DCHECK on reentrant invocations of loop()

Summary:
I had a crash bug where an `EventBase` handler called a function, which tried to use the thread's EventBase as if it were its own:

```
auto evb = folly::EventBaseManager::get()->getEventBase();
// schedule work on evb, which calls evb->terminateLoopSoon() on completion
evb->loopForever();
```

This ended up invoking the `event_base_loop()` reentrantly, and corrupting the heap in a hard-to-debug way.

Reviewed By: djwatson

Differential Revision: D3408155

fbshipit-source-id: b8855aa3b390fa33e032ab295d386122d2cb872a

diff --git a/folly/io/async/EventBase.cpp b/folly/io/async/EventBase.cpp
index 2680d96cd90fb5d4368753fa28689f726cebff1e..9d5949e0a0718746d83eb3253d70027eb60760c6 100644 (file)
--- a/folly/io/async/EventBase.cpp
+++ b/folly/io/async/EventBase.cpp
@@ -303,6 +303,19 @@ bool EventBase::loopOnce(int flags) {
 
 bool EventBase::loopBody(int flags) {
   VLOG(5) << "EventBase(): Starting loop.";
+
+  DCHECK(!invokingLoop_)
+      << "Your code just tried to loop over an event base from inside another "
+      << "event base loop. Since libevent is not reentrant, this leads to "
+      << "undefined behavior in opt builds. Please fix immediately. For the "
+      << "common case of an inner function that needs to do some synchronous "
+      << "computation on an event-base, replace getEventBase() by a new, "
+      << "stack-allocated EvenBase.";
+  invokingLoop_ = true;
+  SCOPE_EXIT {
+    invokingLoop_ = false;
+  };
+
   int res = 0;
   bool ranLoopCallbacks;
   bool blocking = !(flags & EVLOOP_NONBLOCK);

diff --git a/folly/io/async/EventBase.h b/folly/io/async/EventBase.h
index c65628d2ca1bc5bc5d4a13b041cd3fc0a77a1b0c..3fcf60e3a63295af0e14e4aca561e23f137cafb2 100644 (file)
--- a/folly/io/async/EventBase.h
+++ b/folly/io/async/EventBase.h
@@ -723,6 +723,11 @@ class EventBase : private boost::noncopyable,
   uint64_t nextLoopCnt_;
   uint64_t latestLoopCnt_;
   uint64_t startWork_;
+  // Prevent undefined behavior from invoking event_base_loop() reentrantly.
+  // This is needed since many projects use libevent-1.4, which lacks commit
+  // b557b175c00dc462c1fce25f6e7dd67121d2c001 from
+  // https://github.com/libevent/libevent/.
+  bool invokingLoop_{false};
 
   // Observer to export counters
   std::shared_ptr<EventBaseObserver> observer_;