vendor: update go-runc to reduce gc pressure

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

vendor.conf

@@ -1,5 +1,5 @@
 github.com/coreos/go-systemd 48702e0da86bd25e76cfef347e2adeb434a0d0a6
-github.com/containerd/go-runc ed1cbe1fc31f5fb2359d3a54b6330d1a097858b7
+github.com/containerd/go-runc 4f6e87ae043f859a38255247b49c9abc262d002f
 github.com/containerd/console 84eeaae905fa414d03e07bcd6c8d3f19e7cf180e
 github.com/containerd/cgroups 29da22c6171a4316169f9205ab6c49f59b5b852f
 github.com/containerd/typeurl f6943554a7e7e88b3c14aad190bf05932da84788

vendor/github.com/containerd/go-runc/runc.go

@@ -1,7 +1,6 @@
 package runc
 
 import (
-	"bytes"
 	"context"
 	"encoding/json"
 	"errors"
@@ -532,7 +531,9 @@ func (r *Runc) Restore(context context.Context, id, bundle string, opts *Restore
 
 // Update updates the current container with the provided resource spec
 func (r *Runc) Update(context context.Context, id string, resources *specs.LinuxResources) error {
-	buf := bytes.NewBuffer(nil)
+	buf := getBuf()
+	defer putBuf(buf)
+
 	if err := json.NewEncoder(buf).Encode(resources); err != nil {
 		return err
 	}
@@ -638,11 +639,12 @@ func (r *Runc) runOrError(cmd *exec.Cmd) error {
 }
 
 func cmdOutput(cmd *exec.Cmd, combined bool) ([]byte, error) {
-	var b bytes.Buffer
+	b := getBuf()
+	defer putBuf(b)
 
-	cmd.Stdout = &b
+	cmd.Stdout = b
 	if combined {
-		cmd.Stderr = &b
+		cmd.Stderr = b
 	}
 	ec, err := Monitor.Start(cmd)
 	if err != nil {

vendor/github.com/containerd/go-runc/utils.go

@@ -1,8 +1,10 @@
 package runc
 
 import (
+	"bytes"
 	"io/ioutil"
 	"strconv"
+	"sync"
 	"syscall"
 )
 
@@ -26,3 +28,18 @@ func exitStatus(status syscall.WaitStatus) int {
 	}
 	return status.ExitStatus()
 }
+
+var bytesBufferPool = sync.Pool{
+	New: func() interface{} {
+		return bytes.NewBuffer(nil)
+	},
+}
+
+func getBuf() *bytes.Buffer {
+	return bytesBufferPool.Get().(*bytes.Buffer)
+}
+
+func putBuf(b *bytes.Buffer) {
+	b.Reset()
+	bytesBufferPool.Put(b)
+}