fix(Alpha): Immediately take a snapshot if we don't have one (#6458)

This PR would cause the Alpha leader to immediately trigger a snapshot creation if it doesn't already have one.

This is useful when you bring a new cluster from bulk loader. If you remove an Alpha instance and add a new Alpha instance, the new follower won't get a snapshot if the leader doesn't have one. This PR avoids that by causing the leader to get a snapshot asap.

Fixes DGRAPH-2445

(cherry picked from commit 6bd7f8e)

worker/draft.go

@@ -933,12 +933,22 @@ func (n *node) checkpointAndClose(done chan struct{}) {
 			}
 
 			if n.AmLeader() {
-				var calculate bool
+				// If leader doesn't have a snapshot, we should create one immediately. This is very
+				// useful when you bring up the cluster from bulk loader. If you remove an alpha and
+				// add a new alpha, the new follower won't get a snapshot if the leader doesn't have
+				// one.
+				snap, err := n.Store.Snapshot()
+				if err != nil {
+					glog.Errorf("While retrieving snapshot from Store: %v\n", err)
+					continue
+				}
+				calculate := raft.IsEmptySnap(snap) // If no snapshot, then calculate one immediately.
+
 				if chk, err := n.Store.Checkpoint(); err == nil {
 					if first, err := n.Store.FirstIndex(); err == nil {
 						// Save some cycles by only calculating snapshot if the checkpoint has gone
 						// quite a bit further than the first index.
-						calculate = chk >= first+uint64(x.WorkerConfig.SnapshotAfter)
+						calculate = calculate || chk >= first+uint64(x.WorkerConfig.SnapshotAfter)
 						glog.V(3).Infof("Evaluating snapshot first:%d chk:%d (chk-first:%d) "+
 							"snapshotAfter:%d snap:%v", first, chk, chk-first,
 							x.WorkerConfig.SnapshotAfter, calculate)
@@ -956,7 +966,10 @@ func (n *node) checkpointAndClose(done chan struct{}) {
 				// snapshotting.  We just need to do enough, so that we don't have a huge backlog of
 				// entries to process on a restart.
 				if calculate {
-					if err := n.proposeSnapshot(x.WorkerConfig.SnapshotAfter); err != nil {
+					// We can set discardN argument to zero, because we already know that calculate
+					// would be true if either we absolutely needed to calculate the snapshot,
+					// or our checkpoint already crossed the SnapshotAfter threshold.
+					if err := n.proposeSnapshot(0); err != nil {
 						glog.Errorf("While calculating and proposing snapshot: %v", err)
 					}
 				}