preserve old file

internal/file_store.go

@@ -9,6 +9,9 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
 )
 
 // FileStore is a Store backed by a single JSON file on disk. The on-disk
@@ -121,6 +124,16 @@ func (fs *FileStore) Save(ctx context.Context, queue map[string]*FillerQueue) er
 		state.Queues[symbol] = persisted
 	}
 
+	ext := filepath.Ext(fs.filename)
+	name, _ := strings.CutSuffix(fs.filename, ext)
+	backupFilename := name + "." + strconv.FormatInt(time.Now().UnixMilli(), 10) + ext
+	err := os.Rename(fs.filename, backupFilename)
+	if err != nil {
+		if !errors.Is(err, os.ErrNotExist) {
+			return fmt.Errorf("rename old state file: %w", err)
+		}
+	}
+
 	buf := &bytes.Buffer{}
 	enc := json.NewEncoder(buf)
 	enc.SetIndent("", "  ")
@@ -128,27 +141,13 @@ func (fs *FileStore) Save(ctx context.Context, queue map[string]*FillerQueue) er
 		return fmt.Errorf("encoding state: %w", err)
 	}
 
-	dir := filepath.Dir(fs.filename)
-	tmp, err := os.CreateTemp(dir, ".state-*.tmp")
+	dst, err := os.Create(fs.filename)
 	if err != nil {
-		return fmt.Errorf("creating temp file: %w", err)
+		return fmt.Errorf("creating new state file: %w", err)
 	}
 
-	tmpName := tmp.Name()
-	// Best-effort cleanup if rename never runs (e.g. process killed between
-	// CreateTemp and Rename). On success the temp file has been consumed by
-	// Rename and Remove returns ENOENT which we ignore.
-	defer os.Remove(tmpName)
-
-	if _, err := io.Copy(tmp, buf); err != nil {
-		_ = tmp.Close()
-		return fmt.Errorf("writing temp file: %w", err)
-	}
-	if err := tmp.Close(); err != nil {
-		return fmt.Errorf("closing temp file: %w", err)
-	}
-	if err := os.Rename(tmpName, fs.filename); err != nil {
-		return fmt.Errorf("renaming temp file: %w", err)
+	if _, err := io.Copy(dst, buf); err != nil {
+		return fmt.Errorf("writing to new state file: %w", err)
 	}
 	return nil
 }