1 files changed, 53 insertions, 0 deletions
diff --git a/internal/persistence/atomic.go b/internal/persistence/atomic.go
new file mode 100644
index 0000000..a3cc06a
--- /dev/null
+++ b/internal/persistence/atomic.go
@@ -0,0 +1,53 @@
+// Package persistence provides crash-resistant single-file replacement.
+// It does not serialize callers or make multi-file operations transactional.
+package persistence
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// WriteFile writes and syncs a temporary file in the destination directory,
+// atomically renames it over path, then syncs the directory. Existing file
+// permissions are preserved; perm is used for new files. Callers must own the
+// data being serialized and coordinate competing read-modify-write operations.
+// A directory-sync error means replacement occurred but durability is uncertain.
+func WriteFile(path string, data []byte, perm os.FileMode) error {
+	if info, err := os.Stat(path); err == nil {
+		perm = info.Mode().Perm()
+	} else if !os.IsNotExist(err) {
+		return err
+	}
+	dir := filepath.Dir(path)
+	f, err := os.CreateTemp(dir, ".save-*")
+	if err != nil {
+		return err
+	}
+	defer os.Remove(f.Name())
+	defer f.Close()
+	if _, err := f.Write(data); err != nil {
+		return err
+	}
+	if err := f.Chmod(perm); err != nil {
+		return err
+	}
+	if err := f.Sync(); err != nil {
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	if err := os.Rename(f.Name(), path); err != nil {
+		return err
+	}
+	d, err := os.Open(dir)
+	if err != nil {
+		return fmt.Errorf("replacement completed; open directory for sync: %w", err)
+	}
+	defer d.Close()
+	if err := d.Sync(); err != nil {
+		return fmt.Errorf("replacement completed; sync directory: %w", err)
+	}
+	return nil
+}