Multiple threads should be able to get the same snapshot provided its not too old

Former-commit-id: 054331098ee18dfb1887fd2b0a67688ef894823e

src/aof.cpp

@@ -676,6 +676,7 @@ client *createFakeClient(void) {
     c->peerid = NULL;
     c->resp = 2;
     c->puser = NULL;
+    c->mvccCheckpoint = 0;
     listSetFreeMethod(c->reply,freeClientReplyValue);
     listSetDupMethod(c->reply,dupClientReplyValue);
     fastlock_init(&c->lock, "fake client");

src/db.cpp

@@ -724,6 +724,7 @@ void keysCommand(client *c) {
         }
         return true;
     });
+    
     setDeferredArrayLen(c,replylen,numkeys);
 }
 
@@ -2077,10 +2078,18 @@ void redisDbPersistentData::processChanges()
     m_setchanged.clear();
 }
 
-redisDbPersistentData *redisDbPersistentData::createSnapshot()
+redisDbPersistentData *redisDbPersistentData::createSnapshot(uint64_t mvccCheckpoint)
 {
     serverAssert(GlobalLocksAcquired());
-    serverAssert(m_spdbSnapshotHOLDER == nullptr);
+    if (m_spdbSnapshotHOLDER != nullptr)
+    {
+        if (mvccCheckpoint <= m_spdbSnapshotHOLDER->mvccCheckpoint)
+        {
+            ++m_snapshotRefcount;
+            return m_spdbSnapshotHOLDER.get();            
+        }
+        return nullptr;
+    }
     auto spdb = std::make_unique<redisDbPersistentData>();
     
     spdb->m_fAllChanged = false;
@@ -2095,6 +2104,7 @@ redisDbPersistentData *redisDbPersistentData::createSnapshot()
     
     m_spdbSnapshotHOLDER = std::move(spdb);
     m_pdbSnapshot = m_spdbSnapshotHOLDER.get();
+    ++m_snapshotRefcount;
     return m_pdbSnapshot;
 }
 
@@ -2103,6 +2113,11 @@ void redisDbPersistentData::endSnapshot(const redisDbPersistentData *psnapshot)
     if (!GlobalLocksAcquired())
         serverLog(LL_WARNING, "Global locks not acquired");
     serverAssert(m_spdbSnapshotHOLDER.get() == psnapshot);
+
+    --m_snapshotRefcount;
+    if (m_snapshotRefcount > 0)
+        return;
+
     m_spdbSnapshotHOLDER->m_pdict->iterators--;
 
     if (m_pdbSnapshot == nullptr)

src/networking.cpp

@@ -176,6 +176,7 @@ client *createClient(int fd, int iel) {
     c->bufposAsync = 0;
     c->client_tracking_redirection = 0;
     c->casyncOpsPending = 0;
+    c->mvccCheckpoint = 0;
     memset(c->uuid, 0, UUID_BINARY_LEN);
 
     listSetFreeMethod(c->pubsub_patterns,decrRefCountVoid);

src/rdb.cpp

@@ -1403,7 +1403,7 @@ int launchRdbSaveThread(pthread_t &child, rdbSaveInfo *rsi)
     args->rsi.master_repl_offset = g_pserver->master_repl_offset;
         
     for (int idb = 0; idb < cserver.dbnum; ++idb)
-        args->rgpdb[idb] = g_pserver->db[idb].createSnapshot();
+        args->rgpdb[idb] = g_pserver->db[idb].createSnapshot(getMvccTstamp());
 
     g_pserver->rdbThreadVars.tmpfileNum++;
     g_pserver->rdbThreadVars.fRdbThreadCancel = false;
@@ -2585,7 +2585,7 @@ int rdbSaveToSlavesSockets(rdbSaveInfo *rsi) {
     start = ustime();
 
     for (int idb = 0; idb < cserver.dbnum; ++idb)
-        args->rgpdb[idb] = g_pserver->db[idb].createSnapshot();
+        args->rgpdb[idb] = g_pserver->db[idb].createSnapshot(getMvccTstamp());
 
     g_pserver->rdbThreadVars.tmpfileNum++;
     g_pserver->rdbThreadVars.fRdbThreadCancel = false;

src/server.cpp

@@ -3385,6 +3385,9 @@ void call(client *c, int flags) {
     dirty = g_pserver->dirty-dirty;
     if (dirty < 0) dirty = 0;
 
+    if (dirty)
+        c->mvccCheckpoint = getMvccTstamp();
+
     /* When EVAL is called loading the AOF we don't want commands called
      * from Lua to go into the slowlog or to populate statistics. */
     if (g_pserver->loading && c->flags & CLIENT_LUA)

src/server.h

@@ -1211,7 +1211,7 @@ public:
     expireset *setexpireUnsafe() { return m_setexpire; }
     const expireset *setexpire() { return m_setexpire; }
 
-    redisDbPersistentData *createSnapshot();
+    redisDbPersistentData *createSnapshot(uint64_t mvccCheckpoint);
     void endSnapshot(const redisDbPersistentData *psnapshot);
 
 private:
@@ -1227,6 +1227,7 @@ private:
     bool m_fAllChanged = false;
     std::set<std::string> m_setchanged;
     IStorage *m_pstorage = nullptr;
+    uint64_t mvccCheckpoint = 0;
 
     // Expire
     expireset *m_setexpire = nullptr;
@@ -1236,6 +1237,7 @@ private:
     //      in a snapshot
     redisDbPersistentData *m_pdbSnapshot = nullptr;
     std::unique_ptr<redisDbPersistentData> m_spdbSnapshotHOLDER;
+    int m_snapshotRefcount = 0;
 };
 
 /* Redis database representation. There are multiple databases identified
@@ -1472,6 +1474,8 @@ typedef struct client {
     int buflenAsync;
     char *bufAsync;
 
+    uint64_t mvccCheckpoint = 0;    // the MVCC checkpoint of our last write
+
     int iel; /* the event loop index we're registered with */
     struct fastlock lock;
 } client;