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

config/config_source_test.go

@@ -0,0 +1,50 @@
+package config
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFileWatcherSource(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, "example.txt"), []byte{1, 2, 3, 4}, 0o600)
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	src := NewFileWatcherSource(NewStaticSource(&Config{
+		Options: &Options{
+			CAFile: filepath.Join(tmpdir, "example.txt"),
+		},
+	}))
+	var closeOnce sync.Once
+	ch := make(chan struct{})
+	src.OnConfigChange(func(cfg *Config) {
+		closeOnce.Do(func() {
+			close(ch)
+		})
+	})
+
+	err = ioutil.WriteFile(filepath.Join(tmpdir, "example.txt"), []byte{5, 6, 7, 8}, 0o600)
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	select {
+	case <-ch:
+	case <-time.After(time.Second):
+		t.Error("expected OnConfigChange to be fired after modifying a file")
+	}
+}