cmount: fix creating and renaming files on case insensitive backends

Before this fix, we told cgofuse/WinFSP that the backend was case
insensitive but didn't implement the Getpath backend function to
return the normalised case of a file.

Resently cgofuse started implementing case insensitive files properly
but since we hadn't implemented Getpath, the file names were taking
the default of all in UPPER CASE.

This patch implements Getpath for cgofuse which fixes the case
problems.

This problem came to light when we upgraded cgofuse and WinFSP (to
1.12) which had the code to implement Getpath.

Fixes #6682

cmd/cmount/fs.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"os"
 	"path"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -567,6 +568,21 @@ func (fsys *FS) Listxattr(path string, fill func(name string) bool) (errc int) {
 	return -fuse.ENOSYS
 }
 
+// Getpath allows a case-insensitive file system to report the correct case of
+// a file path.
+func (fsys *FS) Getpath(path string, fh uint64) (errc int, normalisedPath string) {
+	defer log.Trace(path, "Getpath fh=%d", fh)("errc=%d, normalisedPath=%q", &errc, &normalisedPath)
+	node, _, errc := fsys.getNode(path, fh)
+	if errc != 0 {
+		return errc, ""
+	}
+	normalisedPath = node.Path()
+	if !strings.HasPrefix("/", normalisedPath) {
+		normalisedPath = "/" + normalisedPath
+	}
+	return 0, normalisedPath
+}
+
 // Translate errors from mountlib
 func translateError(err error) (errc int) {
 	if err == nil {
@@ -631,6 +647,7 @@ func translateOpenFlags(inFlags int) (outFlags int) {
 var (
 	_ fuse.FileSystemInterface = (*FS)(nil)
 	_ fuse.FileSystemOpenEx    = (*FS)(nil)
+	_ fuse.FileSystemGetpath   = (*FS)(nil)
 	//_ fuse.FileSystemChflags    = (*FS)(nil)
 	//_ fuse.FileSystemSetcrtime  = (*FS)(nil)
 	//_ fuse.FileSystemSetchgtime = (*FS)(nil)