应该返回Handler的函数如何返回HandlerFunc？

While chaining handlers, the function has a return type of Handler, but it actually returns a HandlerFunc. This does not throw any error.

How is HandlerFunc accepted in place of a Handler, the prior being a function type and the latter being a interface type?

func log(h http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    ...
  })
}

The HandlerFunc type is an adapter to allow the use of ordinary functions as HTTP handlers. If f is a function with the appropriate signature, HandlerFunc(f) is a Handler that calls f.

The http.Handler is an interface:

type Handler interface {
    ServeHTTP(ResponseWriter, *Request)
}

The http.HandlerFunc is a type:

type HandlerFunc func(ResponseWriter, *Request)

// ServeHTTP calls f(w, r).
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
    f(w, r)
}

http.Handler is an interface. http.HandlerFunc is a concrete type that implements that interface. This is all documented in the http package documentation. If interfaces are new to you, start with A Tour of Go.