darwin: syscallcompat: add RenameatxNp plus flags

Fix macos file saving problem by implementing RENAME_EXCHANGE flag.
With test.

The dummy value for RENAME_WHITEOUT collides with the new
flags. Move it wayyy up.

https://github.com/rfjakob/gocryptfs/issues/914

2 files changed, 71 insertions, 6 deletions

diff --git a/internal/syscallcompat/rename_exchange_test.go b/internal/syscallcompat/rename_exchange_test.go
new file mode 100644
index 0000000..97a95f8
--- /dev/null
+++ b/internal/syscallcompat/rename_exchange_test.go
@@ -0,0 +1,58 @@
+package syscallcompat
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestRenameExchange(t *testing.T) {
+	// Create a temporary directory for testing
+	tmpDir, err := os.MkdirTemp("", "renameat2_test")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Test basic exchange functionality
+	file1 := filepath.Join(tmpDir, "file1.txt")
+	file2 := filepath.Join(tmpDir, "file2.txt")
+
+	content1 := []byte("content of file 1")
+	content2 := []byte("content of file 2")
+
+	if err := os.WriteFile(file1, content1, 0644); err != nil {
+		t.Fatalf("Failed to create file1: %v", err)
+	}
+
+	if err := os.WriteFile(file2, content2, 0644); err != nil {
+		t.Fatalf("Failed to create file2: %v", err)
+	}
+
+	// Test RENAME_EXCHANGE - this is the core functionality for issue #914
+	err = Renameat2(unix.AT_FDCWD, file1, unix.AT_FDCWD, file2, RENAME_EXCHANGE)
+	if err != nil {
+		t.Fatalf("RENAME_EXCHANGE failed: %v", err)
+	}
+
+	// Verify that the files have been swapped
+	newContent1, err := os.ReadFile(file1)
+	if err != nil {
+		t.Fatalf("Failed to read file1 after exchange: %v", err)
+	}
+
+	newContent2, err := os.ReadFile(file2)
+	if err != nil {
+		t.Fatalf("Failed to read file2 after exchange: %v", err)
+	}
+
+	if string(newContent1) != string(content2) {
+		t.Errorf("file1 content after exchange. Expected: %s, Got: %s", content2, newContent1)
+	}
+
+	if string(newContent2) != string(content1) {
+		t.Errorf("file2 content after exchange. Expected: %s, Got: %s", content1, newContent2)
+	}
+}
diff --git a/internal/syscallcompat/sys_darwin.go b/internal/syscallcompat/sys_darwin.go
index cf2f3f0..0ebdd3b 100644
--- a/internal/syscallcompat/sys_darwin.go
+++ b/internal/syscallcompat/sys_darwin.go
@@ -20,11 +20,13 @@ const (
 	// O_PATH is only defined on Linux
 	O_PATH = 0
 
+	// Same meaning, different name
+	RENAME_NOREPLACE = unix.RENAME_EXCL
+	RENAME_EXCHANGE  = unix.RENAME_SWAP
+
 	// Only exists on Linux. Define here to fix build failure, even though
-	// we will never see the flags.
-	RENAME_NOREPLACE = 1
-	RENAME_EXCHANGE  = 2
-	RENAME_WHITEOUT  = 4
+	// we will never see this flag.
+	RENAME_WHITEOUT = 1 << 30
 )
 
 // Unfortunately fsetattrlist does not have a syscall wrapper yet.
@@ -152,7 +154,12 @@ func GetdentsSpecial(fd int) (entries []fuse.DirEntry, entriesSpecial []fuse.Dir
 	return emulateGetdents(fd)
 }
 
-// Renameat2 does not exist on Darwin, so we call Renameat and ignore the flags.
+// Renameat2 does not exist on Darwin, but RenameatxNp does.
 func Renameat2(olddirfd int, oldpath string, newdirfd int, newpath string, flags uint) (err error) {
-	return unix.Renameat(olddirfd, oldpath, newdirfd, newpath)
+	// If no flags are set, use tried and true renameat
+	if flags == 0 {
+		return unix.Renameat(olddirfd, oldpath, newdirfd, newpath)
+	}
+	// Let RenameatxNp handle everything else
+	return unix.RenameatxNp(olddirfd, oldpath, newdirfd, newpath, uint32(flags))
 }