config: detect underlying file changes (#1775)

* wip

* cleanup

* add test

* use uuid for temp dir, derive root CA path from filemgr for tests

* fix comment

* fix double close

* use latest notify

internal/fileutil/watcher_test.go

@@ -0,0 +1,42 @@
+package fileutil
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestWatcher(t *testing.T) {
+	tmpdir := filepath.Join(os.TempDir(), uuid.New().String())
+	err := os.MkdirAll(tmpdir, 0o755)
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	err = ioutil.WriteFile(filepath.Join(tmpdir, "test1.txt"), []byte{1, 2, 3, 4}, 0o666)
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	w := NewWatcher()
+	w.Add(filepath.Join(tmpdir, "test1.txt"))
+
+	ch := w.Bind()
+	defer w.Unbind(ch)
+
+	err = ioutil.WriteFile(filepath.Join(tmpdir, "test1.txt"), []byte{5, 6, 7, 8}, 0o666)
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	select {
+	case <-ch:
+	case <-time.After(time.Second):
+		t.Error("expected change signal when file is modified")
+	}
+}