Fix race when we free an item before we finish writing to disk making it temporarily unavailable

Former-commit-id: 6139cfee295261d0210f2af6217c2a23eeaf52b8

src/IStorage.h

@@ -29,6 +29,9 @@ public:
     virtual void beginWriteBatch() {} // NOP
     virtual void endWriteBatch() {} // NOP
 
+    virtual void batch_lock() {} // NOP
+    virtual void batch_unlock() {} // NOP
+
     virtual void flush() = 0;
 
     /* This is permitted to be a shallow clone */

src/db.cpp

@@ -2514,12 +2514,19 @@ bool redisDbPersistentData::removeCachedValue(const char *key)
 {
     serverAssert(m_spstorage != nullptr);
     // First ensure its not a pending key
+    if (m_spstorage != nullptr)
+        m_spstorage->batch_lock();
+    
     auto itr = m_setchanged.find(key);
     if (itr != m_setchanged.end())
         return false; // can't evict
 
     // since we write ASAP the database already has a valid copy so safe to delete
     dictDelete(m_pdict, key);
+
+    if (m_spstorage != nullptr)
+        m_spstorage->batch_unlock();
+    
     return true;
 }