bool BacktraceThread::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
if (ucontext) {
// Unwind using an already existing ucontext.
return impl_->Unwind(num_ignore_frames, ucontext);
}
// Prevent multiple threads trying to set the trigger action on different
// threads at the same time.
if (pthread_mutex_lock(&g_sigaction_mutex) < 0) {
BACK_LOGW("sigaction failed: %s", strerror(errno));
return false;
}
ThreadEntry* entry = ThreadEntry::Get(Pid(), Tid());
entry->Lock();
struct sigaction act, oldact;
memset(&act, 0, sizeof(act));
act.sa_sigaction = SignalHandler;
act.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK;
sigemptyset(&act.sa_mask);
if (sigaction(THREAD_SIGNAL, &act, &oldact) != 0) {
BACK_LOGW("sigaction failed %s", strerror(errno));
entry->Unlock();
ThreadEntry::Remove(entry);
pthread_mutex_unlock(&g_sigaction_mutex);
return false;
}
if (tgkill(Pid(), Tid(), THREAD_SIGNAL) != 0) {
BACK_LOGW("tgkill %d failed: %s", Tid(), strerror(errno));
sigaction(THREAD_SIGNAL, &oldact, NULL);
entry->Unlock();
ThreadEntry::Remove(entry);
pthread_mutex_unlock(&g_sigaction_mutex);
return false;
}
// Wait for the thread to get the ucontext.
entry->Wait(0);
// After the thread has received the signal, allow other unwinders to
// continue.
sigaction(THREAD_SIGNAL, &oldact, NULL);
pthread_mutex_unlock(&g_sigaction_mutex);
bool unwind_done = impl_->Unwind(num_ignore_frames, entry->GetUcontext());
// Tell the signal handler to exit and release the entry.
entry->Wake();
return unwind_done;
}
static void SignalHandler(int, siginfo_t*, void* sigcontext) {
ThreadEntry* entry = ThreadEntry::Get(getpid(), gettid(), false);
if (!entry) {
BACK_LOGW("Unable to find pid %d tid %d information", getpid(), gettid());
return;
}
entry->CopyUcontext(reinterpret_cast<ucontext_t*>(sigcontext));
// Indicate the ucontext is now valid.
entry->Wake();
// Pause the thread until the unwind is complete. This avoids having
// the thread run ahead causing problems.
entry->Wait(1);
ThreadEntry::Remove(entry);
}