allow overwrite in untar

Docker-DCO-1.1-Signed-off-by: Tibor Vass <teabee89@gmail.com> (github: tiborvass)

archive/archive.go

@@ -383,7 +383,8 @@ func TarWithOptions(srcPath string, options *TarOptions) (io.ReadCloser, error)
 //  identity (uncompressed), gzip, bzip2, xz.
 // If `dest` does not exist, it is created unless there are multiple entries in `archive`.
 // In the latter case, an error is returned.
-// An other error is returned if `dest` exists but is not a directory, to prevent overwriting.
+// If `dest` is an existing file, it gets overwritten.
+// If `dest` is an existing directory, its files get merged (with overwrite for conflicting files).
 func Untar(archive io.Reader, dest string, options *TarOptions) error {
 	if archive == nil {
 		return fmt.Errorf("Empty archive")
@@ -399,7 +400,7 @@ func Untar(archive io.Reader, dest string, options *TarOptions) error {
 
 	var (
 		dirs            []*tar.Header
-		destNotExist    bool
+		create          bool
 		multipleEntries bool
 	)
 
@@ -408,9 +409,10 @@ func Untar(archive io.Reader, dest string, options *TarOptions) error {
 			return err
 		}
 		// destination does not exist, so it is assumed it has to be created.
-		destNotExist = true
+		create = true
 	} else if !fi.IsDir() {
-		return fmt.Errorf("Trying to untar to `%s`: exists but not a directory", dest)
+		// destination exists and is not a directory, so it will be overwritten.
+		create = true
 	}
 
 	// Iterate through the files in the archive.
@@ -425,7 +427,7 @@ func Untar(archive io.Reader, dest string, options *TarOptions) error {
 		}
 
 		// Return an error if destination needs to be created and there is more than 1 entry in the tar stream.
-		if destNotExist && multipleEntries {
+		if create && multipleEntries {
 			return fmt.Errorf("Trying to untar an archive with multiple entries to an inexistant target `%s`: did you mean `%s` instead?", dest, filepath.Dir(dest))
 		}
 
@@ -445,7 +447,7 @@ func Untar(archive io.Reader, dest string, options *TarOptions) error {
 		}
 
 		var path string
-		if destNotExist {
+		if create {
 			path = dest // we are renaming hdr.Name to dest
 		} else {
 			path = filepath.Join(dest, hdr.Name)
@@ -465,6 +467,7 @@ func Untar(archive io.Reader, dest string, options *TarOptions) error {
 				}
 			}
 		}
+
 		if err := createTarFile(path, dest, hdr, tr, options == nil || !options.NoLchown); err != nil {
 			return err
 		}

archive/archive_test.go

@@ -176,6 +176,9 @@ func TestTarUntarFile(t *testing.T) {
 	if err := ioutil.WriteFile(path.Join(origin, "before", "file"), []byte("hello world"), 0700); err != nil {
 		t.Fatal(err)
 	}
+	if err := ioutil.WriteFile(path.Join(origin, "after", "file2"), []byte("please overwrite me"), 0700); err != nil {
+		t.Fatal(err)
+	}
 
 	tar, err := TarWithOptions(path.Join(origin, "before"), &TarOptions{Compression: Uncompressed, Includes: []string{"file"}})
 	if err != nil {