fusefrontend: Fix longname handling for renames with existing target

Fixes https://github.com/rfjakob/gocryptfs/issues/170

Steps to reproduce the problem:

* Create a regular forward mount point
* Create a file with a shortname and one with a long filename
* Try to run 'mv <shortname> <longname>'

This should actually work and replace the existing file, but instead it
fails with:

    mv: cannot move '<shortname>' to '<longname>': File exists

The problem is the creation of the .name file. If the target already exists
we can safely ignore the EEXIST error and just keep the existing .name file.

1 files changed, 7 insertions, 1 deletions

diff --git a/internal/fusefrontend/fs.go b/internal/fusefrontend/fs.go
index 3c442a5..b4819fd 100644
--- a/internal/fusefrontend/fs.go
+++ b/internal/fusefrontend/fs.go
@@ -517,7 +517,13 @@ func (fs *FS) Rename(oldPath string, newPath string, context *fuse.Context) (cod
 		finalNewPath = cNewName
 		// Create destination .name file
 		err = fs.nameTransform.WriteLongName(newDirFd, cNewName, newPath)
-		if err != nil {
+		// Failure to write the .name file is expected when the target path already
+		// exists. Since hashes are pretty unique, there is no need to modify the
+		// file anyway. We still set newDirFd to nil to ensure that we do not delete
+		// the file on error.
+		if err == syscall.EEXIST {
+			newDirFd = nil
+		} else if err != nil {
 			return fuse.ToStatus(err)
 		}
 	}