work around a miscompile crashing DistributedMutex::unlock

Summary: Under gcc8 in some usages, the compiler emits a stack copy of the proxy state but passes `7` instead of the address of the stack copy. Looks like incorrect codegen. Work around that bug by not having a stack copy.

Reviewed By: aary

Differential Revision: D28890467

fbshipit-source-id: 50b16464708c7ef3e360618f3d738264f59cf8a6

folly/synchronization/DistributedMutex-inl.h

@@ -1533,7 +1533,8 @@ bool tryUnlockClean(Atomic& state, Proxy& proxy, Sleepers sleepers) {
 
 template <template <typename> class Atomic, bool Publish>
 void DistributedMutex<Atomic, Publish>::unlock(
-    DistributedMutex::DistributedMutexStateProxy proxy) {
+    DistributedMutex::DistributedMutexStateProxy const& proxy_) {
+  auto proxy = proxy_;
   // we always wake up ready threads and timed waiters if we saw either
   DCHECK(proxy) << "Invalid proxy passed to DistributedMutex::unlock()";
   DCHECK(!proxy.combined_) << "Cannot unlock mutex after a successful combine";

folly/synchronization/DistributedMutex.h

@@ -189,7 +189,7 @@ class DistributedMutex {
    * It is undefined behavior to unlock from a thread that did not lock the
    * mutex
    */
-  void unlock(DistributedMutexStateProxy);
+  void unlock(DistributedMutexStateProxy const&);
 
   /**
    * Try to acquire the mutex