[lldb] Unify/improve MainLoop signal handling (#115197)

Change the signal handler to use a pipe to notify about incoming
signals. This has two benefits:
- the signal no longer has to happen on the MainLoop thread. With the
previous implementation, this had to be the case as that was the only
way to ensure that ppoll gets interrupted correctly. In a multithreaded
process, this would mean that all other threads have to have the signal
blocked at all times.
- we don't need the android-specific workaround, which was necessary due
to the syscall being implemented in a non-atomic way

When the MainLoop class was first implemented, we did not have the
interrupt self-pipe, so syscall interruption was the most
straight-forward implementation. Over time, the class gained new
abilities (the pipe being one of them), so we can now piggy-back on
those.

This patch also changes the kevent-based implementation to use the pipe
for signal notification as well. The motivation there is slightly
different:
- it makes the implementations more uniform
- it makes sure we handle all kinds of signals, like we do with the
linux version (EVFILT_SIGNAL only catches process-directed signals)

Author: labath

lldb/include/lldb/Host/posix/MainLoopPosix.h

@@ -59,6 +59,7 @@ class MainLoopPosix : public MainLoopBase {
 private:
   void ProcessReadObject(IOObject::WaitableHandle handle);
   void ProcessSignal(int signo);
+  void ProcessSignals();
 
   class SignalHandle {
   public:

lldb/source/Host/posix/MainLoopPosix.cpp

@@ -17,29 +17,54 @@
 #include <cerrno>
 #include <csignal>
 #include <ctime>
+#include <fcntl.h>
 #include <vector>
 
 // Multiplexing is implemented using kqueue on systems that support it (BSD
-// variants including OSX). On linux we use ppoll, while android uses pselect
-// (ppoll is present but not implemented properly). On windows we use WSApoll
-// (which does not support signals).
+// variants including OSX). On linux we use ppoll.
 
 #if HAVE_SYS_EVENT_H
 #include <sys/event.h>
-#elif defined(__ANDROID__)
-#include <sys/syscall.h>
 #else
 #include <poll.h>
 #endif
 
 using namespace lldb;
 using namespace lldb_private;
 
-static sig_atomic_t g_signal_flags[NSIG];
+namespace {
+struct GlobalSignalInfo {
+  sig_atomic_t pipe_fd = -1;
+  static_assert(sizeof(sig_atomic_t) >= sizeof(int),
+                "Type too small for a file descriptor");
+  sig_atomic_t flag = 0;
+};
+} // namespace
+static GlobalSignalInfo g_signal_info[NSIG];
 
 static void SignalHandler(int signo, siginfo_t *info, void *) {
   assert(signo < NSIG);
-  g_signal_flags[signo] = 1;
+
+  // Set the flag before writing to the pipe!
+  g_signal_info[signo].flag = 1;
+
+  int fd = g_signal_info[signo].pipe_fd;
+  if (fd < 0) {
+    // This can happen with the following (unlikely) sequence of events:
+    // 1. Thread 1 gets a signal, starts running the signal handler
+    // 2. Thread 2 unregisters the signal handler, setting pipe_fd to -1
+    // 3. Signal handler on thread 1 reads -1 out of pipe_fd
+    // In this case, we can just ignore the signal because we're no longer
+    // interested in it.
+    return;
+  }
+
+  // Write a(ny) character to the pipe to wake up from the poll syscall.
+  char c = '.';
+  ssize_t bytes_written = llvm::sys::RetryAfterSignal(-1, ::write, fd, &c, 1);
+  // We can safely ignore EAGAIN (pipe full), as that means poll will definitely
+  // return.
+  assert(bytes_written == 1 || (bytes_written == -1 && errno == EAGAIN));
 }
 
 class MainLoopPosix::RunImpl {
@@ -48,7 +73,7 @@ class MainLoopPosix::RunImpl {
   ~RunImpl() = default;
 
   Status Poll();
-  void ProcessEvents();
+  void ProcessReadEvents();
 
 private:
   MainLoopPosix &loop;
@@ -58,15 +83,9 @@ class MainLoopPosix::RunImpl {
   struct kevent out_events[4];
   int num_events = -1;
 
-#else
-#ifdef __ANDROID__
-  fd_set read_fd_set;
 #else
   std::vector<struct pollfd> read_fds;
 #endif
-
-  sigset_t get_sigmask();
-#endif
 };
 
 #if HAVE_SYS_EVENT_H
@@ -94,7 +113,7 @@ Status MainLoopPosix::RunImpl::Poll() {
   return Status();
 }
 
-void MainLoopPosix::RunImpl::ProcessEvents() {
+void MainLoopPosix::RunImpl::ProcessReadEvents() {
   assert(num_events >= 0);
   for (int i = 0; i < num_events; ++i) {
     if (loop.m_terminate_request)
@@ -103,74 +122,19 @@ void MainLoopPosix::RunImpl::ProcessEvents() {
     case EVFILT_READ:
       loop.ProcessReadObject(out_events[i].ident);
       break;
-    case EVFILT_SIGNAL:
-      loop.ProcessSignal(out_events[i].ident);
-      break;
     default:
       llvm_unreachable("Unknown event");
     }
   }
 }
 #else
 MainLoopPosix::RunImpl::RunImpl(MainLoopPosix &loop) : loop(loop) {
-#ifndef __ANDROID__
   read_fds.reserve(loop.m_read_fds.size());
-#endif
 }
 
-sigset_t MainLoopPosix::RunImpl::get_sigmask() {
-  sigset_t sigmask;
-  int ret = pthread_sigmask(SIG_SETMASK, nullptr, &sigmask);
-  assert(ret == 0);
-  UNUSED_IF_ASSERT_DISABLED(ret);
-
-  for (const auto &sig : loop.m_signals)
-    sigdelset(&sigmask, sig.first);
-  return sigmask;
-}
-
-#ifdef __ANDROID__
-Status MainLoopPosix::RunImpl::Poll() {
-  // ppoll(2) is not supported on older all android versions. Also, older
-  // versions android (API <= 19) implemented pselect in a non-atomic way, as a
-  // combination of pthread_sigmask and select. This is not sufficient for us,
-  // as we rely on the atomicity to correctly implement signal polling, so we
-  // call the underlying syscall ourselves.
-
-  FD_ZERO(&read_fd_set);
-  int nfds = 0;
-  for (const auto &fd : loop.m_read_fds) {
-    FD_SET(fd.first, &read_fd_set);
-    nfds = std::max(nfds, fd.first + 1);
-  }
-
-  union {
-    sigset_t set;
-    uint64_t pad;
-  } kernel_sigset;
-  memset(&kernel_sigset, 0, sizeof(kernel_sigset));
-  kernel_sigset.set = get_sigmask();
-
-  struct {
-    void *sigset_ptr;
-    size_t sigset_len;
-  } extra_data = {&kernel_sigset, sizeof(kernel_sigset)};
-  if (syscall(__NR_pselect6, nfds, &read_fd_set, nullptr, nullptr, nullptr,
-              &extra_data) == -1) {
-    if (errno != EINTR)
-      return Status(errno, eErrorTypePOSIX);
-    else
-      FD_ZERO(&read_fd_set);
-  }
-
-  return Status();
-}
-#else
 Status MainLoopPosix::RunImpl::Poll() {
   read_fds.clear();
 
-  sigset_t sigmask = get_sigmask();
-
   for (const auto &fd : loop.m_read_fds) {
     struct pollfd pfd;
     pfd.fd = fd.first;
@@ -179,55 +143,39 @@ Status MainLoopPosix::RunImpl::Poll() {
     read_fds.push_back(pfd);
   }
 
-  if (ppoll(read_fds.data(), read_fds.size(), nullptr, &sigmask) == -1 &&
+  if (ppoll(read_fds.data(), read_fds.size(),
+            /*timeout=*/nullptr,
+            /*sigmask=*/nullptr) == -1 &&
       errno != EINTR)
     return Status(errno, eErrorTypePOSIX);
 
   return Status();
 }
-#endif
 
-void MainLoopPosix::RunImpl::ProcessEvents() {
-#ifdef __ANDROID__
-  // Collect first all readable file descriptors into a separate vector and
-  // then iterate over it to invoke callbacks. Iterating directly over
-  // loop.m_read_fds is not possible because the callbacks can modify the
-  // container which could invalidate the iterator.
-  std::vector<IOObject::WaitableHandle> fds;
-  for (const auto &fd : loop.m_read_fds)
-    if (FD_ISSET(fd.first, &read_fd_set))
-      fds.push_back(fd.first);
-
-  for (const auto &handle : fds) {
-#else
+void MainLoopPosix::RunImpl::ProcessReadEvents() {
   for (const auto &fd : read_fds) {
     if ((fd.revents & (POLLIN | POLLHUP)) == 0)
       continue;
     IOObject::WaitableHandle handle = fd.fd;
-#endif
     if (loop.m_terminate_request)
       return;
 
     loop.ProcessReadObject(handle);
   }
-
-  std::vector<int> signals;
-  for (const auto &entry : loop.m_signals)
-    if (g_signal_flags[entry.first] != 0)
-      signals.push_back(entry.first);
-
-  for (const auto &signal : signals) {
-    if (loop.m_terminate_request)
-      return;
-    g_signal_flags[signal] = 0;
-    loop.ProcessSignal(signal);
-  }
 }
 #endif
 
 MainLoopPosix::MainLoopPosix() : m_triggering(false) {
   Status error = m_trigger_pipe.CreateNew(/*child_process_inherit=*/false);
   assert(error.Success());
+
+  // Make the write end of the pipe non-blocking.
+  int result = fcntl(m_trigger_pipe.GetWriteFileDescriptor(), F_SETFL,
+                     fcntl(m_trigger_pipe.GetWriteFileDescriptor(), F_GETFL) |
+                         O_NONBLOCK);
+  assert(result == 0);
+  UNUSED_IF_ASSERT_DISABLED(result);
+
   const int trigger_pipe_fd = m_trigger_pipe.GetReadFileDescriptor();
   m_read_fds.insert({trigger_pipe_fd, [trigger_pipe_fd](MainLoopBase &loop) {
                        char c;
@@ -295,26 +243,17 @@ MainLoopPosix::RegisterSignal(int signo, const Callback &callback,
   sigaddset(&new_action.sa_mask, signo);
   sigset_t old_set;
 
-  g_signal_flags[signo] = 0;
+  // Set signal info before installing the signal handler!
+  g_signal_info[signo].pipe_fd = m_trigger_pipe.GetWriteFileDescriptor();
+  g_signal_info[signo].flag = 0;
 
   // Even if using kqueue, the signal handler will still be invoked, so it's
   // important to replace it with our "benign" handler.
   int ret = sigaction(signo, &new_action, &info.old_action);
   UNUSED_IF_ASSERT_DISABLED(ret);
   assert(ret == 0 && "sigaction failed");
 
-#if HAVE_SYS_EVENT_H
-  struct kevent ev;
-  EV_SET(&ev, signo, EVFILT_SIGNAL, EV_ADD, 0, 0, 0);
-  ret = kevent(m_kqueue, &ev, 1, nullptr, 0, nullptr);
-  assert(ret == 0);
-#endif
-
-  // If we're using kqueue, the signal needs to be unblocked in order to
-  // receive it. If using pselect/ppoll, we need to block it, and later unblock
-  // it as a part of the system call.
-  ret = pthread_sigmask(HAVE_SYS_EVENT_H ? SIG_UNBLOCK : SIG_BLOCK,
-                        &new_action.sa_mask, &old_set);
+  ret = pthread_sigmask(SIG_UNBLOCK, &new_action.sa_mask, &old_set);
   assert(ret == 0 && "pthread_sigmask failed");
   info.was_blocked = sigismember(&old_set, signo);
   auto insert_ret = m_signals.insert({signo, info});
@@ -349,14 +288,8 @@ void MainLoopPosix::UnregisterSignal(
   assert(ret == 0);
   UNUSED_IF_ASSERT_DISABLED(ret);
 
-#if HAVE_SYS_EVENT_H
-  struct kevent ev;
-  EV_SET(&ev, signo, EVFILT_SIGNAL, EV_DELETE, 0, 0, 0);
-  ret = kevent(m_kqueue, &ev, 1, nullptr, 0, nullptr);
-  assert(ret == 0);
-#endif
-
   m_signals.erase(it);
+  g_signal_info[signo] = {};
 }
 
 Status MainLoopPosix::Run() {
@@ -370,7 +303,9 @@ Status MainLoopPosix::Run() {
     if (error.Fail())
       return error;
 
-    impl.ProcessEvents();
+    impl.ProcessReadEvents();
+
+    ProcessSignals();
 
     m_triggering = false;
     ProcessPendingCallbacks();
@@ -384,6 +319,21 @@ void MainLoopPosix::ProcessReadObject(IOObject::WaitableHandle handle) {
     it->second(*this); // Do the work
 }
 
+void MainLoopPosix::ProcessSignals() {
+  std::vector<int> signals;
+  for (const auto &entry : m_signals)
+    if (g_signal_info[entry.first].flag != 0)
+      signals.push_back(entry.first);
+
+  for (const auto &signal : signals) {
+    if (m_terminate_request)
+      return;
+
+    g_signal_info[signal].flag = 0;
+    ProcessSignal(signal);
+  }
+}
+
 void MainLoopPosix::ProcessSignal(int signo) {
   auto it = m_signals.find(signo);
   if (it != m_signals.end()) {

lldb/unittests/Host/MainLoopTest.cpp

@@ -254,6 +254,17 @@ TEST_F(MainLoopTest, Signal) {
   ASSERT_EQ(1u, callback_count);
 }
 
+TEST_F(MainLoopTest, SignalOnOtherThread) {
+  MainLoop loop;
+  Status error;
+
+  auto handle = loop.RegisterSignal(SIGUSR1, make_callback(), error);
+  ASSERT_TRUE(error.Success());
+  std::thread([] { pthread_kill(pthread_self(), SIGUSR1); }).join();
+  ASSERT_TRUE(loop.Run().Success());
+  ASSERT_EQ(1u, callback_count);
+}
+
 // Test that a signal which is not monitored by the MainLoop does not
 // cause a premature exit.
 TEST_F(MainLoopTest, UnmonitoredSignal) {