Triggering 500 Internal Server Error response from a Golang API with a HTTP middleware

13/02/2020 - GO

If you wish to return a 500 Internal Server Error in your HTTP API without duplicating some obvious lines, you can use example below. It registers a server level middleware so all you have to do is to just call middleware.InternalServerResponse() function.


Structure


.
├── cmd
│   └── client
│       └── main.go
├── go.mod
└── internal
    ├── pkg
    │   └── middleware
    │       ├── internalservererror.go
    │       └── middleware.go
    └── user
        └── create.go

Files


main.go


package main

import (
	"log"
	"net/http"

	"internal/pkg/middleware"
	"internal/user"
)

func main() {
	http.HandleFunc("/users", user.Create{}.Handle)

	log.Fatal(http.ListenAndServe(":8888", middleware.Server(nil, middleware.InternalServerError)))
}

middleware.go


package middleware

import "net/http"

// server represents a server level middleware and is used for chaining.
type server func(http.Handler) http.Handler

// Server is used for middleware chaining and dedicated to HTTP server.
func Server(handler http.Handler, middleware ...server) http.Handler {
	for _, m := range middleware {
		handler = m(handler)
	}

	return handler
}

internalservererror.go


package middleware

import "net/http"

// InternalServerError catches all `panic` calls in order to prevent HTTP server
// from going down and return a standardised 500 Internal Server Error response.
func InternalServerError(handler http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		defer func() {
			if err := recover(); err != nil {
				w.WriteHeader(http.StatusInternalServerError)
				_, _ = w.Write([]byte("Standard error message."))
			}
		}()

		handler.ServeHTTP(w, r)
	})
}

// TriggerInternalServerError is called from anywhere in the application in order
// to trigger 500 Internal Server Error response.
func TriggerInternalServerError() {
	panic("")
}

create.go


As you have already guessed, I am purposely calling middleware.TriggerInternalServerError() function below.


package user

import (
	"log"
	"net/http"

	"internal/pkg/middleware"
)

type Create struct {}

func (c Create) Handle(w http.ResponseWriter, r *http.Request) {
	// This will cause 500 Internal Server error on purpose.
	c.create()

	// Since the function above terminates the whole process with an
	// Internal Server error, this line onwards will never be executed.

	w.WriteHeader(http.StatusCreated)
	_, _ = w.Write([]byte("created"))
}

func (Create) create() {
	// Some valid code.
	// ...
	
	// After this line, nothing after gets executed so no need for
	// manual `return` or `exit`.
	log.Println("Database is down.")
	middleware.TriggerInternalServerError()
	
	// Some valid code.
	// ...
}

Test


$ curl -i http://localhost:8888/users
HTTP/1.1 500 Internal Server Error
Date: Thu, 13 Feb 2020 21:32:06 GMT
Content-Length: 23
Content-Type: text/plain; charset=utf-8

Standard error message