Initial QSfera import

Server/pkg/x/io/fsx/fallback.go

@@ -0,0 +1,37 @@
+package fsx
+
+import (
+	"github.com/spf13/afero"
+)
+
+var (
+	// assert interfaces implemented
+	_ afero.Fs = (*FallbackFS)(nil)
+	_ FS       = (*FallbackFS)(nil)
+)
+
+// FallbackFS is a filesystem that layers a primary filesystem on top of a secondary filesystem.
+type FallbackFS struct {
+	FS
+	primary   *BaseFS
+	secondary *BaseFS
+}
+
+// Primary returns the primary filesystem.
+func (d *FallbackFS) Primary() *BaseFS {
+	return d.primary
+}
+
+// Secondary returns the secondary filesystem.
+func (d *FallbackFS) Secondary() *BaseFS {
+	return d.secondary
+}
+
+// NewFallbackFS returns a new FallbackFS instance.
+func NewFallbackFS(primary, secondary FS) *FallbackFS {
+	return &FallbackFS{
+		FS:        FromAfero(afero.NewCopyOnWriteFs(secondary, primary)),
+		primary:   &BaseFS{Fs: primary},
+		secondary: &BaseFS{Fs: secondary},
+	}
+}