gopls/internal/lsp/fake/workdir.go - tools - Git at Google

 // Copyright 2020 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 fake

 import (
 	"bytes"
 	"context"
 	"crypto/sha256"
 	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"runtime"
 	"sort"
 	"strings"
 	"sync"
 	"time"

 	"golang.org/x/tools/gopls/internal/lsp/protocol"
 	"golang.org/x/tools/gopls/internal/span"
 	"golang.org/x/tools/internal/robustio"
 )

 // RelativeTo is a helper for operations relative to a given directory.
 type RelativeTo string

 // AbsPath returns an absolute filesystem path for the workdir-relative path.
 func (r RelativeTo) AbsPath(path string) string {
 	fp := filepath.FromSlash(path)
 	if filepath.IsAbs(fp) {
 		return fp
 	}
 	return filepath.Join(string(r), filepath.FromSlash(path))
 }

 // RelPath returns a '/'-encoded path relative to the working directory (or an
 // absolute path if the file is outside of workdir)
 func (r RelativeTo) RelPath(fp string) string {
 	root := string(r)
 	if rel, err := filepath.Rel(root, fp); err == nil && !strings.HasPrefix(rel, "..") {
 		return filepath.ToSlash(rel)
 	}
 	return filepath.ToSlash(fp)
 }

 // writeFileData writes content to the relative path, replacing the special
 // token $SANDBOX_WORKDIR with the relative root given by rel. It does not
 // trigger any file events.
 func writeFileData(path string, content []byte, rel RelativeTo) error {
 	content = bytes.ReplaceAll(content, []byte("$SANDBOX_WORKDIR"), []byte(rel))
 	fp := rel.AbsPath(path)
 	if err := os.MkdirAll(filepath.Dir(fp), 0755); err != nil {
 		return fmt.Errorf("creating nested directory: %w", err)
 	}
 	backoff := 1 * time.Millisecond
 	for {
 		err := ioutil.WriteFile(fp, []byte(content), 0644)
 		if err != nil {
 			// This lock file violation is not handled by the robustio package, as it
 			// indicates a real race condition that could be avoided.
 			if isWindowsErrLockViolation(err) {
 				time.Sleep(backoff)
 				backoff *= 2
 				continue
 			}
 			return fmt.Errorf("writing %q: %w", path, err)
 		}
 		return nil
 	}
 }

 // isWindowsErrLockViolation reports whether err is ERROR_LOCK_VIOLATION
 // on Windows.
 var isWindowsErrLockViolation = func(err error) bool { return false }

 // Workdir is a temporary working directory for tests. It exposes file
 // operations in terms of relative paths, and fakes file watching by triggering
 // events on file operations.
 type Workdir struct {
 	RelativeTo

 	watcherMu sync.Mutex
 	watchers  []func(context.Context, []protocol.FileEvent)

 	fileMu sync.Mutex
 	// File identities we know about, for the purpose of detecting changes.
 	//
 	// Since files is only used for detecting _changes_, we are tolerant of
 	// fileIDs that may have hash and mtime coming from different states of the
 	// file: if either are out of sync, then the next poll should detect a
 	// discrepancy. It is OK if we detect too many changes, but not OK if we miss
 	// changes.
 	//
 	// For that matter, this mechanism for detecting changes can still be flaky
 	// on platforms where mtime is very coarse (such as older versions of WSL).
 	// It would be much better to use a proper fs event library, but we can't
 	// currently import those into x/tools.
 	//
 	// TODO(golang/go#52284): replace this polling mechanism with a
 	// cross-platform library for filesystem notifications.
 	files map[string]fileID
 }

 // fileID is a file identity for the purposes of detecting on-disk
 // modifications.
 type fileID struct {
 	hash  string
 	mtime time.Time
 }

 // NewWorkdir writes the txtar-encoded file data in txt to dir, and returns a
 // Workir for operating on these files using
 func NewWorkdir(dir string) *Workdir {
 	return &Workdir{RelativeTo: RelativeTo(dir)}
 }

 func hashFile(data []byte) string {
 	return fmt.Sprintf("%x", sha256.Sum256(data))
 }

 func (w *Workdir) writeInitialFiles(files map[string][]byte) error {
 	w.files = map[string]fileID{}
 	for name, data := range files {
 		if err := writeFileData(name, data, w.RelativeTo); err != nil {
 			return fmt.Errorf("writing to workdir: %w", err)
 		}
 		fp := w.AbsPath(name)

 		// We need the mtime of the file just written for the purposes of tracking
 		// file identity. Calling Stat here could theoretically return an mtime
 		// that is inconsistent with the file contents represented by the hash, but
 		// since we "own" this file we assume that the mtime is correct.
 		//
 		// Furthermore, see the documentation for Workdir.files for why mismatches
 		// between identifiers are considered to be benign.
 		fi, err := os.Stat(fp)
 		if err != nil {
 			return fmt.Errorf("reading file info: %v", err)
 		}

 		w.files[name] = fileID{
 			hash:  hashFile(data),
 			mtime: fi.ModTime(),
 		}
 	}
 	return nil
 }

 // RootURI returns the root URI for this working directory of this scratch
 // environment.
 func (w *Workdir) RootURI() protocol.DocumentURI {
 	return toURI(string(w.RelativeTo))
 }

 // AddWatcher registers the given func to be called on any file change.
 func (w *Workdir) AddWatcher(watcher func(context.Context, []protocol.FileEvent)) {
 	w.watcherMu.Lock()
 	w.watchers = append(w.watchers, watcher)
 	w.watcherMu.Unlock()
 }

 // URI returns the URI to a the workdir-relative path.
 func (w *Workdir) URI(path string) protocol.DocumentURI {
 	return toURI(w.AbsPath(path))
 }

 // URIToPath converts a uri to a workdir-relative path (or an absolute path,
 // if the uri is outside of the workdir).
 func (w *Workdir) URIToPath(uri protocol.DocumentURI) string {
 	fp := uri.SpanURI().Filename()
 	return w.RelPath(fp)
 }

 func toURI(fp string) protocol.DocumentURI {
 	return protocol.DocumentURI(span.URIFromPath(fp))
 }

 // ReadFile reads a text file specified by a workdir-relative path.
 func (w *Workdir) ReadFile(path string) (string, error) {
 	backoff := 1 * time.Millisecond
 	for {
 		b, err := ioutil.ReadFile(w.AbsPath(path))
 		if err != nil {
 			if runtime.GOOS == "plan9" && strings.HasSuffix(err.Error(), " exclusive use file already open") {
 				// Plan 9 enforces exclusive access to locked files.
 				// Give the owner time to unlock it and retry.
 				time.Sleep(backoff)
 				backoff *= 2
 				continue
 			}
 			return "", err
 		}
 		return string(b), nil
 	}
 }

 func (w *Workdir) RegexpRange(path, re string) (Pos, Pos, error) {
 	content, err := w.ReadFile(path)
 	if err != nil {
 		return Pos{}, Pos{}, err
 	}
 	return regexpRange(content, re)
 }

 // RegexpSearch searches the file corresponding to path for the first position
 // matching re.
 func (w *Workdir) RegexpSearch(path string, re string) (Pos, error) {
 	content, err := w.ReadFile(path)
 	if err != nil {
 		return Pos{}, err
 	}
 	start, _, err := regexpRange(content, re)
 	return start, err
 }

 // RemoveFile removes a workdir-relative file path and notifies watchers of the
 // change.
 func (w *Workdir) RemoveFile(ctx context.Context, path string) error {
 	fp := w.AbsPath(path)
 	if err := robustio.RemoveAll(fp); err != nil {
 		return fmt.Errorf("removing %q: %w", path, err)
 	}

 	return w.CheckForFileChanges(ctx)
 }

 // WriteFiles writes the text file content to workdir-relative paths and
 // notifies watchers of the changes.
 func (w *Workdir) WriteFiles(ctx context.Context, files map[string]string) error {
 	for path, content := range files {
 		fp := w.AbsPath(path)
 		_, err := os.Stat(fp)
 		if err != nil && !os.IsNotExist(err) {
 			return fmt.Errorf("checking if %q exists: %w", path, err)
 		}
 		if err := writeFileData(path, []byte(content), w.RelativeTo); err != nil {
 			return err
 		}
 	}
 	return w.CheckForFileChanges(ctx)
 }

 // WriteFile writes text file content to a workdir-relative path and notifies
 // watchers of the change.
 func (w *Workdir) WriteFile(ctx context.Context, path, content string) error {
 	return w.WriteFiles(ctx, map[string]string{path: content})
 }

 func (w *Workdir) fileEvent(path string, changeType protocol.FileChangeType) protocol.FileEvent {
 	return protocol.FileEvent{
 		URI:  w.URI(path),
 		Type: changeType,
 	}
 }

 // RenameFile performs an on disk-renaming of the workdir-relative oldPath to
 // workdir-relative newPath, and notifies watchers of the changes.
 //
 // oldPath must either be a regular file or in the same directory as newPath.
 func (w *Workdir) RenameFile(ctx context.Context, oldPath, newPath string) error {
 	oldAbs := w.AbsPath(oldPath)
 	newAbs := w.AbsPath(newPath)

 	// For os.Rename, “OS-specific restrictions may apply when oldpath and newpath
 	// are in different directories.” If that applies here, we may fall back to
 	// ReadFile, WriteFile, and RemoveFile to perform the rename non-atomically.
 	//
 	// However, the fallback path only works for regular files: renaming a
 	// directory would be much more complex and isn't needed for our tests.
 	fallbackOk := false
 	if filepath.Dir(oldAbs) != filepath.Dir(newAbs) {
 		fi, err := os.Stat(oldAbs)
 		if err == nil && !fi.Mode().IsRegular() {
 			return &os.PathError{
 				Op:   "RenameFile",
 				Path: oldPath,
 				Err:  fmt.Errorf("%w: file is not regular and not in the same directory as %s", os.ErrInvalid, newPath),
 			}
 		}
 		fallbackOk = true
 	}

 	var renameErr error
 	const debugFallback = false
 	if fallbackOk && debugFallback {
 		renameErr = fmt.Errorf("%w: debugging fallback path", os.ErrInvalid)
 	} else {
 		renameErr = robustio.Rename(oldAbs, newAbs)
 	}
 	if renameErr != nil {
 		if !fallbackOk {
 			return renameErr // The OS-specific Rename restrictions do not apply.
 		}

 		content, err := w.ReadFile(oldPath)
 		if err != nil {
 			// If we can't even read the file, the error from Rename may be accurate.
 			return renameErr
 		}
 		fi, err := os.Stat(newAbs)
 		if err == nil {
 			if fi.IsDir() {
 				// “If newpath already exists and is not a directory, Rename replaces it.”
 				// But if it is a directory, maybe not?
 				return renameErr
 			}
 			// On most platforms, Rename replaces the named file with a new file,
 			// rather than overwriting the existing file it in place. Mimic that
 			// behavior here.
 			if err := robustio.RemoveAll(newAbs); err != nil {
 				// Maybe we don't have permission to replace newPath?
 				return renameErr
 			}
 		} else if !os.IsNotExist(err) {
 			// If the destination path already exists or there is some problem with it,
 			// the error from Rename may be accurate.
 			return renameErr
 		}
 		if writeErr := writeFileData(newPath, []byte(content), w.RelativeTo); writeErr != nil {
 			// At this point we have tried to actually write the file.
 			// If it still doesn't exist, assume that the error from Rename was accurate:
 			// for example, maybe we don't have permission to create the new path.
 			// Otherwise, return the error from the write, which may indicate some
 			// other problem (such as a full disk).
 			if _, statErr := os.Stat(newAbs); !os.IsNotExist(statErr) {
 				return writeErr
 			}
 			return renameErr
 		}
 		if err := robustio.RemoveAll(oldAbs); err != nil {
 			// If we failed to remove the old file, that may explain the Rename error too.
 			// Make a best effort to back out the write to the new path.
 			robustio.RemoveAll(newAbs)
 			return renameErr
 		}
 	}

 	return w.CheckForFileChanges(ctx)
 }

 // ListFiles returns a new sorted list of the relative paths of files in dir,
 // recursively.
 func (w *Workdir) ListFiles(dir string) ([]string, error) {
 	m, err := w.listFiles(dir)
 	if err != nil {
 		return nil, err
 	}

 	var paths []string
 	for p := range m {
 		paths = append(paths, p)
 	}
 	sort.Strings(paths)
 	return paths, nil
 }

 // listFiles lists files in the given directory, returning a map of relative
 // path to contents and modification time.
 func (w *Workdir) listFiles(dir string) (map[string]fileID, error) {
 	files := make(map[string]fileID)
 	absDir := w.AbsPath(dir)
 	if err := filepath.Walk(absDir, func(fp string, info os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
 		if info.IsDir() {
 			return nil
 		}
 		path := w.RelPath(fp)

 		data, err := ioutil.ReadFile(fp)
 		if err != nil {
 			return err
 		}
 		// The content returned by ioutil.ReadFile could be inconsistent with
 		// info.ModTime(), due to a subsequent modification. See the documentation
 		// for w.files for why we consider this to be benign.
 		files[path] = fileID{
 			hash:  hashFile(data),
 			mtime: info.ModTime(),
 		}
 		return nil
 	}); err != nil {
 		return nil, err
 	}
 	return files, nil
 }

 // CheckForFileChanges walks the working directory and checks for any files
 // that have changed since the last poll.
 func (w *Workdir) CheckForFileChanges(ctx context.Context) error {
 	evts, err := w.pollFiles()
 	if err != nil {
 		return err
 	}
 	if len(evts) == 0 {
 		return nil
 	}
 	w.watcherMu.Lock()
 	watchers := make([]func(context.Context, []protocol.FileEvent), len(w.watchers))
 	copy(watchers, w.watchers)
 	w.watcherMu.Unlock()
 	for _, w := range watchers {
 		w(ctx, evts)
 	}
 	return nil
 }

 // pollFiles updates w.files and calculates FileEvents corresponding to file
 // state changes since the last poll. It does not call sendEvents.
 func (w *Workdir) pollFiles() ([]protocol.FileEvent, error) {
 	w.fileMu.Lock()
 	defer w.fileMu.Unlock()

 	files, err := w.listFiles(".")
 	if err != nil {
 		return nil, err
 	}
 	var evts []protocol.FileEvent
 	// Check which files have been added or modified.
 	for path, id := range files {
 		oldID, ok := w.files[path]
 		delete(w.files, path)
 		var typ protocol.FileChangeType
 		switch {
 		case !ok:
 			typ = protocol.Created
 		case oldID != id:
 			typ = protocol.Changed
 		default:
 			continue
 		}
 		evts = append(evts, protocol.FileEvent{
 			URI:  w.URI(path),
 			Type: typ,
 		})
 	}
 	// Any remaining files must have been deleted.
 	for path := range w.files {
 		evts = append(evts, protocol.FileEvent{
 			URI:  w.URI(path),
 			Type: protocol.Deleted,
 		})
 	}
 	w.files = files
 	return evts, nil
 }
	// Copyright 2020 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 fake

	import (
	"bytes"
	"context"
	"crypto/sha256"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
	"runtime"
	"sort"
	"strings"
	"sync"
	"time"

	"golang.org/x/tools/gopls/internal/lsp/protocol"
	"golang.org/x/tools/gopls/internal/span"
	"golang.org/x/tools/internal/robustio"
	)

	// RelativeTo is a helper for operations relative to a given directory.
	type RelativeTo string

	// AbsPath returns an absolute filesystem path for the workdir-relative path.
	func (r RelativeTo) AbsPath(path string) string {
	fp := filepath.FromSlash(path)
	if filepath.IsAbs(fp) {
	return fp
	}
	return filepath.Join(string(r), filepath.FromSlash(path))
	}

	// RelPath returns a '/'-encoded path relative to the working directory (or an
	// absolute path if the file is outside of workdir)
	func (r RelativeTo) RelPath(fp string) string {
	root := string(r)
	if rel, err := filepath.Rel(root, fp); err == nil && !strings.HasPrefix(rel, "..") {
	return filepath.ToSlash(rel)
	}
	return filepath.ToSlash(fp)
	}

	// writeFileData writes content to the relative path, replacing the special
	// token $SANDBOX_WORKDIR with the relative root given by rel. It does not
	// trigger any file events.
	func writeFileData(path string, content []byte, rel RelativeTo) error {
	content = bytes.ReplaceAll(content, []byte("$SANDBOX_WORKDIR"), []byte(rel))
	fp := rel.AbsPath(path)
	if err := os.MkdirAll(filepath.Dir(fp), 0755); err != nil {
	return fmt.Errorf("creating nested directory: %w", err)
	}
	backoff := 1 * time.Millisecond
	for {
	err := ioutil.WriteFile(fp, []byte(content), 0644)
	if err != nil {
	// This lock file violation is not handled by the robustio package, as it
	// indicates a real race condition that could be avoided.
	if isWindowsErrLockViolation(err) {
	time.Sleep(backoff)
	backoff *= 2
	continue
	}
	return fmt.Errorf("writing %q: %w", path, err)
	}
	return nil
	}
	}

	// isWindowsErrLockViolation reports whether err is ERROR_LOCK_VIOLATION
	// on Windows.
	var isWindowsErrLockViolation = func(err error) bool { return false }

	// Workdir is a temporary working directory for tests. It exposes file
	// operations in terms of relative paths, and fakes file watching by triggering
	// events on file operations.
	type Workdir struct {
	RelativeTo

	watcherMu sync.Mutex
	watchers []func(context.Context, []protocol.FileEvent)

	fileMu sync.Mutex
	// File identities we know about, for the purpose of detecting changes.
	//
	// Since files is only used for detecting _changes_, we are tolerant of
	// fileIDs that may have hash and mtime coming from different states of the
	// file: if either are out of sync, then the next poll should detect a
	// discrepancy. It is OK if we detect too many changes, but not OK if we miss
	// changes.
	//
	// For that matter, this mechanism for detecting changes can still be flaky
	// on platforms where mtime is very coarse (such as older versions of WSL).
	// It would be much better to use a proper fs event library, but we can't
	// currently import those into x/tools.
	//
	// TODO(golang/go#52284): replace this polling mechanism with a
	// cross-platform library for filesystem notifications.
	files map[string]fileID
	}

	// fileID is a file identity for the purposes of detecting on-disk
	// modifications.
	type fileID struct {
	hash string
	mtime time.Time
	}

	// NewWorkdir writes the txtar-encoded file data in txt to dir, and returns a
	// Workir for operating on these files using
	func NewWorkdir(dir string) *Workdir {
	return &Workdir{RelativeTo: RelativeTo(dir)}
	}

	func hashFile(data []byte) string {
	return fmt.Sprintf("%x", sha256.Sum256(data))
	}

	func (w *Workdir) writeInitialFiles(files map[string][]byte) error {
	w.files = map[string]fileID{}
	for name, data := range files {
	if err := writeFileData(name, data, w.RelativeTo); err != nil {
	return fmt.Errorf("writing to workdir: %w", err)
	}
	fp := w.AbsPath(name)

	// We need the mtime of the file just written for the purposes of tracking
	// file identity. Calling Stat here could theoretically return an mtime
	// that is inconsistent with the file contents represented by the hash, but
	// since we "own" this file we assume that the mtime is correct.
	//
	// Furthermore, see the documentation for Workdir.files for why mismatches
	// between identifiers are considered to be benign.
	fi, err := os.Stat(fp)
	if err != nil {
	return fmt.Errorf("reading file info: %v", err)
	}

	w.files[name] = fileID{
	hash: hashFile(data),
	mtime: fi.ModTime(),
	}
	}
	return nil
	}

	// RootURI returns the root URI for this working directory of this scratch
	// environment.
	func (w *Workdir) RootURI() protocol.DocumentURI {
	return toURI(string(w.RelativeTo))
	}

	// AddWatcher registers the given func to be called on any file change.
	func (w *Workdir) AddWatcher(watcher func(context.Context, []protocol.FileEvent)) {
	w.watcherMu.Lock()
	w.watchers = append(w.watchers, watcher)
	w.watcherMu.Unlock()
	}

	// URI returns the URI to a the workdir-relative path.
	func (w *Workdir) URI(path string) protocol.DocumentURI {
	return toURI(w.AbsPath(path))
	}

	// URIToPath converts a uri to a workdir-relative path (or an absolute path,
	// if the uri is outside of the workdir).
	func (w *Workdir) URIToPath(uri protocol.DocumentURI) string {
	fp := uri.SpanURI().Filename()
	return w.RelPath(fp)
	}

	func toURI(fp string) protocol.DocumentURI {
	return protocol.DocumentURI(span.URIFromPath(fp))
	}

	// ReadFile reads a text file specified by a workdir-relative path.
	func (w *Workdir) ReadFile(path string) (string, error) {
	backoff := 1 * time.Millisecond
	for {
	b, err := ioutil.ReadFile(w.AbsPath(path))
	if err != nil {
	if runtime.GOOS == "plan9" && strings.HasSuffix(err.Error(), " exclusive use file already open") {
	// Plan 9 enforces exclusive access to locked files.
	// Give the owner time to unlock it and retry.
	time.Sleep(backoff)
	backoff *= 2
	continue
	}
	return "", err
	}
	return string(b), nil
	}
	}

	func (w *Workdir) RegexpRange(path, re string) (Pos, Pos, error) {
	content, err := w.ReadFile(path)
	if err != nil {
	return Pos{}, Pos{}, err
	}
	return regexpRange(content, re)
	}

	// RegexpSearch searches the file corresponding to path for the first position
	// matching re.
	func (w *Workdir) RegexpSearch(path string, re string) (Pos, error) {
	content, err := w.ReadFile(path)
	if err != nil {
	return Pos{}, err
	}
	start, _, err := regexpRange(content, re)
	return start, err
	}

	// RemoveFile removes a workdir-relative file path and notifies watchers of the
	// change.
	func (w *Workdir) RemoveFile(ctx context.Context, path string) error {
	fp := w.AbsPath(path)
	if err := robustio.RemoveAll(fp); err != nil {
	return fmt.Errorf("removing %q: %w", path, err)
	}

	return w.CheckForFileChanges(ctx)
	}

	// WriteFiles writes the text file content to workdir-relative paths and
	// notifies watchers of the changes.
	func (w *Workdir) WriteFiles(ctx context.Context, files map[string]string) error {
	for path, content := range files {
	fp := w.AbsPath(path)
	_, err := os.Stat(fp)
	if err != nil && !os.IsNotExist(err) {
	return fmt.Errorf("checking if %q exists: %w", path, err)
	}
	if err := writeFileData(path, []byte(content), w.RelativeTo); err != nil {
	return err
	}
	}
	return w.CheckForFileChanges(ctx)
	}

	// WriteFile writes text file content to a workdir-relative path and notifies
	// watchers of the change.
	func (w *Workdir) WriteFile(ctx context.Context, path, content string) error {
	return w.WriteFiles(ctx, map[string]string{path: content})
	}

	func (w *Workdir) fileEvent(path string, changeType protocol.FileChangeType) protocol.FileEvent {
	return protocol.FileEvent{
	URI: w.URI(path),
	Type: changeType,
	}
	}

	// RenameFile performs an on disk-renaming of the workdir-relative oldPath to
	// workdir-relative newPath, and notifies watchers of the changes.
	//
	// oldPath must either be a regular file or in the same directory as newPath.
	func (w *Workdir) RenameFile(ctx context.Context, oldPath, newPath string) error {
	oldAbs := w.AbsPath(oldPath)
	newAbs := w.AbsPath(newPath)

	// For os.Rename, “OS-specific restrictions may apply when oldpath and newpath
	// are in different directories.” If that applies here, we may fall back to
	// ReadFile, WriteFile, and RemoveFile to perform the rename non-atomically.
	//
	// However, the fallback path only works for regular files: renaming a
	// directory would be much more complex and isn't needed for our tests.
	fallbackOk := false
	if filepath.Dir(oldAbs) != filepath.Dir(newAbs) {
	fi, err := os.Stat(oldAbs)
	if err == nil && !fi.Mode().IsRegular() {
	return &os.PathError{
	Op: "RenameFile",
	Path: oldPath,
	Err: fmt.Errorf("%w: file is not regular and not in the same directory as %s", os.ErrInvalid, newPath),
	}
	}
	fallbackOk = true
	}

	var renameErr error
	const debugFallback = false
	if fallbackOk && debugFallback {
	renameErr = fmt.Errorf("%w: debugging fallback path", os.ErrInvalid)
	} else {
	renameErr = robustio.Rename(oldAbs, newAbs)
	}
	if renameErr != nil {
	if !fallbackOk {
	return renameErr // The OS-specific Rename restrictions do not apply.
	}

	content, err := w.ReadFile(oldPath)
	if err != nil {
	// If we can't even read the file, the error from Rename may be accurate.
	return renameErr
	}
	fi, err := os.Stat(newAbs)
	if err == nil {
	if fi.IsDir() {
	// “If newpath already exists and is not a directory, Rename replaces it.”
	// But if it is a directory, maybe not?
	return renameErr
	}
	// On most platforms, Rename replaces the named file with a new file,
	// rather than overwriting the existing file it in place. Mimic that
	// behavior here.
	if err := robustio.RemoveAll(newAbs); err != nil {
	// Maybe we don't have permission to replace newPath?
	return renameErr
	}
	} else if !os.IsNotExist(err) {
	// If the destination path already exists or there is some problem with it,
	// the error from Rename may be accurate.
	return renameErr
	}
	if writeErr := writeFileData(newPath, []byte(content), w.RelativeTo); writeErr != nil {
	// At this point we have tried to actually write the file.
	// If it still doesn't exist, assume that the error from Rename was accurate:
	// for example, maybe we don't have permission to create the new path.
	// Otherwise, return the error from the write, which may indicate some
	// other problem (such as a full disk).
	if _, statErr := os.Stat(newAbs); !os.IsNotExist(statErr) {
	return writeErr
	}
	return renameErr
	}
	if err := robustio.RemoveAll(oldAbs); err != nil {
	// If we failed to remove the old file, that may explain the Rename error too.
	// Make a best effort to back out the write to the new path.
	robustio.RemoveAll(newAbs)
	return renameErr
	}
	}

	return w.CheckForFileChanges(ctx)
	}

	// ListFiles returns a new sorted list of the relative paths of files in dir,
	// recursively.
	func (w *Workdir) ListFiles(dir string) ([]string, error) {
	m, err := w.listFiles(dir)
	if err != nil {
	return nil, err
	}

	var paths []string
	for p := range m {
	paths = append(paths, p)
	}
	sort.Strings(paths)
	return paths, nil
	}

	// listFiles lists files in the given directory, returning a map of relative
	// path to contents and modification time.
	func (w *Workdir) listFiles(dir string) (map[string]fileID, error) {
	files := make(map[string]fileID)
	absDir := w.AbsPath(dir)
	if err := filepath.Walk(absDir, func(fp string, info os.FileInfo, err error) error {
	if err != nil {
	return err
	}
	if info.IsDir() {
	return nil
	}
	path := w.RelPath(fp)

	data, err := ioutil.ReadFile(fp)
	if err != nil {
	return err
	}
	// The content returned by ioutil.ReadFile could be inconsistent with
	// info.ModTime(), due to a subsequent modification. See the documentation
	// for w.files for why we consider this to be benign.
	files[path] = fileID{
	hash: hashFile(data),
	mtime: info.ModTime(),
	}
	return nil
	}); err != nil {
	return nil, err
	}
	return files, nil
	}

	// CheckForFileChanges walks the working directory and checks for any files
	// that have changed since the last poll.
	func (w *Workdir) CheckForFileChanges(ctx context.Context) error {
	evts, err := w.pollFiles()
	if err != nil {
	return err
	}
	if len(evts) == 0 {
	return nil
	}
	w.watcherMu.Lock()
	watchers := make([]func(context.Context, []protocol.FileEvent), len(w.watchers))
	copy(watchers, w.watchers)
	w.watcherMu.Unlock()
	for _, w := range watchers {
	w(ctx, evts)
	}
	return nil
	}

	// pollFiles updates w.files and calculates FileEvents corresponding to file
	// state changes since the last poll. It does not call sendEvents.
	func (w *Workdir) pollFiles() ([]protocol.FileEvent, error) {
	w.fileMu.Lock()
	defer w.fileMu.Unlock()

	files, err := w.listFiles(".")
	if err != nil {
	return nil, err
	}
	var evts []protocol.FileEvent
	// Check which files have been added or modified.
	for path, id := range files {
	oldID, ok := w.files[path]
	delete(w.files, path)
	var typ protocol.FileChangeType
	switch {
	case !ok:
	typ = protocol.Created
	case oldID != id:
	typ = protocol.Changed
	default:
	continue
	}
	evts = append(evts, protocol.FileEvent{
	URI: w.URI(path),
	Type: typ,
	})
	}
	// Any remaining files must have been deleted.
	for path := range w.files {
	evts = append(evts, protocol.FileEvent{
	URI: w.URI(path),
	Type: protocol.Deleted,
	})
	}
	w.files = files
	return evts, nil
	}