linux/shim: reduce memory overhead by using ttrpc

By replacing grpc with ttrpc, we can reduce total memory runtime
requirements and binary size. With minimal code changes, the shim can
now be controlled by the much lightweight protocol, reducing the total
memory required per container.

When reviewing this change, take particular notice of the generated shim
code.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/ctr/commands/shim/shim.go

@@ -6,12 +6,9 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net"
-	"time"
 
 	gocontext "context"
 
-	"google.golang.org/grpc"
-
 	"github.com/containerd/console"
 	"github.com/containerd/containerd/cmd/ctr/commands"
 	shim "github.com/containerd/containerd/linux/shim/v1"
@@ -20,6 +17,7 @@ import (
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
+	"github.com/stevvooe/ttrpc"
 	"github.com/urfave/cli"
 )
 
@@ -212,21 +210,21 @@ var execCommand = cli.Command{
 	},
 }
 
-func getShimService(context *cli.Context) (shim.ShimClient, error) {
+func getShimService(context *cli.Context) (shim.ShimService, error) {
 	bindSocket := context.GlobalString("socket")
 	if bindSocket == "" {
 		return nil, errors.New("socket path must be specified")
 	}
 
-	dialOpts := []grpc.DialOption{grpc.WithInsecure(), grpc.WithTimeout(100 * time.Second)}
-	dialOpts = append(dialOpts,
-		grpc.WithDialer(func(addr string, timeout time.Duration) (net.Conn, error) {
-			return net.DialTimeout("unix", "\x00"+bindSocket, timeout)
-		},
-		))
-	conn, err := grpc.Dial(fmt.Sprintf("unix://%s", bindSocket), dialOpts...)
+	conn, err := net.Dial("unix", "\x00"+bindSocket)
 	if err != nil {
 		return nil, err
 	}
-	return shim.NewShimClient(conn), nil
+
+	client := ttrpc.NewClient(conn)
+
+	// TODO(stevvooe): This actually leaks the connection. We were leaking it
+	// before, so may not be a huge deal.
+
+	return shim.NewShimClient(client), nil
 }