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: D52921205

fbshipit-source-id: 966782bbe1372f83b720ce9a1dddaf1642288964

cachelib/allocator/CacheAllocator.h

@@ -2124,7 +2124,7 @@ class CacheAllocator : public CacheBase {
   typename Item::PtrCompressor compressor_;
 
   // Lock to synchronize addition of a new pool and its resizing/rebalancing
-  folly::SharedMutex poolsResizeAndRebalanceLock_;
+  mutable folly::SharedMutex poolsResizeAndRebalanceLock_;
 
   // container for the allocations which are currently being memory managed by
   // the cache allocator.
@@ -2167,7 +2167,7 @@ class CacheAllocator : public CacheBase {
 
   // lock to serilize access of isCompactCachePool_ array, including creation of
   // compact cache pools
-  folly::SharedMutex compactCachePoolsLock_;
+  mutable folly::SharedMutex compactCachePoolsLock_;
 
   // mutex protecting the creation and destruction of workers poolRebalancer_,
   // poolResizer_, poolOptimizer_, memMonitor_, reaper_

cachelib/benchmarks/SpeedUpExistenceCheckBenchmark.cpp

@@ -90,14 +90,14 @@ struct BucketWithKey {
 struct SharedMutex {
   auto getReadLock() { return folly::SharedMutex::ReadHolder{l}; }
   auto getWriteLock() { return std::unique_lock{l}; }
-  folly::SharedMutex l{};
+  mutable folly::SharedMutex l{};
 };
 
 struct alignas(folly::hardware_destructive_interference_size)
     SharedMutexAligned {
   auto getReadLock() { return folly::SharedMutex::ReadHolder{l}; }
   auto getWriteLock() { return std::unique_lock{l}; }
-  folly::SharedMutex l{};
+  mutable folly::SharedMutex l{};
 };
 
 struct SpinLock {

cachelib/compact_cache/CCache.h

@@ -555,8 +555,8 @@ class CompactCache : public ICompactCache {
   RemoveCb removeCb_;
   ReplaceCb replaceCb_;
   ValidCb validCb_;
-  facebook::cachelib::Cohort cohort_; /**< resize cohort synchronization */
-  folly::SharedMutex resizeLock_;     /**< Lock to prevent resize conflicts. */
+  facebook::cachelib::Cohort cohort_;     /**< resize cohort synchronization */
+  mutable folly::SharedMutex resizeLock_; /**< Lock to synchronize resize. */
   const size_t bucketsPerChunk_;
   util::FastStats<CCacheStats> stats_;
   const bool allowPromotions_; /**< Whether promotions are allowed on read