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, 5 insertions, 1 deletions

diff --git a/internal/nametransform/longnames.go b/internal/nametransform/longnames.go
index 8af191d..7cd904e 100644
--- a/internal/nametransform/longnames.go
+++ b/internal/nametransform/longnames.go
@@ -114,7 +114,11 @@ func (n *NameTransform) WriteLongName(dirfd *os.File, hashName string, plainName
 	fdRaw, err := syscallcompat.Openat(int(dirfd.Fd()), hashName+LongNameSuffix,
 		syscall.O_WRONLY|syscall.O_CREAT|syscall.O_EXCL, 0600)
 	if err != nil {
-		tlog.Warn.Printf("WriteLongName: Openat: %v", err)
+		// Don't warn if the file already exists - this is allowed for renames
+		// and should be handled by the caller.
+		if err != syscall.EEXIST {
+			tlog.Warn.Printf("WriteLongName: Openat: %v", err)
+		}
 		return err
 	}
 	fd := os.NewFile(uintptr(fdRaw), hashName+LongNameSuffix)