在init程序的main函數中有調用sigchld_handler_init() 設定SIGCHLD signal。這裡設定SIGCHLD的處理函數時，在sa_flags中有加SA_NOCLDSTOP flag，這個flag表示隻有當子程序終止時父程序才接受這個signal，當子程序在暫停或者繼續運作的狀态時是不會收到這個signal的。
void sigchld_handler_init() {
// Create a signalling mechanism for SIGCHLD.
int s[2];
if (socketpair(AF_UNIX, SOCK_STREAM | SOCK_NONBLOCK | SOCK_CLOEXEC, 0, s) == -1) {
PLOG(FATAL) << "socketpair failed in sigchld_handler_init";
}
signal_write_fd = s[0];
signal_read_fd = s[1];
// Write to signal_write_fd if we catch SIGCHLD.
struct sigaction act;
memset(&act, 0, sizeof(act));
act.sa_handler = SIGCHLD_handler;
act.sa_flags = SA_NOCLDSTOP;
sigaction(SIGCHLD, &act, 0);
ReapAnyOutstandingChildren();
register_epoll_handler(signal_read_fd, handle_signal);
}
1. 注冊的SIGCHLD處理函數是SIGCHLD_handler。
2. 調用register_epoll_handler()
init.cpp
void register_epoll_handler(int fd, void (*fn)()) {
epoll_event ev;
ev.events = EPOLLIN;
ev.data.ptr = reinterpret_cast(fn);
if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, fd, &ev) == -1) {
PLOG(ERROR) << "epoll_ctl failed";
}
}
1. 在子程序退出時，init程序會收到SIGCHLD signal，然後調用SIGCHLD_handler()進行處理。這個函數隻是簡單地調用write()
2. 由于上面有進行write，是以在init的main函數中的epoll_wait()會傳回，然後調用通過register_epoll_handler()注冊的函數，即hand_signal()
epoll_event ev;
int nr = TEMP_FAILURE_RETRY(epoll_wait(epoll_fd, &ev, 1, epoll_timeout_ms));
if (nr == -1) {
PLOG(ERROR) << "epoll_wait failed";
} else if (nr == 1) {
((void (*)()) ev.data.ptr)();
}
在hand_signal()中，首先調用read()将資料讀出，然後會調用到ReapOneProcess()。在這個函數中，調用waitpid()等待子程序退出，waitpid()會拿到一個siginfo，這個siginfo中有退出的子程序的pid。
然後根據這個pid從ServiceList中找到這個退出的子程序的Service對象；
然後調用這個Service對象的Reap()。
在Reap函數中，首先調用KillProcessGroup(SIGKILL)。這個函數會調到system/core/libprocessgroup/processgroup.cpp中的killProcessGroup()。這個調用的目的是kill掉退出的子程序的所有子程序。
void Service::KillProcessGroup(int signal) {
// If we've already seen a successful result from killProcessGroup*(), then we have removed
// the cgroup already and calling these functions a second time will simply result in an error.
// This is true regardless of which signal was sent.
// These functions handle their own logging, so no additional logging is needed.
if (!process_cgroup_empty_) {
LOG(INFO) << "Sending signal " << signal << " to service '" << name_ << "' (pid " << pid_
<< ") process group...";
int r;
if (signal == SIGTERM) {
r = killProcessGroupOnce(uid_, pid_, signal);
} else {
r = killProcessGroup(uid_, pid_, signal);
}
if (r == 0) process_cgroup_empty_ = true;
}
}
然後調用UnsetExec()将is_exec_service_running_設定為false并将SVC_EXEC flag清掉；
然後會将flags_中的SVC_RESTARTING flag設定上；
然後調用onrestart_.ExecuteAllCommands()
void Service::Reap(const siginfo_t& siginfo) {
if (!(flags_ & SVC_ONESHOT) || (flags_ & SVC_RESTART)) {
KillProcessGroup(SIGKILL);
}
// Remove any descriptor resources we may have created.
std::for_each(descriptors_.begin(), descriptors_.end(),
std::bind(&DescriptorInfo::Clean, std::placeholders::_1));
for (const auto& f : reap_callbacks_) {
f(siginfo);
}
if (flags_ & SVC_EXEC) UnSetExec();
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_DISABLED;
}
// Disabled and reset processes do not get restarted automatically.
if (flags_ & (SVC_DISABLED | SVC_RESET)) {
NotifyStateChange("stopped");
return;
}
// If we crash > 4 times in 4 minutes, reboot into recovery.
boot_clock::time_point now = boot_clock::now();
if ((flags_ & SVC_CRITICAL) && !(flags_ & SVC_RESTART)) {
if (now < time_crashed_ + 4min) {
if (++crash_count_ > 4) {
LOG(FATAL) << "critical process '" << name_ << "' exited 4 times in 4 minutes";
}
} else {
time_crashed_ = now;
crash_count_ = 1;
}
}
flags_ &= (~SVC_RESTART);
flags_ |= SVC_RESTARTING;
// Execute all onrestart commands for this service.
onrestart_.ExecuteAllCommands();
NotifyStateChange("restarting");
return;
}
然後在init的main函數的死循環中，會調用Service::is_exec_service_running()函數，這個函數是直接return Service class的is_exec_service_running_ static bool變量(注意這個變量是static變量)。
因為上面有将is_exec_service_running_變量設定為false，是以這個if條件成立，是以會調用RestartProcesses()。在這個函數中，會從ServiceList中取出每一個Service對象，然後判斷其flags_變量中是否有SVC_RESTARTING flag，如果有，最終會調到這個service的Start()去再次啟動這個service。