add close injector to allow auto-closing the wrapped writers

internal/ext/addcloser.go

@@ -0,0 +1,31 @@
+// Copyright 2024 Matthew Rich <matthewrich.conf@gmail.com>. All rights reserved.
+
+package ext
+
+import (
+	"io"
+)
+
+type Closer func() error
+
+func WriteAddCloser(w io.WriteCloser, c Closer) io.WriteCloser {
+	a := writeAddCloser{ WriteCloser: w, AddClose: func() (err error) {
+		if err = w.Close(); err != nil {
+			return
+		}
+		if c != nil {
+			return c()
+		}
+		return
+	} }
+	return a
+}
+
+type writeAddCloser struct {
+	io.WriteCloser
+	AddClose Closer
+}
+
+func (w writeAddCloser) Close() error {
+	return w.AddClose()
+}

internal/ext/addcloser_test.go

@@ -0,0 +1,19 @@
+// Copyright 2024 Matthew Rich <matthewrich.conf@gmail.com>. All rights reserved.
+
+package ext
+
+import (
+	"testing"
+	"github.com/stretchr/testify/assert"
+	"strings"
+_	"fmt"
+_	"log"
+)
+
+func TestNewWriteAddCloser(t *testing.T) {
+	var testWriter strings.Builder
+
+	closer := WriteAddCloser(WriteNopCloser(&testWriter), func() error { testWriter.Write([]byte("foo")); return nil })
+	closer.Close()
+	assert.Equal(t, "foo", testWriter.String())
+}