This is mid, a collection of useful middleware for HTTP services.

The Err function turns a func(http.ResponseWriter, *http.Request) error into an http.Handler, allowing handler functions to return errors in a more natural fashion. These errors result in the handler producing an HTTP 500 status code, but CodeErr and Responder allow you to control this behavior (see below). A nil return produces a 200, or a 204 (“no content”) if no bytes were written to the response object.

Usage:

func main() {
  http.Handle("/foo", Err(fooHandler))
  http.ListenAndServe(":8080", nil)
}

func fooHandler(w http.ResponseWriter, req *http.Request) error {
  if x := req.FormValue("x"); x != "secret password" {
    return CodeErr{C: http.StatusUnauthorized}
  }
  fmt.Fprintf(w, "You know the secret password")
  return nil
}

The JSON function turns a func(context.Context, X) (Y, error) into an http.Handler, where X is the type of a parameter into which the HTTP request body is automatically JSON-unmarshaled, and Y is the type of a result that is automatically JSON-marshaled into the HTTP response. Any error is handled as with Err (see above). All parts of the func signature are optional: the context.Context parameter, the X parameter, the Y result, and the error result.

Usage:

func main() {
  // Parses the request body as a JSON-encoded array of strings,
  // then sorts, re-encodes, and returns that array.
  http.Handle("/bar", JSON(barHandler))

  http.ListenAndServe(":8080", nil)
}

func barHandler(inp []string) []string {
  sort.Strings(inp)
  return inp
}

CodeErr is an error type suitable for returning from Err- and JSON-wrapped handlers that can control the HTTP status code that gets returned. It contains an HTTP status code field and a nested error.

Responder is an interface (implemented by CodeErr) that allows an error type to control how Err- and JSON-wrapped handlers respond to the pending request.

ResponseWrapper is an http.ResponseWriter that wraps a nested http.ResponseWriter and also records the status code and number of bytes sent in the response.

The Trace function wraps an http.Handler and decorates the context.Context in its *http.Request with any “trace ID” string found in the request header.

The Log function wraps an http.Handler with a function that writes a simple log line on the way into and out of the handler. The log line includes any “trace ID” found in the request’s context.Context.