Pass excludes/options to tar unarchiver via environment

Fixes #10426

Because of the ability to easily overload the shell max argument list
length once the image count is several hundred, `docker load` will
start to fail because of this as it passes an excludes list of all
images in the graph.  This patch uses an environment variable with the
json marshalled through it to get around the arg length limitation.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

pkg/chrootarchive/archive.go

@@ -1,7 +1,6 @@
 package chrootarchive
 
 import (
-	"bytes"
 	"encoding/json"
 	"flag"
 	"fmt"
@@ -9,7 +8,6 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
-	"strings"
 	"syscall"
 
 	"github.com/docker/docker/pkg/archive"
@@ -28,11 +26,14 @@ func chroot(path string) error {
 func untar() {
 	runtime.LockOSThread()
 	flag.Parse()
-	if err := chroot(flag.Arg(0)); err != nil {
+
+	var options *archive.TarOptions
+
+	if err := json.Unmarshal([]byte(os.Getenv("OPT")), &options); err != nil {
 		fatal(err)
 	}
-	var options *archive.TarOptions
-	if err := json.NewDecoder(strings.NewReader(flag.Arg(1))).Decode(&options); err != nil {
+
+	if err := chroot(flag.Arg(0)); err != nil {
 		fatal(err)
 	}
 	if err := archive.Unpack(os.Stdin, "/", options); err != nil {
@@ -54,27 +55,32 @@ func Untar(tarArchive io.Reader, dest string, options *archive.TarOptions) error
 		options.ExcludePatterns = []string{}
 	}
 
-	var (
-		buf bytes.Buffer
-		enc = json.NewEncoder(&buf)
-	)
-	if err := enc.Encode(options); err != nil {
-		return fmt.Errorf("Untar json encode: %v", err)
-	}
+	dest = filepath.Clean(dest)
 	if _, err := os.Stat(dest); os.IsNotExist(err) {
 		if err := os.MkdirAll(dest, 0777); err != nil {
 			return err
 		}
 	}
-	dest = filepath.Clean(dest)
+
+	// We can't pass the exclude list directly via cmd line
+	// because we easily overrun the shell max argument list length
+	// when the full image list is passed (e.g. when this is used
+	// by `docker load`). Instead we will add the JSON marshalled
+	// and placed in the env, which has significantly larger
+	// max size
+	data, err := json.Marshal(options)
+	if err != nil {
+		return fmt.Errorf("Untar json encode: %v", err)
+	}
 	decompressedArchive, err := archive.DecompressStream(tarArchive)
 	if err != nil {
 		return err
 	}
 	defer decompressedArchive.Close()
 
-	cmd := reexec.Command("docker-untar", dest, buf.String())
+	cmd := reexec.Command("docker-untar", dest)
 	cmd.Stdin = decompressedArchive
+	cmd.Env = append(cmd.Env, fmt.Sprintf("OPT=%s", data))
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		return fmt.Errorf("Untar %s %s", err, out)