Workaround for broken output

This avoids calling `os.Newfile` since it seems to break the
file referenced by it.
This workaround expects the `io.Writer` to be also a `*os.File`.
The default case is true since it uses an `os.Stdout` to
construct the `dockerCli`.
If any of this works, it prints a warning and returns `os.Stdout`.

Signed-off-by: Ulysses Souza <ulysses.souza@docker.com>

internal/commands/build/build.go

@@ -8,8 +8,10 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"reflect"
 	"strconv"
 	"strings"
+	"unsafe"
 
 	"github.com/deislabs/cnab-go/bundle"
 	cnab "github.com/deislabs/cnab-go/driver"
@@ -23,6 +25,7 @@ import (
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	compose "github.com/docker/cli/cli/compose/types"
+	"github.com/docker/cli/cli/streams"
 	"github.com/docker/cnab-to-oci/remotes"
 	"github.com/docker/distribution/reference"
 	"github.com/moby/buildkit/client"
@@ -136,6 +139,28 @@ func runBuild(dockerCli command.Cli, contextPath string, opt buildOptions) error
 	return err
 }
 
+// FIXME (ulyssessouza): This is a workaround to avoid calling os.NewFile on an already referenced file
+// Calling `os.NewFile` with for example `os.Stdout.Fd()` invalidates the descriptor.
+func getOutputFile(realOut *streams.Out, quiet bool) (*os.File, error) {
+	if quiet {
+		out, err := os.Create(os.DevNull)
+		if err != nil {
+			return nil, err
+		}
+		return out, nil
+	}
+	// Gets the encapsulated `io.Writer` from inside the realOut (default for dockerCli is `os.Stdout`)
+	// Expects the internal io.Writer to be a `*os.File` and not just an `io.Writer`
+	f := reflect.ValueOf(realOut).Elem().FieldByName("out")
+	f = reflect.NewAt(f.Type(), unsafe.Pointer(f.UnsafeAddr())).Elem()
+	out, ok := f.Interface().(*os.File)
+	if !ok {
+		fmt.Fprintln(os.Stderr, "WARNING: Could not retrieve the output stream, using stdout instead")
+		return os.Stdout, nil
+	}
+	return out, nil
+}
+
 func buildImageUsingBuildx(app *types.App, contextPath string, opt buildOptions, dockerCli command.Cli) (*bundle.Bundle, error) {
 	buildopts, pulledServices, err := parseCompose(app, contextPath, opt)
 	if err != nil {
@@ -161,12 +186,8 @@ func buildImageUsingBuildx(app *types.App, contextPath string, opt buildOptions,
 	}
 
 	var out *os.File
-	if opt.quiet {
-		if out, err = os.Create(os.DevNull); err != nil {
-			return nil, err
-		}
-	} else {
-		out = os.NewFile(dockerCli.Out().FD(), "/dev/stdout")
+	if out, err = getOutputFile(dockerCli.Out(), opt.quiet); err != nil {
+		return nil, err
 	}
 
 	pw := progress.NewPrinter(ctx, out, opt.progress)