Fix a potential deadlock in the cache

ChangeLog.txt

@@ -3,6 +3,7 @@ Version 0.9.4 (unreleased)
 Fixed Bugs:
 * Renaming a file to an existing file (i.e. overwriting an existing file) didn't free the allocated memory for the overwritten file
 * Renaming a file to an existing file could hurt an invariant in the directory layout (directory entries have to be sorted) and doing so could cause files to seemingly disappear.
+* Fix a potential deadlock in the cache
 
 Compatibility:
 * The generated .deb packages work for Ubuntu 16.04

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

@@ -102,6 +102,7 @@ void Cache<Key, Value, MAX_ENTRIES>::_makeSpaceForEntry(std::unique_lock<std::mu
 
 template<class Key, class Value, uint32_t MAX_ENTRIES>
 void Cache<Key, Value, MAX_ENTRIES>::_deleteEntry(std::unique_lock<std::mutex> *lock) {
+  ASSERT(lock->owns_lock(), "The operations in this function require a locked mutex");
   auto key = _cachedBlocks.peekKey();
   ASSERT(key != boost::none, "There was no entry to delete");
   cpputils::MutexPoolLock<Key> lockEntryFromBeingPopped(&_currentlyFlushingEntries, *key);
@@ -154,6 +155,7 @@ bool Cache<Key, Value, MAX_ENTRIES>::_deleteMatchingEntryAtBeginning(std::functi
   std::unique_lock<std::mutex> lock(_mutex);
   if (_cachedBlocks.size() > 0 && matches(*_cachedBlocks.peek())) {
     _deleteEntry(&lock);
+    ASSERT(lock.owns_lock(), "Something strange happened with the lock. It should be locked again when we come back.");
     return true;
   } else {
     return false;

src/cpp-utils/lock/CombinedLock.h

@@ -0,0 +1,36 @@
+#ifndef MESSMER_CPPUTILS_LOCK_COMBINEDLOCK_H
+#define MESSMER_CPPUTILS_LOCK_COMBINEDLOCK_H
+
+#include "../macros.h"
+
+namespace cpputils {
+
+    /**
+     * This class is used to combine multiple locks into one, taking care that they are locked/unlocked
+     * in the order they were given to the constructor.
+     */
+    class CombinedLock final {
+    public:
+        CombinedLock(std::unique_lock<std::mutex> *outer, std::unique_lock<std::mutex> *inner)
+                : _outer(outer), _inner(inner) {
+        }
+
+        void lock() {
+            _outer->lock();
+            _inner->lock();
+        }
+
+        void unlock() {
+            _inner->unlock();
+            _outer->unlock();
+        }
+
+    private:
+        std::unique_lock<std::mutex> *_outer;
+        std::unique_lock<std::mutex> *_inner;
+
+        DISALLOW_COPY_AND_ASSIGN(CombinedLock);
+    };
+}
+
+#endif

src/cpp-utils/lock/LockPool.h

@@ -8,11 +8,13 @@
 #include <algorithm>
 #include "../assert/assert.h"
 #include "../macros.h"
+#include "CombinedLock.h"
 
 //TODO Test
 //TODO Rename package to synchronization
 //TODO Rename to MutexPool
 namespace cpputils {
+
     template<class LockName>
     class LockPool final {
     public:
@@ -26,7 +28,7 @@ namespace cpputils {
 
         std::vector<LockName> _lockedLocks;
         std::mutex _mutex;
-        std::condition_variable _cv;
+        std::condition_variable_any _cv;
 
         DISALLOW_COPY_AND_ASSIGN(LockPool);
     };
@@ -42,15 +44,14 @@ namespace cpputils {
     inline void LockPool<LockName>::lock(const LockName &lock, std::unique_lock<std::mutex> *lockToFreeWhileWaiting) {
         std::unique_lock<std::mutex> mutexLock(_mutex); // TODO Is shared_lock enough here?
         if (_isLocked(lock)) {
-            if(lockToFreeWhileWaiting != nullptr) {
-                lockToFreeWhileWaiting->unlock();
-            }
-            _cv.wait(mutexLock, [this, &lock]{
+            // Order of locking/unlocking is important and should be the same order as everywhere else to prevent deadlocks.
+            // Since when entering the function, lockToFreeWhileWaiting is already locked and mutexLock is locked afterwards,
+            // the condition variable should do it in the same order. We use combinedLock for this.
+            CombinedLock combinedLock(lockToFreeWhileWaiting, &mutexLock);
+            _cv.wait(combinedLock, [this, &lock]{
                 return !_isLocked(lock);
             });
-            if(lockToFreeWhileWaiting != nullptr) {
-                lockToFreeWhileWaiting->lock();
-            }
+            ASSERT(mutexLock.owns_lock() && lockToFreeWhileWaiting->owns_lock(), "Locks haven't been correctly relocked");
         }
         _lockedLocks.push_back(lock);
     }