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)
+}

internal/fsx/fsx_test.go

@@ -0,0 +1,34 @@
+package fsx_test
+
+import (
+	"testing"
+
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/lukaszraczylo/harness-sync/internal/fsx"
+)
+
+func TestOSReturnsAferoOsFs(t *testing.T) {
+	fs := fsx.OS()
+	require.NotNil(t, fs)
+	_, ok := fs.(*afero.OsFs)
+	assert.True(t, ok, "OS() should return *afero.OsFs")
+}
+
+func TestMemReturnsAferoMemFs(t *testing.T) {
+	fs := fsx.Mem()
+	require.NotNil(t, fs)
+	_, ok := fs.(*afero.MemMapFs)
+	assert.True(t, ok, "Mem() should return *afero.MemMapFs")
+}
+
+func TestWriteReadRoundtrip(t *testing.T) {
+	fs := fsx.Mem()
+	data := []byte("hello world")
+	require.NoError(t, fsx.WriteFile(fs, "/test.txt", data, 0o644))
+	got, err := fsx.ReadFile(fs, "/test.txt")
+	require.NoError(t, err)
+	assert.Equal(t, data, got)
+}