migrate fields of type folly::SharedMutex to be marked as mutable in …

…fbcode (assorted)

Summary:
The nested-name lock-holder `folly::SharedMutex::ReadHolder` accepts `folly::SharedMutex const&`, i.e., as ref-to-const, and it internally does a `const_cast` to remove the `const`. This internal `const_cast` is necessary because members `folly::SharedMutex::lock_shared` and related member functions are not `const`-qualified.

This lock-holder interface is a convenience since a shared-lock on a shared-mutex instance are commonly acquired in `const`-qualified class member functions, where the shared-mutex instance is a class data member and access to it from a `const`-qualified member function implicitly has only `const`-access to the shared-mutex instance.

But `std::shared_lock` does not have this convenience. It is the canonical lock-holder for shared locks and we wish to migrate all uses of `folly::SharedMutex::ReadHolder` to `std::shared_lock`. So we must mark relevant shared-mutex instances with `mutable`.

As a small shortcut, we search for and mark all class data members of type `folly::SharedMutex` as `mutable`.

Reviewed By: luciang

Differential Revision: D52921022

fbshipit-source-id: bca727c89d153cee583b2862fdf70fdd4e22ff05

mcrouter/AsyncWriter.h

@@ -85,7 +85,7 @@ class AsyncWriter {
   size_t maxQueueSize_;
   std::atomic<size_t> queueSize_{0};
   std::atomic<bool> stopped_{false};
-  folly::SharedMutex runLock_;
+  mutable folly::SharedMutex runLock_;
 
   folly::fibers::FiberManager fiberManager_;
   folly::EventBase eventBase_;

mcrouter/CallbackPool-inl.h

@@ -42,7 +42,7 @@ struct CallbackPool<Args...>::CallbackHandleImpl {
 template <typename... Args>
 struct CallbackPool<Args...>::Data {
   std::set<CallbackHandleImpl*> callbacks;
-  folly::SharedMutex callbackLock;
+  mutable folly::SharedMutex callbackLock;
 };
 
 /* CallbackPool */

mcrouter/Observable.h

@@ -82,7 +82,7 @@ class Observable {
   CallbackPool<const Data&, const Data&> pool_;
   Data data_;
 
-  folly::SharedMutex dataLock_;
+  mutable folly::SharedMutex dataLock_;
 };
 } // namespace mcrouter
 } // namespace memcache

mcrouter/Proxy.h

@@ -192,7 +192,7 @@ class Proxy : public ProxyBase {
   bool beingDestroyed_{false};
 
   /** Read/write lock for config pointer */
-  folly::SharedMutex configLock_;
+  mutable folly::SharedMutex configLock_;
   std::shared_ptr<ProxyConfig<RouterInfo>> config_;
 
   typename RouterInfo::RouterStats requestStats_;