Fix premature close of build output on pull

The build job will sometimes trigger a pull job when the base image
does not exist. Now that engine jobs properly close their output by default
the pull job would also close the build job's stdout in a cascading close
upon completion of the pull.

This patch corrects this by wrapping the `pull` job's stdout with a
nopCloseWriter which will not close the stdout of the `build` job.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: e662775ffb096fdb4a7f247bb25b8e3022006c1b
Component: engine

components/engine/builder/internals.go

@@ -25,6 +25,7 @@ import (
 	imagepkg "github.com/docker/docker/image"
 	"github.com/docker/docker/pkg/archive"
 	"github.com/docker/docker/pkg/chrootarchive"
+	"github.com/docker/docker/pkg/ioutils"
 	"github.com/docker/docker/pkg/parsers"
 	"github.com/docker/docker/pkg/symlink"
 	"github.com/docker/docker/pkg/system"
@@ -433,7 +434,7 @@ func (b *Builder) pullImage(name string) (*imagepkg.Image, error) {
 	job.SetenvBool("json", b.StreamFormatter.Json())
 	job.SetenvBool("parallel", true)
 	job.SetenvJson("authConfig", pullRegistryAuth)
-	job.Stdout.Add(b.OutOld)
+	job.Stdout.Add(ioutils.NopWriteCloser(b.OutOld))
 	if err := job.Run(); err != nil {
 		return nil, err
 	}