mark lock holders with nodiscard

Summary: Acquiring a lock into an unused temporary is generally a bug. Anyone wanting to do this can suppress their compiler's warning by explicitly `static_cast`ing the lock holder to void.

Reviewed By: vitaut

Differential Revision: D15504563

fbshipit-source-id: 1033f6a1d4044b9698934b73eefebfe83602a2a5

folly/SharedMutex.h

@@ -278,9 +278,9 @@ class SharedMutexImpl {
 
   typedef SharedMutexToken Token;
 
-  class ReadHolder;
-  class UpgradeHolder;
-  class WriteHolder;
+  class FOLLY_NODISCARD ReadHolder;
+  class FOLLY_NODISCARD UpgradeHolder;
+  class FOLLY_NODISCARD WriteHolder;
 
   constexpr SharedMutexImpl() noexcept : state_(0) {}
 
@@ -1301,7 +1301,7 @@ class SharedMutexImpl {
   }
 
  public:
-  class ReadHolder {
+  class FOLLY_NODISCARD ReadHolder {
     ReadHolder() : lock_(nullptr) {}
 
    public:
@@ -1363,7 +1363,7 @@ class SharedMutexImpl {
     SharedMutexToken token_;
   };
 
-  class UpgradeHolder {
+  class FOLLY_NODISCARD UpgradeHolder {
     UpgradeHolder() : lock_(nullptr) {}
 
    public:
@@ -1413,7 +1413,7 @@ class SharedMutexImpl {
     SharedMutexImpl* lock_;
   };
 
-  class WriteHolder {
+  class FOLLY_NODISCARD WriteHolder {
     WriteHolder() : lock_(nullptr) {}
 
    public:

folly/fibers/TimedMutex.h

@@ -16,6 +16,7 @@
 #pragma once
 
 #include <folly/IntrusiveList.h>
+#include <folly/Portability.h>
 #include <folly/SpinLock.h>
 #include <folly/fibers/GenericBaton.h>
 
@@ -149,7 +150,7 @@ class TimedRWMutexImpl {
   // are any.
   void downgrade();
 
-  class ReadHolder {
+  class FOLLY_NODISCARD ReadHolder {
    public:
     explicit ReadHolder(TimedRWMutexImpl& lock) : lock_(&lock) {
       lock_->read_lock();
@@ -170,7 +171,7 @@ class TimedRWMutexImpl {
     TimedRWMutexImpl* lock_;
   };
 
-  class WriteHolder {
+  class FOLLY_NODISCARD WriteHolder {
    public:
     explicit WriteHolder(TimedRWMutexImpl& lock) : lock_(&lock) {
       lock_->write_lock();

folly/io/async/DelayedDestructionBase.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <assert.h>
+#include <folly/Portability.h>
 #include <cstddef>
 #include <cstdint>
 #include <functional>
@@ -59,7 +60,7 @@ class DelayedDestructionBase {
    * object, causing problems when the callback function returns and the
    * guarded object's method resumes execution.
    */
-  class DestructorGuard {
+  class FOLLY_NODISCARD DestructorGuard {
    public:
     explicit DestructorGuard(DelayedDestructionBase* dd) : dd_(dd) {
       if (dd_ != nullptr) {

folly/synchronization/RWSpinLock.h

@@ -307,11 +307,11 @@ class RWSpinLock {
     return bits_.load(std::memory_order_acquire);
   }
 
-  class ReadHolder;
-  class UpgradedHolder;
-  class WriteHolder;
+  class FOLLY_NODISCARD ReadHolder;
+  class FOLLY_NODISCARD UpgradedHolder;
+  class FOLLY_NODISCARD WriteHolder;
 
-  class ReadHolder {
+  class FOLLY_NODISCARD ReadHolder {
    public:
     explicit ReadHolder(RWSpinLock* lock) : lock_(lock) {
       if (lock_) {
@@ -380,7 +380,7 @@ class RWSpinLock {
     RWSpinLock* lock_;
   };
 
-  class UpgradedHolder {
+  class FOLLY_NODISCARD UpgradedHolder {
    public:
     explicit UpgradedHolder(RWSpinLock* lock) : lock_(lock) {
       if (lock_) {
@@ -443,7 +443,7 @@ class RWSpinLock {
     RWSpinLock* lock_;
   };
 
-  class WriteHolder {
+  class FOLLY_NODISCARD WriteHolder {
    public:
     explicit WriteHolder(RWSpinLock* lock) : lock_(lock) {
       if (lock_) {
@@ -742,10 +742,10 @@ class RWTicketSpinLockT {
     __sync_fetch_and_add(&ticket.write, 1);
   }
 
-  class WriteHolder;
+  class FOLLY_NODISCARD WriteHolder;
 
   typedef RWTicketSpinLockT<kBitWidth, kFavorWriter> RWSpinLock;
-  class ReadHolder {
+  class FOLLY_NODISCARD ReadHolder {
    public:
     ReadHolder(ReadHolder const&) = delete;
     ReadHolder& operator=(ReadHolder const&) = delete;
@@ -794,7 +794,7 @@ class RWTicketSpinLockT {
     RWSpinLock* lock_;
   };
 
-  class WriteHolder {
+  class FOLLY_NODISCARD WriteHolder {
    public:
     WriteHolder(WriteHolder const&) = delete;
     WriteHolder& operator=(WriteHolder const&) = delete;

folly/synchronization/Utility.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/Portability.h>
 #include <mutex>
 
 namespace folly {
@@ -39,7 +40,9 @@ namespace folly {
  * of std::unique_lock
  */
 template <typename Mutex, typename... Args>
-std::unique_lock<Mutex> make_unique_lock(Mutex& mutex, Args&&... args) {
+FOLLY_NODISCARD std::unique_lock<Mutex> make_unique_lock(
+    Mutex& mutex,
+    Args&&... args) {
   return std::unique_lock<Mutex>{mutex, std::forward<Args>(args)...};
 }