Disable exception checks if it wasn't thrown by a fiber

Summary: If fiber was activated with an exception it's safe for that fiber to be preempted.

Differential Revision: D15640534

fbshipit-source-id: d908aacd81e1ec57874324a58b180587a1fe2eeb

folly/fibers/Fiber.cpp

@@ -174,8 +174,10 @@ void Fiber::preempt(State state) {
     DCHECK_EQ(fiberManager_.activeFiber_, this);
     DCHECK_EQ(state_, RUNNING);
     DCHECK_NE(state, RUNNING);
-    CHECK(state == AWAITING_IMMEDIATE || !std::current_exception());
-    CHECK(state == AWAITING_IMMEDIATE || !std::uncaught_exception());
+    if (state != AWAITING_IMMEDIATE) {
+      CHECK(fiberManager_.currentException_ == std::current_exception());
+      CHECK_EQ(fiberManager_.numUncaughtExceptions_, uncaught_exceptions());
+    }
 
     state_ = state;
 

folly/fibers/FiberManagerInternal-inl.h

@@ -198,6 +198,9 @@ void FiberManager::runFibersHelper(LoopFunc&& loopFunc) {
   // Support nested FiberManagers
   auto originalFiberManager = std::exchange(currentFiberManager_, this);
 
+  numUncaughtExceptions_ = uncaught_exceptions();
+  currentException_ = std::current_exception();
+
   // Save current context, and reset it after executing all fibers.
   // This can avoid a lot of context swapping,
   // if the Fibers share the same context

folly/fibers/FiberManagerInternal.h

@@ -548,6 +548,15 @@ class FiberManager : public ::folly::Executor {
 
   ssize_t remoteCount_{0};
 
+  /**
+   * Number of uncaught exceptions when FiberManager loop was called.
+   */
+  ssize_t numUncaughtExceptions_{0};
+  /**
+   * Current exception when FiberManager loop was called.
+   */
+  std::exception_ptr currentException_;
+
   class FibersPoolResizer final : private HHWheelTimer::Callback {
    public:
     explicit FibersPoolResizer(FiberManager& fm) : fiberManager_(fm) {}

folly/fibers/test/FibersTest.cpp

@@ -2460,3 +2460,57 @@ TEST(FiberManager, swapWithException) {
   });
   evb.loop();
 }
+
+TEST(FiberManager, loopInCatch) {
+  folly::EventBase evb;
+  auto& fm = getFiberManager(evb);
+  bool started = false;
+  folly::fibers::Baton baton;
+  bool done = false;
+
+  fm.addTask([&] {
+    started = true;
+    baton.wait();
+    done = true;
+  });
+
+  try {
+    throw std::logic_error("expected");
+  } catch (...) {
+    EXPECT_FALSE(started);
+    evb.drive();
+    EXPECT_TRUE(started);
+    EXPECT_FALSE(done);
+    baton.post();
+    evb.drive();
+    EXPECT_TRUE(done);
+  }
+}
+
+TEST(FiberManager, loopInUnwind) {
+  folly::EventBase evb;
+  auto& fm = getFiberManager(evb);
+  bool started = false;
+  folly::fibers::Baton baton;
+  bool done = false;
+
+  fm.addTask([&] {
+    started = true;
+    baton.wait();
+    done = true;
+  });
+
+  try {
+    SCOPE_EXIT {
+      EXPECT_FALSE(started);
+      evb.drive();
+      EXPECT_TRUE(started);
+      EXPECT_FALSE(done);
+      baton.post();
+      evb.drive();
+      EXPECT_TRUE(done);
+    };
+    throw std::logic_error("expected");
+  } catch (...) {
+  }
+}