v3rpc: return leader loss error if lease stream is canceled

Canceling the stream won't cancel the receive since it's using the internal
grpc context, not the one assigned by etcd.

etcdserver/api/v3rpc/lease.go

@@ -18,6 +18,7 @@ import (
 	"io"
 
 	"github.com/coreos/etcd/etcdserver"
+	"github.com/coreos/etcd/etcdserver/api/v3rpc/rpctypes"
 	pb "github.com/coreos/etcd/etcdserver/etcdserverpb"
 	"github.com/coreos/etcd/lease"
 	"golang.org/x/net/context"
@@ -67,7 +68,24 @@ func (ls *LeaseServer) LeaseTimeToLive(ctx context.Context, rr *pb.LeaseTimeToLi
 	return resp, nil
 }
 
-func (ls *LeaseServer) LeaseKeepAlive(stream pb.Lease_LeaseKeepAliveServer) error {
+func (ls *LeaseServer) LeaseKeepAlive(stream pb.Lease_LeaseKeepAliveServer) (err error) {
+	errc := make(chan error, 1)
+	go func() {
+		errc <- ls.leaseKeepAlive(stream)
+	}()
+	select {
+	case err = <-errc:
+	case <-stream.Context().Done():
+		// the only server-side cancellation is noleader for now.
+		err = stream.Context().Err()
+		if err == context.Canceled {
+			err = rpctypes.ErrGRPCNoLeader
+		}
+	}
+	return err
+}
+
+func (ls *LeaseServer) leaseKeepAlive(stream pb.Lease_LeaseKeepAliveServer) error {
 	for {
 		req, err := stream.Recv()
 		if err == io.EOF {