SERVER-39961 Update proxy scope to use two cvs

Our proxyscope type currently uses one condition variable to wait/notify
from the caller and the callee threads. It's been pointed out that this
may not be safe on older versions of glibc, as there used to be a
different understanding of whether or not it was acceptable for a
condvar notify to notify the thread which called notify (if it later
called wait).

See http://austingroupbugs.net/view.php?id=609

2 files changed, 11 insertions, 8 deletions

diff --git a/src/mongo/scripting/mozjs/proxyscope.cpp b/src/mongo/scripting/mozjs/proxyscope.cpp
index afc21eca1eb..b6ccdf26d08 100644
--- a/src/mongo/scripting/mozjs/proxyscope.cpp
+++ b/src/mongo/scripting/mozjs/proxyscope.cpp
@@ -51,7 +51,6 @@ MozJSProxyScope::MozJSProxyScope(MozJSScriptEngine* engine)
       _mutex(),
       _state(State::Idle),
       _status(Status::OK()),
-      _condvar(),
       // Despite calling PR_CreateThread, we're actually using our own
       // implementation of PosixNSPR.cpp in this directory. So these threads
       // are actually hosted on top of stdx::threads and most of the flags
@@ -292,19 +291,21 @@ void MozJSProxyScope::runOnImplThread(unique_function<void()> f) {
     invariant(_state == State::Idle);
     _state = State::ProxyRequest;
 
-    _condvar.notify_one();
+    lk.unlock();
+    _implCondvar.notify_one();
+    lk.lock();
 
     Interruptible* interruptible = _opCtx ? _opCtx : Interruptible::notInterruptible();
 
     auto pred = [&] { return _state == State::ImplResponse; };
 
     try {
-        interruptible->waitForConditionOrInterrupt(_condvar, lk, pred);
+        interruptible->waitForConditionOrInterrupt(_proxyCondvar, lk, pred);
     } catch (const DBException& ex) {
         _status = ex.toStatus();
 
         _implScope->kill();
-        _condvar.wait(lk, pred);
+        _proxyCondvar.wait(lk, pred);
     }
 
     _state = State::Idle;
@@ -327,7 +328,7 @@ void MozJSProxyScope::shutdownThread() {
         _state = State::Shutdown;
     }
 
-    _condvar.notify_one();
+    _implCondvar.notify_one();
 
     PR_JoinThread(_thread);
 }
@@ -370,7 +371,7 @@ void MozJSProxyScope::implThread(void* arg) {
         stdx::unique_lock<stdx::mutex> lk(proxy->_mutex);
         {
             MONGO_IDLE_THREAD_BLOCK;
-            proxy->_condvar.wait(lk, [proxy] {
+            proxy->_implCondvar.wait(lk, [proxy] {
                 return proxy->_state == State::ProxyRequest || proxy->_state == State::Shutdown;
             });
         }
@@ -388,7 +389,8 @@ void MozJSProxyScope::implThread(void* arg) {
 
         proxy->_state = State::ImplResponse;
 
-        proxy->_condvar.notify_one();
+        lk.unlock();
+        proxy->_proxyCondvar.notify_one();
     }
 }
 
diff --git a/src/mongo/scripting/mozjs/proxyscope.h b/src/mongo/scripting/mozjs/proxyscope.h
index 607c3d2b4b2..ddaa9c815ec 100644
--- a/src/mongo/scripting/mozjs/proxyscope.h
+++ b/src/mongo/scripting/mozjs/proxyscope.h
@@ -200,7 +200,8 @@ private:
     Status _status;
     OperationContext* _opCtx = nullptr;
 
-    stdx::condition_variable _condvar;
+    stdx::condition_variable _proxyCondvar;
+    stdx::condition_variable _implCondvar;
     PRThread* _thread;
 };