Cache: Make Cache abstract

The methods that need to be different depending on the thread model
are now pure virtual.

server/modules/filter/cache/cache.cc

@@ -143,36 +143,6 @@ bool Cache::shouldUse(const SESSION* pSession)
     return cache_rules_should_use(m_pRules, pSession);
 }
 
-bool Cache::mustRefresh(const char* pKey, const SessionCache* pSessionCache)
-{
-    long key = hash_of_key(pKey);
-
-    spinlock_acquire(&m_lockPending);
-    // TODO: Remove the internal locking of hashtable. The internal
-    // TODO: locking is no good if you need transactional behaviour.
-    // TODO: Now we lock twice.
-    void *pValue = hashtable_fetch(m_pPending, (void*)pKey);
-    if (!pValue)
-    {
-        // It's not being fetched, so we make a note that we are.
-        hashtable_add(m_pPending, (void*)pKey, (void*)pSessionCache);
-    }
-    spinlock_release(&m_lockPending);
-
-    return pValue == NULL;
-}
-
-void Cache::refreshed(const char* pKey,  const SessionCache* pSessionCache)
-{
-    long key = hash_of_key(pKey);
-
-    spinlock_acquire(&m_lockPending);
-    ss_dassert(hashtable_fetch(m_pPending, (void*)pKey) == pSessionCache);
-    ss_debug(int n =) hashtable_delete(m_pPending, (void*)pKey);
-    ss_dassert(n == 1);
-    spinlock_release(&m_lockPending);
-}
-
 cache_result_t Cache::getKey(const char* zDefaultDb,
                              const GWBUF* pQuery,
                              char* pKey)

server/modules/filter/cache/cache.h

@@ -53,7 +53,7 @@ public:
      *
      * @return True, if the session cache should refresh the data.
      */
-    virtual bool mustRefresh(const char* pKey, const SessionCache* pSessionCache);
+    virtual bool mustRefresh(const char* pKey, const SessionCache* pSessionCache) = 0;
 
     /**
      * To inform the cache that a particular item has been updated upon request.
@@ -61,7 +61,7 @@ public:
      * @param pKey           The hashed key for a query.
      * @param pSessionCache  The session cache informing.
      */
-    virtual void refreshed(const char* pKey,  const SessionCache* pSessionCache);
+    virtual void refreshed(const char* pKey,  const SessionCache* pSessionCache) = 0;
 
     const CACHE_CONFIG& config() const { return m_config; }
 
@@ -103,5 +103,4 @@ protected:
     StorageFactory* m_pFactory;    // The storage factory.
     Storage*        m_pStorage;    // The storage instance to use.
     HASHTABLE*      m_pPending;    // Pending items; being fetched from the backend.
-    SPINLOCK        m_lockPending; // Lock used for protecting 'pending'.
 };

server/modules/filter/cache/cachemt.h

@@ -13,6 +13,7 @@
  */
 
 #include <maxscale/cdefs.h>
+#include <maxscale/spinlock.h>
 #include "cache.h"
 
 class CacheMT : public Cache