Prevent unnecessary copy when overwriting a value from a snapshot

Former-commit-id: 654a7bc6ea82f4ac45a1c1a25c794e1c27c0d902

src/db.cpp

@@ -338,6 +338,7 @@ int dbMerge(redisDb *db, robj *key, robj *val, int fReplace)
  * The client 'c' argument may be set to NULL if the operation is performed
  * in a context where there is no clear client performing the operation. */
 void genericSetKey(client *c, redisDb *db, robj *key, robj *val, int keepttl, int signal) {
+    db->prepOverwriteForSnapshot(szFromObj(key));
     if (!dbAddCore(db, key, val)) {
         dbOverwrite(db, key, val, !keepttl);
     }
@@ -2358,6 +2359,24 @@ bool redisDbPersistentData::insert(char *key, robj *o, bool fAssumeNew)
     return (res == DICT_OK);
 }
 
+// This is a performance tool to prevent us copying over an object we're going to overwrite anyways
+void redisDbPersistentData::prepOverwriteForSnapshot(char *key)
+{
+    if (g_pserver->maxmemory_policy & MAXMEMORY_FLAG_LFU)
+        return;
+
+    if (m_pdbSnapshot != nullptr)
+    {
+        auto itr = m_pdbSnapshot->find_cached_threadsafe(key);
+        if (itr.key() != nullptr)
+        {
+            sds keyNew = sdsdupshared(itr.key());
+            if (dictAdd(m_pdictTombstone, keyNew, (void*)dictHashKey(m_pdict, key)) != DICT_OK)
+                sdsfree(keyNew);
+        }
+    }
+}
+
 void redisDbPersistentData::tryResize()
 {
     if (htNeedsResize(m_pdict))

src/server.h

@@ -1326,6 +1326,9 @@ public:
     void setExpire(robj *key, robj *subkey, long long when);
     void setExpire(expireEntry &&e);
     void initialize();
+    void prepOverwriteForSnapshot(char *key);
+
+    bool FRehashing() const { return dictIsRehashing(m_pdict) || dictIsRehashing(m_pdictTombstone); }
 
     void setStorageProvider(StorageCache *pstorage);
 
@@ -1527,6 +1530,8 @@ struct redisDb : public redisDbPersistentDataSnapshot
     using redisDbPersistentData::dictUnsafeKeyOnly;
     using redisDbPersistentData::resortExpire;
     using redisDbPersistentData::prefetchKeysAsync;
+    using redisDbPersistentData::prepOverwriteForSnapshot;
+    using redisDbPersistentData::FRehashing;
 
 public:
     expireset::setiter expireitr;