Fix potential (although improbable) deadlock

ChangeLog.txt

@@ -1,5 +1,8 @@
 Version 0.9.6 (unreleased)
 ---------------
+Fixed bugs:
+* Fix potential (although very improbable) deadlock
+
 Compatibility:
 * Compatible with libcurl version >= 7.50.0, and <= 7.21.6 (tested down to 7.19.0)
 * Compatible with Crypto++ 5.6.4

src/blockstore/implementations/caching/cache/Cache.h

@@ -111,6 +111,7 @@ void Cache<Key, Value, MAX_ENTRIES>::_deleteEntry(std::unique_lock<std::mutex> *
   // i.e. pop() and push() can be called here, except for pop() on the element in _currentlyFlushingEntries
   lock->unlock();
   value = boost::none; // Call destructor
+  lockEntryFromBeingPopped.unlock();  // unlock this one first to keep same locking oder (preventing potential deadlock)
   lock->lock();
 };
 

src/cpp-utils/lock/LockPool.h

@@ -42,6 +42,7 @@ namespace cpputils {
 
     template<class LockName>
     inline void LockPool<LockName>::lock(const LockName &lock, std::unique_lock<std::mutex> *lockToFreeWhileWaiting) {
+        ASSERT(lockToFreeWhileWaiting->owns_lock(), "Given lock must be locked");
         std::unique_lock<std::mutex> mutexLock(_mutex); // TODO Is shared_lock enough here?
         if (_isLocked(lock)) {
             // Order of locking/unlocking is important and should be the same order as everywhere else to prevent deadlocks.

src/cpp-utils/lock/MutexPoolLock.h

@@ -23,10 +23,14 @@ namespace cpputils {
 
         ~MutexPoolLock() {
             if (_pool != nullptr) {
+                unlock();
+            }
+        }
+
+        void unlock() {
             _pool->release(_lockName);
             _pool = nullptr;
         }
-        }
 
     private:
         LockPool<LockName> *_pool;