Support atomic directory promotion across platforms

internal/fileops/directory.go

@@ -8,7 +8,6 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
-	"syscall"
 )
 
 const (
@@ -16,10 +15,17 @@ const (
 	promotedFileMode      = 0o644
 )
 
+// ErrAtomicDirectoryPromotionUnsupported indicates that the current operating
+// system lacks the atomic no-replace primitive required by PromoteDirectory.
+var ErrAtomicDirectoryPromotionUnsupported = errors.New("atomic no-replace directory promotion is unsupported")
+
 // PromoteDirectory copies an existing regular-file tree into a new directory
 // and installs the complete copy atomically. It never removes the source or
 // replaces an existing destination.
 func PromoteDirectory(src, dst string) error {
+	if err := checkAtomicDirectoryPromotionSupport(); err != nil {
+		return err
+	}
 	return promoteDirectory(src, dst, renameDirectoryNoReplace)
 }
 
@@ -84,7 +90,9 @@ func promoteDirectory(src, dst string, install func(string, string) error) error
 	}
 
 	removeTemporary = false
-	_ = syncDirectory(destinationParent)
+	if err := syncDirectory(destinationParent); err != nil {
+		return fmt.Errorf("sync destination parent: %w", err)
+	}
 	return nil
 }
 
@@ -173,20 +181,6 @@ func copyRegularFile(src, dst string, inspected os.FileInfo) error {
 	return nil
 }
 
-func syncDirectory(path string) error {
-	directory, err := os.Open(path)
-	if err != nil {
-		return err
-	}
-	defer func() { _ = directory.Close() }()
-
-	err = directory.Sync()
-	if errors.Is(err, syscall.EINVAL) || errors.Is(err, syscall.ENOTSUP) {
-		return nil
-	}
-	return err
-}
-
 func pathWithin(parent, candidate string) (bool, error) {
 	absoluteParent, err := filepath.Abs(parent)
 	if err != nil {

internal/fileops/directory_special_unix_test.go

@@ -1,4 +1,4 @@
-//go:build !windows
+//go:build linux || darwin
 
 package fileops
 

internal/fileops/directory_test.go

@@ -1,3 +1,5 @@
+//go:build linux || darwin || windows
+
 package fileops
 
 import (
@@ -5,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -32,22 +35,24 @@ func TestPromoteDirectoryCopiesNestedRegularTree(t *testing.T) {
 	assertFileBytes(t, filepath.Join(dst, "nested", "binary.dat"), []byte{0, 1, 2, 0xff})
 	assertFileBytes(t, filepath.Join(dst, "z-last.txt"), []byte("last"))
 
-	for _, path := range []string{dst, filepath.Join(dst, "nested"), filepath.Join(dst, "empty")} {
-		info, err := os.Stat(path)
-		if err != nil {
-			t.Fatalf("Stat(%q) error = %v", path, err)
+	if runtime.GOOS != "windows" {
+		for _, path := range []string{dst, filepath.Join(dst, "nested"), filepath.Join(dst, "empty")} {
+			info, err := os.Stat(path)
+			if err != nil {
+				t.Fatalf("Stat(%q) error = %v", path, err)
+			}
+			if got := info.Mode().Perm(); got != promotedDirectoryMode {
+				t.Fatalf("directory mode for %q = %o, want %o", path, got, promotedDirectoryMode)
+			}
 		}
-		if got := info.Mode().Perm(); got != promotedDirectoryMode {
-			t.Fatalf("directory mode for %q = %o, want %o", path, got, promotedDirectoryMode)
-		}
-	}
-	for _, path := range []string{filepath.Join(dst, "a-first.txt"), filepath.Join(dst, "nested", "binary.dat"), filepath.Join(dst, "z-last.txt")} {
-		info, err := os.Stat(path)
-		if err != nil {
-			t.Fatalf("Stat(%q) error = %v", path, err)
-		}
-		if got := info.Mode().Perm(); got != promotedFileMode {
-			t.Fatalf("file mode for %q = %o, want %o", path, got, promotedFileMode)
+		for _, path := range []string{filepath.Join(dst, "a-first.txt"), filepath.Join(dst, "nested", "binary.dat"), filepath.Join(dst, "z-last.txt")} {
+			info, err := os.Stat(path)
+			if err != nil {
+				t.Fatalf("Stat(%q) error = %v", path, err)
+			}
+			if got := info.Mode().Perm(); got != promotedFileMode {
+				t.Fatalf("file mode for %q = %o, want %o", path, got, promotedFileMode)
+			}
 		}
 	}
 

internal/fileops/directory_unsupported_test.go

@@ -0,0 +1,37 @@
+//go:build !linux && !darwin && !windows
+
+package fileops
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestPromoteDirectoryFailsBeforeCreatingTemporaryTree(t *testing.T) {
+	root := t.TempDir()
+	src := filepath.Join(root, "source")
+	dst := filepath.Join(root, "destination")
+	if err := os.Mkdir(src, 0o755); err != nil {
+		t.Fatalf("Mkdir(source) error = %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(src, "value.txt"), []byte("source"), 0o644); err != nil {
+		t.Fatalf("WriteFile(source) error = %v", err)
+	}
+
+	err := PromoteDirectory(src, dst)
+	if !errors.Is(err, ErrAtomicDirectoryPromotionUnsupported) {
+		t.Fatalf("PromoteDirectory() error = %v, want unsupported capability", err)
+	}
+	entries, readErr := os.ReadDir(root)
+	if readErr != nil {
+		t.Fatalf("ReadDir(root) error = %v", readErr)
+	}
+	for _, entry := range entries {
+		if entry.Name() == filepath.Base(dst) || strings.HasPrefix(entry.Name(), ".destination.tmp-") {
+			t.Fatalf("unsupported promotion created %q", entry.Name())
+		}
+	}
+}

internal/fileops/rename_noreplace_darwin.go

@@ -0,0 +1,11 @@
+//go:build darwin
+
+package fileops
+
+import "golang.org/x/sys/unix"
+
+func checkAtomicDirectoryPromotionSupport() error { return nil }
+
+func renameDirectoryNoReplace(src, dst string) error {
+	return unix.RenamexNp(src, dst, unix.RENAME_EXCL)
+}

internal/fileops/rename_noreplace_linux.go

@@ -4,6 +4,8 @@ package fileops
 
 import "golang.org/x/sys/unix"
 
+func checkAtomicDirectoryPromotionSupport() error { return nil }
+
 func renameDirectoryNoReplace(src, dst string) error {
 	return unix.Renameat2(unix.AT_FDCWD, src, unix.AT_FDCWD, dst, unix.RENAME_NOREPLACE)
 }

internal/fileops/rename_noreplace_other.go

@@ -1,9 +1,16 @@
-//go:build !linux
+//go:build !linux && !darwin && !windows
 
 package fileops
 
-import "fmt"
+import (
+	"fmt"
+	"runtime"
+)
+
+func checkAtomicDirectoryPromotionSupport() error {
+	return fmt.Errorf("%w on %s", ErrAtomicDirectoryPromotionUnsupported, runtime.GOOS)
+}
 
 func renameDirectoryNoReplace(_, _ string) error {
-	return fmt.Errorf("atomic no-replace directory rename is unsupported on this platform")
+	return checkAtomicDirectoryPromotionSupport()
 }

internal/fileops/rename_noreplace_supported_test.go

@@ -0,0 +1,26 @@
+//go:build linux || darwin || windows
+
+package fileops
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestRenameDirectoryNoReplacePreservesExistingDestination(t *testing.T) {
+	root := t.TempDir()
+	src := filepath.Join(root, "source")
+	dst := filepath.Join(root, "destination")
+	mustWriteFile(t, filepath.Join(src, "value.txt"), []byte("source"), 0o644)
+	mustWriteFile(t, filepath.Join(dst, "value.txt"), []byte("existing"), 0o644)
+
+	if err := renameDirectoryNoReplace(src, dst); err == nil {
+		t.Fatal("renameDirectoryNoReplace() error = nil, want existing destination failure")
+	}
+	assertFileBytes(t, filepath.Join(src, "value.txt"), []byte("source"))
+	assertFileBytes(t, filepath.Join(dst, "value.txt"), []byte("existing"))
+	if info, err := os.Stat(src); err != nil || !info.IsDir() {
+		t.Fatalf("source directory was not preserved: info=%v err=%v", info, err)
+	}
+}

internal/fileops/rename_noreplace_windows.go

@@ -0,0 +1,19 @@
+//go:build windows
+
+package fileops
+
+import "golang.org/x/sys/windows"
+
+func checkAtomicDirectoryPromotionSupport() error { return nil }
+
+func renameDirectoryNoReplace(src, dst string) error {
+	from, err := windows.UTF16PtrFromString(src)
+	if err != nil {
+		return err
+	}
+	to, err := windows.UTF16PtrFromString(dst)
+	if err != nil {
+		return err
+	}
+	return windows.MoveFileEx(from, to, 0)
+}

internal/fileops/sync_directory_unix.go

@@ -0,0 +1,25 @@
+//go:build linux || darwin
+
+package fileops
+
+import (
+	"errors"
+	"os"
+	"syscall"
+)
+
+func syncDirectory(path string) error {
+	directory, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = directory.Close() }()
+
+	err = directory.Sync()
+	// Some Unix filesystems do not implement directory syncing. Only their
+	// explicit unsupported-operation errors are safe to treat as best effort.
+	if errors.Is(err, syscall.EINVAL) || errors.Is(err, syscall.ENOTSUP) {
+		return nil
+	}
+	return err
+}

internal/fileops/sync_directory_unsupported.go

@@ -0,0 +1,7 @@
+//go:build !linux && !darwin && !windows
+
+package fileops
+
+func syncDirectory(string) error {
+	return checkAtomicDirectoryPromotionSupport()
+}

internal/fileops/sync_directory_windows.go

@@ -0,0 +1,40 @@
+//go:build windows
+
+package fileops
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+func syncDirectory(path string) error {
+	pathPointer, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return err
+	}
+	directory, err := windows.CreateFile(
+		pathPointer,
+		windows.GENERIC_WRITE,
+		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
+		nil,
+		windows.OPEN_EXISTING,
+		windows.FILE_FLAG_BACKUP_SEMANTICS,
+		0,
+	)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = windows.CloseHandle(directory) }()
+
+	err = windows.FlushFileBuffers(directory)
+	// Windows filesystems may reject flushing a directory handle even when it
+	// was opened correctly. Preserve every error except the documented forms
+	// that mean this operation is unavailable for the handle or filesystem.
+	if errors.Is(err, windows.ERROR_INVALID_FUNCTION) ||
+		errors.Is(err, windows.ERROR_INVALID_HANDLE) ||
+		errors.Is(err, windows.ERROR_NOT_SUPPORTED) {
+		return nil
+	}
+	return err
+}