[MainLoop] Fix possible use of an invalid iterator

[petpav01]

Summary

Store file descriptors from loop.m_read_fds (if FORCE_PSELECT is defined) and signals from loop.m_signals that need to be processed in MainLoop::RunImpl::ProcessEvents() into a separate vector and then iterate over this container to invoke the callbacks.

This prevents a problem where when the code iterated directly over m_read_fds/m_signals, a callback invoked from within the loop could modify these variables and invalidate the loop iterator. This would then result in the following assertion failure:

llvm/include/llvm/ADT/DenseMap.h:1099: llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>& llvm::DenseMapIterator<KeyT, ValueT, KeyInfoT, Bucket, IsConst>::operator++() [with KeyT = int; ValueT = std::function<void(lldb_private::MainLoopBase&)>; KeyInfoT = llvm::DenseMapInfo<int>; Bucket = llvm::detail::DenseMapPair<int, std::function<void(lldb_private::MainLoopBase&)> >; bool IsConst = false]: assertion "isHandleInSync() && "invalid iterator access!"" failed

[labath]

Looks great, thanks for catching this.

The code was doing it this way initially, but then this got lost with all the frantic refactors.

lldb/trunk/source/Host/common/MainLoop.cpp

if (ppoll(read_fds.data(), read_fds.size(), nullptr, &sigmask) == -1 &&
     return Status(errno, eErrorTypePOSIX);
 
   return Status();
 }
 #endif
 
 void MainLoop::RunImpl::ProcessEvents() {
 #ifdef FORCE_PSELECT
-  for (const auto &fd : loop.m_read_fds) {
-    if (!FD_ISSET(fd.first, &read_fd_set))
-      continue;
-    IOObject::WaitableHandle handle = fd.first;
+  // 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
   for (const auto &fd : read_fds) {
     if ((fd.revents & POLLIN) == 0)
       continue;
     IOObject::WaitableHandle handle = fd.fd;
 #endif
     if (loop.m_terminate_request)
       return;
 
     loop.ProcessReadObject(handle);
   }
 
-  for (const auto &entry : loop.m_signals) {
+  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;
-    if (g_signal_flags[entry.first] == 0)
-      continue; // No signal
-    g_signal_flags[entry.first] = 0;
-    loop.ProcessSignal(entry.first);
+    g_signal_flags[signal] = 0;
+    loop.ProcessSignal(signal);
   }
 }
 #endif
 
 MainLoop::MainLoop() {
 #if HAVE_SYS_EVENT_H
   m_kqueue = kqueue();
   assert(m_kqueue >= 0);