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.

3 files changed, 54 insertions, 5 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)
 		}
 	}
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)
diff --git a/tests/matrix/matrix_test.go b/tests/matrix/matrix_test.go
index 88c255e..9b9cb1b 100644
--- a/tests/matrix/matrix_test.go
+++ b/tests/matrix/matrix_test.go
@@ -544,7 +544,7 @@ func TestLongNames(t *testing.T) {
 	if !test_helpers.VerifyExistence(wd + n255x) {
 		t.Errorf("n255x is not in directory listing")
 	}
-	// Rename long to long
+	// Rename long to long (target does not exist)
 	n255y := string(bytes.Repeat([]byte("y"), 255))
 	err = os.Rename(wd+n255x, wd+n255y)
 	if err != nil {
@@ -553,7 +553,20 @@ func TestLongNames(t *testing.T) {
 	if !test_helpers.VerifyExistence(wd + n255y) {
 		t.Errorf("n255y is not in directory listing")
 	}
-	// Rename long to short
+	// Rename long to long (target exists)
+	f, err = os.Create(wd + n255x)
+	if err != nil {
+		t.Fatalf("Could not create n255x: %v", err)
+	}
+	f.Close()
+	err = os.Rename(wd+n255x, wd+n255y)
+	if err != nil {
+		t.Fatalf("Could not rename n255x to n255y: %v", err)
+	}
+	if !test_helpers.VerifyExistence(wd + n255y) {
+		t.Errorf("n255y is not in directory listing")
+	}
+	// Rename long to short (target does not exist)
 	err = os.Rename(wd+n255y, wd+"short")
 	if err != nil {
 		t.Fatalf("Could not rename n255y to short: %v", err)
@@ -561,7 +574,20 @@ func TestLongNames(t *testing.T) {
 	if !test_helpers.VerifyExistence(wd + "short") {
 		t.Errorf("short is not in directory listing")
 	}
-	// Rename short to long
+	// Rename long to short (target exists)
+	f, err = os.Create(wd + n255y)
+	if err != nil {
+		t.Fatalf("Could not create n255y: %v", err)
+	}
+	f.Close()
+	err = os.Rename(wd+n255y, wd+"short")
+	if err != nil {
+		t.Fatalf("Could not rename n255y to short: %v", err)
+	}
+	if !test_helpers.VerifyExistence(wd + "short") {
+		t.Errorf("short is not in directory listing")
+	}
+	// Rename short to long (target does not exist)
 	err = os.Rename(wd+"short", wd+n255x)
 	if err != nil {
 		t.Fatalf("Could not rename short to n255x: %v", err)
@@ -569,6 +595,19 @@ func TestLongNames(t *testing.T) {
 	if !test_helpers.VerifyExistence(wd + n255x) {
 		t.Errorf("255x is not in directory listing II")
 	}
+	// Rename short to long (target exists)
+	f, err = os.Create(wd + "short")
+	if err != nil {
+		t.Fatalf("Could not create short: %v", err)
+	}
+	f.Close()
+	err = os.Rename(wd+"short", wd+n255x)
+	if err != nil {
+		t.Fatalf("Could not rename short to n255x: %v", err)
+	}
+	if !test_helpers.VerifyExistence(wd + n255x) {
+		t.Errorf("n255x is not in directory listing")
+	}
 	// Unlink
 	err = syscall.Unlink(wd + n255x)
 	if err != nil {