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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package buildutil
import (
// OverlayContext overlays a build.Context with additional files from
// a map. Files in the map take precedence over other files.
// In addition to plain string comparison, two file names are
// considered equal if their base names match and their directory
// components point at the same directory on the file system. That is,
// symbolic links are followed for directories, but not files.
// A common use case for OverlayContext is to allow editors to pass in
// a set of unsaved, modified files.
// Currently, only the Context.OpenFile function will respect the
// overlay. This may change in the future.
func OverlayContext(orig *build.Context, overlay map[string][]byte) *build.Context {
// TODO(dominikh): Implement IsDir, HasSubdir and ReadDir
rc := func(data []byte) (io.ReadCloser, error) {
return io.NopCloser(bytes.NewBuffer(data)), nil
copy := *orig // make a copy
ctxt := &copy
ctxt.OpenFile = func(path string) (io.ReadCloser, error) {
// Fast path: names match exactly.
if content, ok := overlay[path]; ok {
return rc(content)
// Slow path: check for same file under a different
// alias, perhaps due to a symbolic link.
for filename, content := range overlay {
if sameFile(path, filename) {
return rc(content)
return OpenFile(orig, path)
return ctxt
// ParseOverlayArchive parses an archive containing Go files and their
// contents. The result is intended to be used with OverlayContext.
// # Archive format
// The archive consists of a series of files. Each file consists of a
// name, a decimal file size and the file contents, separated by
// newlines. No newline follows after the file contents.
func ParseOverlayArchive(archive io.Reader) (map[string][]byte, error) {
overlay := make(map[string][]byte)
r := bufio.NewReader(archive)
for {
// Read file name.
filename, err := r.ReadString('\n')
if err != nil {
if err == io.EOF {
break // OK
return nil, fmt.Errorf("reading archive file name: %v", err)
filename = filepath.Clean(strings.TrimSpace(filename))
// Read file size.
sz, err := r.ReadString('\n')
if err != nil {
return nil, fmt.Errorf("reading size of archive file %s: %v", filename, err)
sz = strings.TrimSpace(sz)
size, err := strconv.ParseUint(sz, 10, 32)
if err != nil {
return nil, fmt.Errorf("parsing size of archive file %s: %v", filename, err)
// Read file content.
content := make([]byte, size)
if _, err := io.ReadFull(r, content); err != nil {
return nil, fmt.Errorf("reading archive file %s: %v", filename, err)
overlay[filename] = content
return overlay, nil