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/controlplane/filemgr/config.go

@@ -0,0 +1,35 @@
+package filemgr
+
+import (
+	"os"
+	"path/filepath"
+
+	"github.com/google/uuid"
+)
+
+type config struct {
+	cacheDir string
+}
+
+// An Option updates the config.
+type Option = func(*config)
+
+// WithCacheDir returns an Option that sets the cache dir.
+func WithCacheDir(cacheDir string) Option {
+	return func(cfg *config) {
+		cfg.cacheDir = cacheDir
+	}
+}
+
+func newConfig(options ...Option) *config {
+	cfg := new(config)
+	cacheDir, err := os.UserCacheDir()
+	if err != nil {
+		cacheDir = filepath.Join(os.TempDir(), uuid.New().String())
+	}
+	WithCacheDir(filepath.Join(cacheDir, "pomerium", "envoy", "files"))(cfg)
+	for _, o := range options {
+		o(cfg)
+	}
+	return cfg
+}