fix(conn): JoinCluster loop should use latest conn (#7950)

JoinCluster loop was getting the connection from pool upfront, and then looping over it. This opened up a bug because in #7918 , we close the connection in case it becomes unhealthy.

This PR gets the latest connection available in the loop. This was the only place in the codebase where I found this issue.

(cherry picked from commit 7531e95)
(cherry picked from commit c4098ed)

Author: manishrjain

dgraph/cmd/zero/raft.go

@@ -576,10 +576,9 @@ func (n *node) initAndStartNode() error {
 			return errors.Errorf("Unhealthy connection to %v", opts.peer)
 		}
 
-		gconn := p.Get()
-		c := pb.NewRaftClient(gconn)
 		timeout := 8 * time.Second
 		for {
+			c := pb.NewRaftClient(p.Get())
 			ctx, cancel := context.WithTimeout(n.ctx, timeout)
 			// JoinCluster can block indefinitely, raft ignores conf change proposal
 			// if it has pending configuration.

worker/draft.go

@@ -1628,8 +1628,7 @@ func (n *node) joinPeers() error {
 		return err
 	}
 
-	gconn := pl.Get()
-	c := pb.NewRaftClient(gconn)
+	c := pb.NewRaftClient(pl.Get())
 	glog.Infof("Calling JoinCluster via leader: %s", pl.Addr)
 	if _, err := c.JoinCluster(n.ctx, n.RaftContext); err != nil {
 		return errors.Wrapf(err, "error while joining cluster")

worker/mutation.go

@@ -546,10 +546,9 @@ func proposeOrSend(ctx context.Context, gid uint32, m *pb.Mutations, chr chan re
 		chr <- res
 		return
 	}
-	con := pl.Get()
 
 	var tc *api.TxnContext
-	c := pb.NewWorkerClient(con)
+	c := pb.NewWorkerClient(pl.Get())
 
 	ch := make(chan error, 1)
 	go func() {

worker/schema.go

@@ -173,8 +173,7 @@ func getSchemaOverNetwork(ctx context.Context, gid uint32, s *pb.SchemaRequest,
 		ch <- resultErr{err: conn.ErrNoConnection}
 		return
 	}
-	conn := pl.Get()
-	c := pb.NewWorkerClient(conn)
+	c := pb.NewWorkerClient(pl.Get())
 	schema, e := c.Schema(ctx, s)
 	ch <- resultErr{result: schema, err: e}
 }

worker/snapshot.go

@@ -47,8 +47,7 @@ type badgerWriter interface {
 
 // populateSnapshot gets data for a shard from the leader and writes it to BadgerDB on the follower.
 func (n *node) populateSnapshot(snap pb.Snapshot, pl *conn.Pool) error {
-	con := pl.Get()
-	c := pb.NewWorkerClient(con)
+	c := pb.NewWorkerClient(pl.Get())
 
 	// We should absolutely cancel the context when we return from this function, that way, the
 	// leader who is sending the snapshot would stop sending.

worker/task.go

@@ -53,11 +53,10 @@ func invokeNetworkRequest(ctx context.Context, addr string,
 		return nil, errors.Wrapf(err, "dispatchTaskOverNetwork: while retrieving connection.")
 	}
 
-	con := pl.Get()
 	if span := otrace.FromContext(ctx); span != nil {
 		span.Annotatef(nil, "invokeNetworkRequest: Sending request to %v", addr)
 	}
-	c := pb.NewWorkerClient(con)
+	c := pb.NewWorkerClient(pl.Get())
 	return f(ctx, c)
 }