Optimize uid allocation in live loader. (#5132)

The live loader is having trouble loading exported data with the
existing uids because there are too many requests for new uids.
The current version requests new Uids to be allocated for every
uids greater than the maximum. In the exported data, the uids can come
in increasing order, which causes a new request for uids with every
NQuad.

This PR changes the code to pre-allocate the uids, once per batch of
NQuad received from the NQuad buffer channel.

Tested it with the 1 million movie data set and now I am getting times
similar to the live loader with the --new_uids option enabled.

Fixes #4996

dgraph/cmd/live/run.go

@@ -205,7 +205,6 @@ func (l *loader) uid(val string) string {
 	// later to another node. It is up to the user to avoid this.
 	if !opt.newUids {
 		if uid, err := strconv.ParseUint(val, 0, 64); err == nil {
-			l.alloc.BumpTo(uid)
 			return fmt.Sprintf("%#x", uid)
 		}
 	}
@@ -214,6 +213,34 @@ func (l *loader) uid(val string) string {
 	return fmt.Sprintf("%#x", uint64(uid))
 }
 
+// allocateUids looks for the maximum uid value in the given NQuads and bumps the
+// maximum seen uid to that value.
+func (l *loader) allocateUids(nqs []*api.NQuad) {
+	if opt.newUids {
+		return
+	}
+
+	var maxUid uint64
+	for _, nq := range nqs {
+		sUid, err := strconv.ParseUint(nq.Subject, 0, 64)
+		if err != nil {
+			continue
+		}
+		if sUid > maxUid {
+			maxUid = sUid
+		}
+
+		oUid, err := strconv.ParseUint(nq.ObjectId, 0, 64)
+		if err != nil {
+			continue
+		}
+		if oUid > maxUid {
+			maxUid = oUid
+		}
+	}
+	l.alloc.BumpTo(maxUid)
+}
+
 // processFile forwards a file to the RDF or JSON processor as appropriate
 func (l *loader) processFile(ctx context.Context, filename string) error {
 	fmt.Printf("Processing data file %q\n", filename)
@@ -281,6 +308,8 @@ func (l *loader) processLoadFile(ctx context.Context, rd *bufio.Reader, ck chunk
 			if len(nqs) == 0 {
 				continue
 			}
+
+			l.allocateUids(nqs)
 			for _, nq := range nqs {
 				nq.Subject = l.uid(nq.Subject)
 				if len(nq.ObjectId) > 0 {