperty("sys.init.updatable_crashing", "1");}}} else {time_crashed_ = now;crash_count_ = 1;}}flags_ &= (~SVC_RESTART);//设置重启标志位flags_ |= SVC_RESTARTING;// Execute all onrestart commands for this service.//启动所有rc中标志位onrestart的服务onrestart_.ExecuteAllCommands();NotifyStateChange("restarting");return;
}

这里清除了进程的资源，并设置该进程标志为SVC_RESTARTING，并启动rc中定义的onrestart相关服务

while (true) {...if (!IsShuttingDown()) {auto next_process_action_time = HandleProcessActions();// If there's a process that needs restarting, wake up in time for that.if (next_process_action_time) {epoll_timeout = std::chrono::ceil<std::chrono::milliseconds>(*next_process_action_time - boot_clock::now());if (*epoll_timeout < 0ms) epoll_timeout = 0ms;}}...}

init进程的死循环中HandleProcessActions会处理服务重启

static std::optional<boot_clock::time_point> HandleProcessActions() {std::optional<boot_clock::time_point> next_process_action_time;//遍历所有的servicefor (const auto& s : ServiceList::GetInstance()) {if ((s->flags() & SVC_RUNNING) && s->timeout_period()) {auto timeout_time = s->time_started() + *s->timeout_period();if (boot_clock::now() > timeout_time) {s->Timeout();} else {if (!next_process_action_time || timeout_time < *next_process_action_time) {next_process_action_time = timeout_time;}}}//如果没有标志为SVC_RESTARTING，则跳过if (!(s->flags() & SVC_RESTARTING)) continue;auto restart_time = s->time_started() + s->restart_period();if (boot_clock::now() > restart_time) {//调用Start重启服务if (auto result = s->Start(); !result.ok()) {LOG(ERROR) << "Could not restart process '" << s->name() << "': " << result.error();}} else {if (!next_process_action_time || restart_time < *next_process_action_time) {next_process_action_time = restart_time;}}}return next_process_action_time;
}

调用Start函数执行

Result<void> Service::Start() {...pid_t pid = -1;if (namespaces_.flags) {pid = clone(nullptr, nullptr, namespaces_.flags | SIGCHLD, nullptr);} else {//新建一个进程pid = fork();}if (pid == 0) {//子进程中执行，运行子进程服务umask(077);RunService(override_mount_namespace, descriptors, std::move(pipefd));_exit(127);}if (pid < 0) {pid_ = 0;return ErrnoError() << "Failed to fork";}...
}

fork子进程，并调用RunService执行service

void Service::RunService(const std::optional<MountNamespace>& override_mount_namespace,const std::vector<Descriptor>& descriptors,std::unique_ptr<std::array<int, 2>, decltype(&ClosePipe)> pipefd) {...if (!ExpandArgsAndExecv(args_, sigstop_)) {PLOG(ERROR) << "cannot execv('" << args_[0]<< "'). See the 'Debugging init' section of init's README.md for tips";}
}

调用ExpandArgsAndExecv执行

static bool ExpandArgsAndExecv(const std::vector<std::string>& args, bool sigstop) {std::vector<std::string> expanded_args;std::vector<char*> c_strings;expanded_args.resize(args.size());c_strings.push_back(const_cast<char*>(args[0].data()));for (std::size_t i = 1; i < args.size(); ++i) {auto expanded_arg = ExpandProps(args[i]);if (!expanded_arg.ok()) {LOG(FATAL) << args[0] << ": cannot expand arguments': " << expanded_arg.error();}expanded_args[i] = *expanded_arg;c_strings.push_back(expanded_args[i].data());}c_strings.push_back(nullptr);if (sigstop) {kill(getpid(), SIGSTOP);}return execv(c_strings[0], c_strings.data()) == 0;
}

调用execv执行可执行文件