Fix for character incompatibility corner case on Windows.

We generally rely on system calls rejecting invalid path names so that
we can avoid checking every path for invalid characters, but backslashes
on Windows can be problematic if synchronizing against a POSIX system
where a directory at the same level as the invalid filename has a name
that is the prefix (pre-'\' string) of the invalid filename. This commit
adds an explicit check for this case with negligible overhead.

pkg/sync/transition.go

@@ -190,6 +190,11 @@ func (t *transitioner) ensureNotExists(path string) error {
 }
 
 func (t *transitioner) removeFile(path string, target *Entry) error {
+	// Watch for special characters.
+	if pathContainsInvalidCharacters(path) {
+		return errors.New("path contains invalid characters")
+	}
+
 	// Ensure that the existing entry hasn't been modified from what we're
 	// expecting.
 	if _, _, _, err := t.ensureExpectedFile(path, target); err != nil {
@@ -201,6 +206,11 @@ func (t *transitioner) removeFile(path string, target *Entry) error {
 }
 
 func (t *transitioner) removeSymlink(path string, target *Entry) error {
+	// Watch for special characters.
+	if pathContainsInvalidCharacters(path) {
+		return errors.New("path contains invalid characters")
+	}
+
 	// Ensure that the existing symlink hasn't been modified from what we're
 	// expecting.
 	if err := t.ensureExpectedSymlink(path, target); err != nil {
@@ -212,6 +222,12 @@ func (t *transitioner) removeSymlink(path string, target *Entry) error {
 }
 
 func (t *transitioner) removeDirectory(path string, target *Entry) bool {
+	// Watch for special characters.
+	if pathContainsInvalidCharacters(path) {
+		t.recordProblem(path, errors.New("path contains invalid characters"))
+		return false
+	}
+
 	// Compute the full path to this directory.
 	fullPath := filepath.Join(t.root, path)
 
@@ -332,6 +348,11 @@ func (t *transitioner) remove(path string, target *Entry) *Entry {
 }
 
 func (t *transitioner) swapFile(path string, oldEntry, newEntry *Entry) error {
+	// Watch for special characters.
+	if pathContainsInvalidCharacters(path) {
+		return errors.New("path contains invalid characters")
+	}
+
 	// Compute the full path to this file.
 	fullPath := filepath.Join(t.root, path)
 
@@ -394,6 +415,11 @@ func (t *transitioner) swapFile(path string, oldEntry, newEntry *Entry) error {
 }
 
 func (t *transitioner) createFile(path string, target *Entry) error {
+	// Watch for special characters.
+	if pathContainsInvalidCharacters(path) {
+		return errors.New("path contains invalid characters")
+	}
+
 	// Ensure that the target path doesn't exist, e.g. due to a case conflict or
 	// modification since the last scan.
 	if err := t.ensureNotExists(path); err != nil {
@@ -429,6 +455,11 @@ func (t *transitioner) createFile(path string, target *Entry) error {
 }
 
 func (t *transitioner) createSymlink(path string, target *Entry) error {
+	// Watch for special characters.
+	if pathContainsInvalidCharacters(path) {
+		return errors.New("path contains invalid characters")
+	}
+
 	// Ensure that the target path doesn't exist, e.g. due to a case conflict or
 	// modification since the last scan.
 	if err := t.ensureNotExists(path); err != nil {
@@ -445,6 +476,12 @@ func (t *transitioner) createSymlink(path string, target *Entry) error {
 }
 
 func (t *transitioner) createDirectory(path string, target *Entry) *Entry {
+	// Watch for special characters.
+	if pathContainsInvalidCharacters(path) {
+		t.recordProblem(path, errors.New("path contains invalid characters"))
+		return nil
+	}
+
 	// Ensure that the target path doesn't exist, e.g. due to a case conflict or
 	// modification since the last scan.
 	if err := t.ensureNotExists(path); err != nil {

pkg/sync/transition_posix.go

@@ -0,0 +1,7 @@
+// +build !windows
+
+package sync
+
+func pathContainsInvalidCharacters(path string) bool {
+	return false
+}

pkg/sync/transition_windows.go

@@ -0,0 +1,9 @@
+package sync
+
+import (
+	"strings"
+)
+
+func pathContainsInvalidCharacters(path string) bool {
+	return strings.Contains(path, "\\")
+}