Fix replication log usage in HA

Summary:
Instead of calling the wanted method, this diff takes copies of both `is_replicated(x)` and `is_active(x)` results so that they can't change during the method execution.

The problem with the previous implementation was that the information about a
log could change during two consecutive queries as ilustrated in the example
below.
```
      Thread 1                                  Thread2
rlog->set_active(1);
if (rlog->is_replicated(1)) return true;
                                          rlog->set_replicated(1);
if (rlog->is_active(1)) return false;
throw InvalidLogReplicationLookup();
```

The snippet above would throw as we don't have any information about the log
`1`, because the `set_replicated` call would "shadow" the active bit.

Reviewers: ipaljak

Reviewed By: ipaljak

Subscribers: pullbot, mferencevic

Differential Revision: https://phabricator.memgraph.io/D2165

src/raft/raft_server.cpp

@@ -481,16 +481,24 @@ bool RaftServer::SafeToCommit(const tx::TransactionId &tx_id) {
       // circuit the check to always return true if in follower mode.
       return true;
     case Mode::LEADER:
-			// If we are shutting down, but we know that the Raft Log replicated
-			// successfully, we return true. This will eventually commit since we
-			// replicate NoOp on leader election.
-      if (rlog_->is_replicated(tx_id)) return true;
+      // We are taking copies of the rlog_ status here so we avoid the case
+      // where the call to `set_replicated` shadows the `active` bit that is
+      // checked after the `replicated` bit. It is possible that both `active`
+      // and `replicated` are `true` but  since we check `replicated` first this
+      // shouldn't be a problem.
+      bool active = rlog_->is_active(tx_id);
+      bool replicated = rlog_->is_replicated(tx_id);
 
-	    // Only if the transaction isn't replicated, thrown an exception to inform
+      // If we are shutting down, but we know that the Raft Log replicated
+      // successfully, we return true. This will eventually commit since we
+      // replicate NoOp on leader election.
+      if (replicated) return true;
+
+      // Only if the transaction isn't replicated, thrown an exception to inform
       // the client.
-			if (exiting_) throw RaftShutdownException();
+      if (exiting_) throw RaftShutdownException();
 
-      if (rlog_->is_active(tx_id)) {
+      if (active) {
         if (replication_timeout_.CheckTimeout(tx_id)) {
           throw ReplicationTimeoutException();
         }

src/raft/replication_log.hpp

@@ -62,10 +62,7 @@ class ReplicationLog final {
       }
     }
 
-    bool is_active() const {
-      if (flags_ & REPLICATED) return false;
-      return flags_ & ACTIVE;
-    }
+    bool is_active() const { return flags_ & ACTIVE; }
 
     bool is_replicated() const { return flags_ & REPLICATED; }