fileutil: add directory helpers, atomic file writing (#5477)

internal/fileutil/atomic_test.go

@@ -0,0 +1,30 @@
+package fileutil_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/pomerium/pomerium/internal/fileutil"
+)
+
+func TestWriteFileAtomically(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+
+	require.NoError(t, fileutil.WriteFileAtomically(filepath.Join(dir, "temp1.txt"), []byte("TEST"), 0o600))
+
+	entries, err := os.ReadDir(dir)
+	require.NoError(t, err)
+
+	names := make([]string, len(entries))
+	for i := range entries {
+		names[i] = entries[i].Name()
+	}
+
+	assert.Equal(t, []string{"temp1.txt"}, names)
+}