perf(txn): de-duplicate the context keys and predicates (#7478)

The zero raft loop got stuck because the proposals took a long time to process. Hence, the goroutine which checks the quorum gets struck leading to leadership change (even in single zero single alpha).

The real issue was the Keys in txnContext. While merging the conflict keys, deduplication was not done. Do deduplication before sending the request to zero.

dgraph/cmd/zero/oracle.go

@@ -367,12 +367,7 @@ func (s *Server) commit(ctx context.Context, src *api.TxnContext) error {
 
 	checkPreds := func() error {
 		// Check if any of these tablets is being moved. If so, abort the transaction.
-		preds := make(map[string]struct{})
-
-		for _, k := range src.Preds {
-			preds[k] = struct{}{}
-		}
-		for pkey := range preds {
+		for _, pkey := range src.Preds {
 			splits := strings.Split(pkey, "-")
 			if len(splits) < 2 {
 				return errors.Errorf("Unable to find group id in %s", pkey)

worker/mutation.go

@@ -820,6 +820,11 @@ func CommitOverNetwork(ctx context.Context, tc *api.TxnContext) (uint64, error)
 	if pl == nil {
 		return 0, conn.ErrNoConnection
 	}
+
+	// Do de-duplication before sending the request to zero.
+	tc.Keys = x.Unique(tc.Keys)
+	tc.Preds = x.Unique(tc.Preds)
+
 	zc := pb.NewZeroClient(pl.Get())
 	tctx, err := zc.CommitOrAbort(ctx, tc)