address comments from code review

config/config.go

@@ -118,9 +118,16 @@ type Config struct {
 	// Consul Template in another application
 	RendererFunc renderer.Renderer `mapstructure:"-" json:"-"`
 
+	// ReaderFunc is called whenever the template source is read, and will
+	// default to os.ReadFile. This is intended for use when embedding Consul
+	// Template in another application.
 	ReaderFunc Reader `mapstructure:"-" json:"-"`
 }
 
+// Reader is an interface that is implemented by os.OpenFile. The
+// Config.ReaderFunc requires this interface so that applications that embed
+// Consul Template can have an alternative implementation of os.OpenFile
+// (ex. virtual file, sandboxed reads)
 type Reader func(src string) ([]byte, error)
 
 // Copy returns a deep copy of the current configuration. This is useful because

manager/runner.go

@@ -114,8 +114,14 @@ type Runner struct {
 	// stopped is a boolean of whether the runner is stopped
 	stopped bool
 
+	// rendererFn is called whenever the template needs to be written, and will
+	// default to renderer.Render. This is intended for use when embedding
+	// Consul Template in another application
 	rendererFn renderer.Renderer
 
+	// readerFn is called whenever the template source is read, and will default
+	// to os.ReadFile. This is intended for use when embedding Consul Template
+	// in another application.
 	readerFn config.Reader
 
 	// finalConfigCopy provides access to a static copy of the finalized

renderer/renderer.go

@@ -64,7 +64,6 @@ type Renderer func(*RenderInput) (*RenderResult, error)
 // Render atomically renders a file contents to disk, returning a result of
 // whether it would have rendered and actually did render.
 func Render(i *RenderInput) (*RenderResult, error) {
-
 	existing, err := os.ReadFile(i.Path)
 	fileExists := !os.IsNotExist(err)
 	if err != nil && fileExists {

template/template.go

@@ -27,6 +27,10 @@ var (
 	// does not specify either a "source" or "content" argument, which is not
 	// valid.
 	ErrTemplateMissingContentsAndSource = errors.New("template: must specify exactly one of 'source' or 'contents'")
+
+	// ErrMissingReaderFunction is the error returned when the template
+	// configuration is missing a reader function.
+	ErrMissingReaderFunction = errors.New("template: missing a reader function")
 )
 
 // Template is the internal representation of an individual template to process.
@@ -156,6 +160,9 @@ func NewTemplate(i *NewTemplateInput) (*Template, error) {
 	}
 
 	if i.Source != "" {
+		if i.ReaderFunc == nil {
+			return nil, ErrMissingReaderFunction
+		}
 		contents, err := i.ReaderFunc(i.Source)
 		if err != nil {
 			return nil, errors.Wrap(err, "failed to read template")