Index: include/lldb/Host/MainLoop.h
===================================================================
--- include/lldb/Host/MainLoop.h
+++ include/lldb/Host/MainLoop.h
@@ -93,6 +93,7 @@
 #endif
     bool was_blocked : 1;
   };
+  class RunImpl;
 
   llvm::DenseMap<IOObject::WaitableHandle, Callback> m_read_fds;
   llvm::DenseMap<int, SignalInfo> m_signals;
Index: source/Host/common/MainLoop.cpp
===================================================================
--- source/Host/common/MainLoop.cpp
+++ source/Host/common/MainLoop.cpp
@@ -32,6 +32,10 @@
 #define POLL poll
 #endif
 
+#ifdef __ANDROID__
+#define FORCE_PSELECT
+#endif
+
 #if SIGNAL_POLLING_UNSUPPORTED
 #ifdef LLVM_ON_WIN32
 typedef int sigset_t;
@@ -59,6 +63,189 @@
   g_signal_flags[signo] = 1;
 }
 
+class MainLoop::RunImpl {
+public:
+  // TODO: Use llvm::Expected<T>
+  static std::unique_ptr<RunImpl> Create(MainLoop &loop, Error &error);
+  ~RunImpl();
+
+  Error Poll();
+
+  template <typename F> void ForEachReadFD(F &&f);
+  template <typename F> void ForEachSignal(F &&f);
+
+private:
+  MainLoop &loop;
+
+#if HAVE_SYS_EVENT_H
+  int queue_id;
+  std::vector<struct kevent> in_events;
+  struct kevent out_events[4];
+  int num_events = -1;
+
+  RunImpl(MainLoop &loop, int queue_id) : loop(loop), queue_id(queue_id) {
+    in_events.reserve(loop.m_read_fds.size() + loop.m_signals.size());
+  }
+#else
+  std::vector<int> signals;
+#ifdef FORCE_PSELECT
+  fd_set read_fd_set;
+#else
+  std::vector<struct pollfd> read_fds;
+#endif
+
+  RunImpl(MainLoop &loop) : loop(loop) {
+    signals.reserve(loop.m_signals.size());
+  }
+
+  sigset_t get_sigmask();
+#endif
+};
+
+#if HAVE_SYS_EVENT_H
+MainLoop::RunImpl::~RunImpl() {
+  int r = close(queue_id);
+  assert(r == 0);
+  (void)r;
+}
+std::unique_ptr<MainLoop::RunImpl> MainLoop::RunImpl::Create(MainLoop &loop, Error &error)
+{
+  error.Clear();
+  int queue_id = kqueue();
+  if(queue_id < 0) {
+    error = Error(errno, eErrorTypePOSIX);
+    return nullptr;
+  }
+  return std::unique_ptr<RunImpl>(new RunImpl(loop, queue_id));
+}
+
+Error MainLoop::RunImpl::Poll() {
+  in_events.resize(loop.m_read_fds.size() + loop.m_signals.size());
+  unsigned i = 0;
+  for (auto &fd : loop.m_read_fds)
+    EV_SET(&in_events[i++], fd.first, EVFILT_READ, EV_ADD, 0, 0, 0);
+
+  for (const auto &sig : loop.m_signals)
+    EV_SET(&in_events[i++], sig.first, EVFILT_SIGNAL, EV_ADD, 0, 0, 0);
+
+  num_events = kevent(queue_id, in_events.data(), in_events.size(), out_events,
+                      llvm::array_lengthof(out_events), nullptr);
+
+  if (num_events < 0)
+    return Error("kevent() failed with error %d\n", num_events);
+  return Error();
+}
+
+template <typename F> void MainLoop::RunImpl::ForEachReadFD(F &&f) {
+  assert(num_events >= 0);
+  for (int i = 0; i < num_events; ++i) {
+    f(out_events[i].ident);
+    if (loop.m_terminate_request)
+      return;
+  }
+}
+template <typename F> void MainLoop::RunImpl::ForEachSignal(F && f) {}
+#else
+MainLoop::RunImpl::~RunImpl() {}
+std::unique_ptr<MainLoop::RunImpl> MainLoop::RunImpl::Create(MainLoop &loop, Error &error)
+{
+  error.Clear();
+  return std::unique_ptr<RunImpl>(new RunImpl(loop));
+}
+
+sigset_t MainLoop::RunImpl::get_sigmask() {
+#if SIGNAL_POLLING_UNSUPPORTED
+  return 0;
+#else
+  sigset_t sigmask;
+  int ret = pthread_sigmask(SIG_SETMASK, nullptr, &sigmask);
+  assert(ret == 0);
+  (void) ret;
+
+  for (const auto &sig : loop.m_signals) {
+    signals.push_back(sig.first);
+    sigdelset(&sigmask, sig.first);
+  }
+  return sigmask;
+#endif
+}
+
+#ifdef FORCE_PSELECT
+Error MainLoop::RunImpl::Poll() {
+  signals.clear();
+
+  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);
+  }
+
+  sigset_t sigmask = get_sigmask();
+  if (pselect(nfds, &read_fd_set, nullptr, nullptr, nullptr, &sigmask) == -1 &&
+      errno != EINTR)
+    return Error(errno, eErrorTypePOSIX);
+
+  return Error();
+}
+
+template <typename F> void MainLoop::RunImpl::ForEachReadFD(F &&f) {
+  for (const auto &fd : loop.m_read_fds) {
+    if(!FD_ISSET(fd.first, &read_fd_set))
+      continue;
+
+    f(fd.first);
+    if (loop.m_terminate_request)
+      return;
+  }
+}
+#else
+Error MainLoop::RunImpl::Poll() {
+  signals.clear();
+  read_fds.clear();
+
+  sigset_t sigmask = get_sigmask();
+
+  for (const auto &fd : loop.m_read_fds) {
+    struct pollfd pfd;
+    pfd.fd = fd.first;
+    pfd.events = POLLIN;
+    pfd.revents = 0;
+    read_fds.push_back(pfd);
+  }
+
+  if (ppoll(read_fds.data(), read_fds.size(), nullptr, &sigmask) == -1 &&
+      errno != EINTR)
+    return Error(errno, eErrorTypePOSIX);
+
+  return Error();
+}
+
+template <typename F> void MainLoop::RunImpl::ForEachReadFD(F &&f) {
+  for (const auto &fd : read_fds) {
+    if ((fd.revents & POLLIN) == 0)
+      continue;
+
+    f(fd.fd);
+    if (loop.m_terminate_request)
+      return;
+  }
+}
+#endif
+
+template <typename F> void MainLoop::RunImpl::ForEachSignal(F &&f) {
+  for (int sig : signals) {
+    if (g_signal_flags[sig] == 0)
+      continue; // No signal
+    g_signal_flags[sig] = 0;
+    f(sig);
+
+    if (loop.m_terminate_request)
+      return;
+  }
+}
+#endif
+
 MainLoop::~MainLoop() {
   assert(m_read_fds.size() == 0);
   assert(m_signals.size() == 0);
@@ -161,108 +348,35 @@
 }
 
 Error MainLoop::Run() {
-  std::vector<int> signals;
   m_terminate_request = false;
-  signals.reserve(m_signals.size());
   
-#if HAVE_SYS_EVENT_H
-  int queue_id = kqueue();
-  if (queue_id < 0)
-    Error("kqueue failed with error %d\n", queue_id);
-
-  std::vector<struct kevent> events;
-  events.reserve(m_read_fds.size() + m_signals.size());
-#else
-  sigset_t sigmask;
-  std::vector<struct pollfd> read_fds;
-  read_fds.reserve(m_read_fds.size());
-#endif
+  Error error;
+  auto impl = RunImpl::Create(*this, error);
+  if (!impl)
+    return error;
 
   // run until termination or until we run out of things to listen to
   while (!m_terminate_request && (!m_read_fds.empty() || !m_signals.empty())) {
-    // To avoid problems with callbacks changing the things we're supposed to
-    // listen to, we
-    // will store the *real* list of events separately.
-    signals.clear();
-
-#if HAVE_SYS_EVENT_H
-    events.resize(m_read_fds.size() + m_signals.size());
-    int i = 0;
-    for (auto &fd: m_read_fds) {
-      EV_SET(&events[i++], fd.first, EVFILT_READ, EV_ADD, 0, 0, 0);
-    }
-
-    for (const auto &sig : m_signals) {
-      signals.push_back(sig.first);
-      EV_SET(&events[i++], sig.first, EVFILT_SIGNAL, EV_ADD, 0, 0, 0);
-    }
-
-    struct kevent event_list[4];
-    int num_events =
-        kevent(queue_id, events.data(), events.size(), event_list, 4, NULL);
-
-    if (num_events < 0)
-      return Error("kevent() failed with error %d\n", num_events);
-
-#else
-    read_fds.clear();
 
-#if !SIGNAL_POLLING_UNSUPPORTED
-    if (int ret = pthread_sigmask(SIG_SETMASK, nullptr, &sigmask))
-      return Error("pthread_sigmask failed with error %d\n", ret);
-
-    for (const auto &sig : m_signals) {
-      signals.push_back(sig.first);
-      sigdelset(&sigmask, sig.first);
-    }
-#endif
-
-    for (const auto &fd : m_read_fds) {
-      struct pollfd pfd;
-      pfd.fd = fd.first;
-      pfd.events = POLLIN;
-      pfd.revents = 0;
-      read_fds.push_back(pfd);
-    }
-
-    if (ppoll(read_fds.data(), read_fds.size(), nullptr, &sigmask) == -1 &&
-        errno != EINTR)
-      return Error(errno, eErrorTypePOSIX);
-#endif
-
-    for (int sig : signals) {
-      if (g_signal_flags[sig] == 0)
-        continue; // No signal
-      g_signal_flags[sig] = 0;
+    error = impl->Poll();
+    if (error.Fail())
+      return error;
 
+    impl->ForEachSignal([&](int sig) {
       auto it = m_signals.find(sig);
-      if (it == m_signals.end())
-        continue; // Signal must have gotten unregistered in the meantime
-
-      it->second.callback(*this); // Do the work
-
-      if (m_terminate_request)
-        return Error();
-    }
-
-#if HAVE_SYS_EVENT_H
-    for (int i = 0; i < num_events; ++i) {
-      auto it = m_read_fds.find(event_list[i].ident);
-#else
-    for (auto fd : read_fds) {
-      if ((fd.revents & POLLIN) == 0)
-        continue;
-
-      auto it = m_read_fds.find(fd.fd);
-#endif
-      if (it == m_read_fds.end())
-        continue; // File descriptor must have gotten unregistered in the
-                  // meantime
-      it->second(*this); // Do the work
-
-      if (m_terminate_request)
-        return Error();
-    }
+      if (it != m_signals.end())
+        it->second.callback(*this); // Do the work
+    });
+    if (m_terminate_request)
+      return Error();
+
+    impl->ForEachReadFD([&](int fd) {
+      auto it = m_read_fds.find(fd);
+      if (it != m_read_fds.end())
+        it->second(*this); // Do the work
+    });
+    if (m_terminate_request)
+      return Error();
   }
   return Error();
 }