Confine local file installation paths

internal/adapters/storage/download_writer.go

@@ -0,0 +1,23 @@
+package storage
+
+import (
+	"context"
+	"fmt"
+	"io"
+)
+
+// WriterDownloader is implemented by storage backends that stream an object
+// into a caller-owned file handle.
+type WriterDownloader interface {
+	DownloadTo(ctx context.Context, key string, destination io.Writer) error
+}
+
+// DownloadTo streams one object into destination. Destination-confined callers
+// require this capability rather than granting a backend a mutable pathname.
+func DownloadTo(ctx context.Context, store ObjectStore, key string, destination io.Writer) error {
+	writer, ok := store.(WriterDownloader)
+	if !ok {
+		return fmt.Errorf("object store does not support handle-confined downloads")
+	}
+	return writer.DownloadTo(ctx, key, destination)
+}