Merge "Restore errno in signal handlers."

static pthread_mutex_t g_sigaction_mutex = PTHREAD_MUTEX_INITIALIZER;

// Since errno is stored per thread, changing it in the signal handler

// modifies the value on the thread in which the signal handler executes.

// If a signal occurs between a call and an errno check, it's possible

// to get the errno set here. Always save and restore it just in case

// code would modify it.

class ErrnoRestorer {

 public:

  ErrnoRestorer() : saved_errno_(errno) {}

  ~ErrnoRestorer() {

    errno = saved_errno_;

  }

 private:

  int saved_errno_;

};

static void SignalLogOnly(int, siginfo_t*, void*) {

  ErrnoRestorer restore;

  BACK_LOGE("pid %d, tid %d: Received a spurious signal %d\n", getpid(), gettid(), THREAD_SIGNAL);

}

static void SignalHandler(int, siginfo_t*, void* sigcontext) {

  ErrnoRestorer restore;

  ThreadEntry* entry = ThreadEntry::Get(getpid(), gettid(), false);

  if (!entry) {

    BACK_LOGE("pid %d, tid %d entry not found", getpid(), gettid());