This patch improve kubectl cp command from two aspects

A.support soft link better
before this patch
  "kubectl cp" command will copy the soft link to destination as an empty regular file
after this patch
  "kubectl cp" command will behave the same as tar command
  this patch improves it on both from container and to container

B.fix some bugs
1.from container to host
  a.when copy a file ends with '/', it will cause a panic.
    for example, container gakki has a regular file /tmp/test, then run command
    kubectl cp gakki:/tmp/test/ /tmp
    panic happens
  b.when copy a file which does not exist in container, the command ends up without
    any error information

2.from host to container
  a.when run command like
    kubectl cp "" gakki:/tmp
    it will try cp current directory to container, in other words, this command works
    the same as kubectl cp . gakki:/tmp
  b.current cp command will omit an empty directory

modified:   pkg/kubectl/cmd/cp.go
modified:   pkg/kubectl/cmd/cp_test.go

pkg/kubectl/cmd/cp.go

@@ -20,6 +20,7 @@ import (
 	"archive/tar"
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"os"
@@ -82,7 +83,10 @@ type fileSpec struct {
 	File         string
 }
 
-var errFileSpecDoesntMatchFormat = errors.New("Filespec must match the canonical format: [[namespace/]pod:]file/path")
+var (
+	errFileSpecDoesntMatchFormat = errors.New("Filespec must match the canonical format: [[namespace/]pod:]file/path")
+	errFileCannotBeEmpty         = errors.New("Filepath can not be empty")
+)
 
 func extractFileSpec(arg string) (fileSpec, error) {
 	pieces := strings.Split(arg, ":")
@@ -157,6 +161,9 @@ func checkDestinationIsDir(dest fileSpec, f cmdutil.Factory, cmd *cobra.Command)
 }
 
 func copyToPod(f cmdutil.Factory, cmd *cobra.Command, stdout, stderr io.Writer, src, dest fileSpec) error {
+	if len(src.File) == 0 {
+		return errFileCannotBeEmpty
+	}
 	reader, writer := io.Pipe()
 
 	// strip trailing slash (if any)
@@ -201,6 +208,10 @@ func copyToPod(f cmdutil.Factory, cmd *cobra.Command, stdout, stderr io.Writer,
 }
 
 func copyFromPod(f cmdutil.Factory, cmd *cobra.Command, cmderr io.Writer, src, dest fileSpec) error {
+	if len(src.File) == 0 {
+		return errFileCannotBeEmpty
+	}
+
 	reader, outStream := io.Pipe()
 	options := &ExecOptions{
 		StreamOptions: StreamOptions{
@@ -222,7 +233,7 @@ func copyFromPod(f cmdutil.Factory, cmd *cobra.Command, cmderr io.Writer, src, d
 		execute(f, cmd, options)
 	}()
 	prefix := getPrefix(src.File)
-
+	prefix = path.Clean(prefix)
 	return untarAll(reader, dest.File, prefix)
 }
 
@@ -238,7 +249,7 @@ func makeTar(srcPath, destPath string, writer io.Writer) error {
 
 func recursiveTar(srcBase, srcFile, destBase, destFile string, tw *tar.Writer) error {
 	filepath := path.Join(srcBase, srcFile)
-	stat, err := os.Stat(filepath)
+	stat, err := os.Lstat(filepath)
 	if err != nil {
 		return err
 	}
@@ -247,28 +258,55 @@ func recursiveTar(srcBase, srcFile, destBase, destFile string, tw *tar.Writer) e
 		if err != nil {
 			return err
 		}
+		if len(files) == 0 {
+			//case empty directory
+			hdr, _ := tar.FileInfoHeader(stat, filepath)
+			hdr.Name = destFile
+			if err := tw.WriteHeader(hdr); err != nil {
+				return err
+			}
+		}
 		for _, f := range files {
 			if err := recursiveTar(srcBase, path.Join(srcFile, f.Name()), destBase, path.Join(destFile, f.Name()), tw); err != nil {
 				return err
 			}
 		}
 		return nil
-	}
-	hdr, err := tar.FileInfoHeader(stat, filepath)
-	if err != nil {
+	} else if stat.Mode()&os.ModeSymlink != 0 {
+		//case soft link
+		hdr, _ := tar.FileInfoHeader(stat, filepath)
+		target, err := os.Readlink(filepath)
+		if err != nil {
+			return err
+		}
+
+		hdr.Linkname = target
+		hdr.Name = destFile
+		if err := tw.WriteHeader(hdr); err != nil {
+			return err
+		}
+	} else {
+		//case regular file or other file type like pipe
+		hdr, err := tar.FileInfoHeader(stat, filepath)
+		if err != nil {
+			return err
+		}
+		hdr.Name = destFile
+
+		if err := tw.WriteHeader(hdr); err != nil {
+			return err
+		}
+
+		f, err := os.Open(filepath)
+		if err != nil {
+			return err
+		}
+
+		defer f.Close()
+		_, err = io.Copy(tw, f)
 		return err
 	}
-	hdr.Name = destFile
-	if err := tw.WriteHeader(hdr); err != nil {
-		return err
-	}
-	f, err := os.Open(filepath)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-	_, err = io.Copy(tw, f)
-	return err
+	return nil
 }
 
 func untarAll(reader io.Reader, destFile, prefix string) error {
@@ -285,6 +323,7 @@ func untarAll(reader io.Reader, destFile, prefix string) error {
 			break
 		}
 		entrySeq++
+		mode := header.FileInfo().Mode()
 		outFileName := path.Join(destFile, header.Name[len(prefix):])
 		baseName := path.Dir(outFileName)
 		if err := os.MkdirAll(baseName, 0755); err != nil {
@@ -308,15 +347,27 @@ func untarAll(reader io.Reader, destFile, prefix string) error {
 				outFileName = filepath.Join(outFileName, path.Base(header.Name))
 			}
 		}
-		outFile, err := os.Create(outFileName)
-		if err != nil {
-			return err
-		}
-		defer outFile.Close()
-		if _, err := io.Copy(outFile, tarReader); err != nil {
-			return err
+
+		if mode&os.ModeSymlink != 0 {
+			err := os.Symlink(header.Linkname, outFileName)
+			if err != nil {
+				return err
+			}
+		} else {
+			outFile, err := os.Create(outFileName)
+			if err != nil {
+				return err
+			}
+			defer outFile.Close()
+			io.Copy(outFile, tarReader)
 		}
 	}
+
+	if entrySeq == -1 {
+		//if no file was copied
+		errInfo := fmt.Sprintf("error: %s no such file or directory", prefix)
+		return errors.New(errInfo)
+	}
 	return nil
 }