test: introduce afero-based mock filesystem for hermetic tests

internal/fsx/fsx.go

@@ -0,0 +1,29 @@
+// Package fsx provides a small filesystem abstraction so file-touching code
+// can be tested against an in-memory filesystem.
+package fsx
+
+import (
+	"os"
+
+	"github.com/spf13/afero"
+)
+
+// FS is the harness-sync filesystem interface — alias for afero.Fs so the
+// dependency stays in one place.
+type FS = afero.Fs
+
+// OS returns the production OS-backed filesystem.
+func OS() FS { return afero.NewOsFs() }
+
+// Mem returns an empty in-memory filesystem for tests.
+func Mem() FS { return afero.NewMemMapFs() }
+
+// WriteFile writes data to path on fs.
+func WriteFile(fs FS, path string, data []byte, perm os.FileMode) error {
+	return afero.WriteFile(fs, path, data, perm)
+}
+
+// ReadFile reads the named file from fs and returns its contents.
+func ReadFile(fs FS, path string) ([]byte, error) {
+	return afero.ReadFile(fs, path)
+}