SERVER-33413 nia countdownlatch misues condvars

You have to hold the mutex that a condvar is waiting on while modifying
the shared state the predicate checks for, or you can see a misordered
write and hang forever.

1 files changed, 9 insertions, 7 deletions

diff --git a/src/mongo/executor/network_interface_asio_test_utils.h b/src/mongo/executor/network_interface_asio_test_utils.h
index 54424735b9b..a95da3dfc5f 100644
--- a/src/mongo/executor/network_interface_asio_test_utils.h
+++ b/src/mongo/executor/network_interface_asio_test_utils.h
@@ -34,7 +34,6 @@
 #include <vector>
 
 #include "mongo/executor/task_executor.h"
-#include "mongo/platform/atomic_word.h"
 #include "mongo/stdx/condition_variable.h"
 #include "mongo/stdx/future.h"
 #include "mongo/stdx/mutex.h"
@@ -120,26 +119,29 @@ private:
 
 class CountdownLatch {
 public:
-    CountdownLatch(uint32_t count) : _count(count) {}
+    explicit CountdownLatch(uint32_t count) : _count(count) {}
 
     void countDown() {
-        if (_count.load() == 0) {
+        stdx::lock_guard<stdx::mutex> lk(_mtx);
+        if (_count == 0) {
             return;
         }
-        if (_count.subtractAndFetch(1) == 0) {
+
+        --_count;
+        if (_count == 0) {
             _cv.notify_all();
         }
     }
 
     void await() {
         stdx::unique_lock<stdx::mutex> lk(_mtx);
-        _cv.wait(lk, [&] { return _count.load() == 0; });
+        _cv.wait(lk, [&] { return _count == 0; });
     }
 
 private:
-    stdx::condition_variable _cv;
     stdx::mutex _mtx;
-    AtomicUInt32 _count;
+    stdx::condition_variable _cv;
+    size_t _count;
 };
 
 namespace helpers {