[lldb] [MainLoopPosix] Fix crash upon adding lots of pending callbacks

If lots of pending callbacks are added while the main loop has exited
already, the trigger pipe buffer fills in, causing the write to fail
and the related assertion to fail.  To avoid this, add a boolean member
indicating whether the callbacks have been triggered already.
If the trigger was done, avoid writing to the pipe until loops proceeds
to run them and resets the variable.

Besides fixing the issue, this also avoids writing to the pipe multiple
times if callbacks are added faster than the loop is able to process
them.  Previously, this would lead to the loop performing multiple read
iterations from pipe unnecessarily.

Sponsored by: The FreeBSD Foundation

Differential Revision: https://reviews.llvm.org/D135516

diff --git a/lldb/include/lldb/Host/posix/MainLoopPosix.h b/lldb/include/lldb/Host/posix/MainLoopPosix.h
index 18185c3..07497b7 100644 (file)
--- a/lldb/include/lldb/Host/posix/MainLoopPosix.h
+++ b/lldb/include/lldb/Host/posix/MainLoopPosix.h
@@ -13,6 +13,7 @@
 #include "lldb/Host/MainLoopBase.h"
 #include "lldb/Host/Pipe.h"
 #include "llvm/ADT/DenseMap.h"
+#include <atomic>
 #include <csignal>
 #include <list>
 #include <vector>
@@ -87,6 +88,7 @@ private:
   llvm::DenseMap<IOObject::WaitableHandle, Callback> m_read_fds;
   llvm::DenseMap<int, SignalInfo> m_signals;
   Pipe m_trigger_pipe;
+  std::atomic<bool> m_triggering;
 #if HAVE_SYS_EVENT_H
   int m_kqueue;
 #endif

diff --git a/lldb/source/Host/posix/MainLoopPosix.cpp b/lldb/source/Host/posix/MainLoopPosix.cpp
index 37d339d..b185c3d 100644 (file)
--- a/lldb/source/Host/posix/MainLoopPosix.cpp
+++ b/lldb/source/Host/posix/MainLoopPosix.cpp
@@ -222,7 +222,7 @@ void MainLoopPosix::RunImpl::ProcessEvents() {
 }
 #endif
 
-MainLoopPosix::MainLoopPosix() {
+MainLoopPosix::MainLoopPosix() : m_triggering(false) {
   Status error = m_trigger_pipe.CreateNew(/*child_process_inherit=*/false);
   assert(error.Success());
   const int trigger_pipe_fd = m_trigger_pipe.GetReadFileDescriptor();
@@ -371,6 +371,7 @@ Status MainLoopPosix::Run() {
 
     impl.ProcessEvents();
 
+    m_triggering = false;
     ProcessPendingCallbacks();
   }
   return Status();
@@ -395,6 +396,9 @@ void MainLoopPosix::ProcessSignal(int signo) {
 }
 
 void MainLoopPosix::TriggerPendingCallbacks() {
+  if (m_triggering.exchange(true))
+    return;
+
   char c = '.';
   size_t bytes_written;
   Status error = m_trigger_pipe.Write(&c, 1, bytes_written);

diff --git a/lldb/unittests/Host/MainLoopTest.cpp b/lldb/unittests/Host/MainLoopTest.cpp
index 00e8551..b1857fe 100644 (file)
--- a/lldb/unittests/Host/MainLoopTest.cpp
+++ b/lldb/unittests/Host/MainLoopTest.cpp
@@ -171,6 +171,17 @@ TEST_F(MainLoopTest, PendingCallbackTrigger) {
   ASSERT_TRUE(callback2_called);
 }
 
+// Regression test for assertion failure if a lot of callbacks end up
+// being queued after loop exits.
+TEST_F(MainLoopTest, PendingCallbackAfterLoopExited) {
+  MainLoop loop;
+  Status error;
+  ASSERT_TRUE(loop.Run().Success());
+  // Try to fill the pipe buffer in.
+  for (int i = 0; i < 65536; ++i)
+    loop.AddPendingCallback([&](MainLoopBase &loop) {});
+}
+
 #ifdef LLVM_ON_UNIX
 TEST_F(MainLoopTest, DetectsEOF) {