Gulter

Gulter is a Go HTTP middleware designed to simplify the process of uploading files for your web apps. It follows the standard http.Handler and http.HandlerFunc interfaces so you can always use with any of framework or the standard library router.

[!NOTE] Name and idea was gotten from the insanely popular multer package in NodeJS that does the same

• Installation
• Usage
• Standard HTTP router
• Chi Router and others
• API
• FAQs
• customizing http error
• ignoring keys
• custom validation logic

Installation

go get -u -v github.com/adelowo/gulter

Usage

Assuming you have a HTML form like this:

<form action="/" method="post" enctype="multipart/form-data">
  <input type="file" name="form-field-1" />
  <input type="file" name="form-field-2" />
</form>

To create a new Gulter instance, you can do something like this:

 handler, _ := gulter.New(
  gulter.WithMaxFileSize(10<<20),
  gulter.WithValidationFunc(
   gulter.ChainValidators(gulter.MimeTypeValidator("image/jpeg", "image/png"),
    func(f gulter.File) error {
     // Your own custom validation function on the file here
     // Else you can really just drop the ChainValidators and use only the MimeTypeValidator or just
     // one custom validator alone
     return nil
    })),
  gulter.WithStorage(s3Store),
 )

The handler is really just a HTTP middleware with the following signature Upload(keys ...string) func(next http.Handler) http.Handler. keys here are the input names from the HTML form, so you can chain this into almost any HTTP router.

Standard HTTP router

package main

import (
 "fmt"
 "net/http"

 "github.com/adelowo/gulter"
 "github.com/adelowo/gulter/storage"
)

func main() {
 s3Store, err := storage.NewS3FromEnvironment(storage.S3Options{
  Bucket: "std-router",
 })
 if err != nil {
  panic(err.Error())
 }

 handler, err := gulter.New(
  gulter.WithMaxFileSize(10<<20),
  gulter.WithStorage(s3Store),
 )

 mux := http.NewServeMux()

 // upload all files with the "name" and "lanre" fields on this route
  // will be uploaded into the bucket_name bucket
  // This allows you handle both public and private buckets or dynamically route files
 mux.Handle("/", handler.Upload("bucket_name","name", "lanre")(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
  fmt.Println("Uploaded file")

  // return all uploaded files
  f, err := gulter.FilesFromContext(r)
  if err != nil {
   fmt.Println(err)
   return
  }

  // return uploaded files with the form field "lanre"
  ff, err := gulter.FilesFromContextWithKey(r, "lanre")
  if err != nil {
   fmt.Println(err)
   return
  }

  fmt.Printf("%+v", ff)

  for _, v := range f {
   fmt.Printf("%+v", v)
   fmt.Println()
  }
 })))

 http.ListenAndServe(":3300", mux)

Chi router and other compatible HTTP handlers

 s3Store, err := storage.NewS3FromEnvironment(storage.S3Options{
  Bucket: "chi-router",
 })
 if err != nil {
  panic(err.Error())
 }

 handler := gulter.New(
  gulter.WithMaxFileSize(10<<20),
  gulter.WithValidationFunc(gulter.ChainValidators(gulter.MimeTypeValidator("image/jpeg", "image/png"))),
  gulter.WithStorage(s3Store),
 )

 router := chi.NewMux()

  // upload all files in the form fields called "form-field-1" and "form-field-2"
  // will be uploaded into the bucket_name bucket
  // This allows you handle both public and private buckets or dynamically route files
 router.With(handler.Upload("bucket_name", "form-field-1", "form-field-2")).Post("/", func(w http.ResponseWriter, r *http.Request) {
  fmt.Println("Uploaded file")

  f, err := gulter.FilesFromContext(r)
  if err != nil {
   fmt.Println(err)
   return
  }

  ff, err := gulter.FilesFromContextWithKey(r, "form-field-1") // or form-field-2
  if err != nil {
   fmt.Println(err)
   return
  }

  fmt.Printf("%+v", ff)

  for _, v := range f {
   fmt.Printf("%+v", v)
   fmt.Println()
  }
 })

API

While this middleware automatically uploads your files, sometimes you need details about the uploaded file to show to the user, this could be making up the image url or path to the image. To get that in your HTTP handler, you can use either:

• FilesFromContextWithKey: retrieve a named input uploaded file.
• FilesFromContext: retrieve all uploaded files

Gulter also ships with two storage implementations at the moment:

• S3Store : supports S3 or any compatible service like Minio,R2 & others
• DiskStore: uses a local filesystem backed store to upload files
• CloudinaryStore: uploads file to cloudinary

FAQs

Ignoring non existent keys in the multipart Request

Sometimes, the keys you have configured the middleware might get dropped from the frontend for some reason, ideally the middleware fails if it cannot find a configured key in the request. To disable this behavior and ignore the missing key, you can make use of the WithIgnoreNonExistentKey(true) option to prevent the middleware from causing an error when such keys do not exists

Customizing the error response

Since Gulter is a middleware that runs, it returns an error to the client if found, this might not match your existing structure, so to configure the response, use the WithErrorResponseHandler. The default is shown below and can be used as a template to define yours.

 errHandler ErrResponseHandler = func(err error) http.HandlerFunc {
  return func(w http.ResponseWriter, _ *http.Request) {
   w.Header().Set("Content-Type", "application/json")
   w.WriteHeader(http.StatusInternalServerError)
   fmt.Fprintf(w, `{"message" : "could not upload file", "error" : %s}`, err.Error())
  }
 }

Writing your custom validator logic

Sometimes, you could have some custom logic to validate uploads, in this example below, we limit the size of the upload based on the mimeypes of the uploaded files

var customValidator gulter.ValidationFunc = func(f gulter.File) error {
 switch f.MimeType {
 case "image/png":
  if f.Size > 4096 {
   return errors.New("file size too large")
  }

  return nil

 case "application/pdf":
  if f.Size > (1024 * 10) {
   return errors.New("file size too large")
  }

  return nil
 default:
  return nil
 }
}