Error handling on a call stack - http request handler - http

Question

In the below code:
func (p *ProductHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// handle the request for a list of products
if r.Method == http.MethodGet {
p.getProductHandler(w, r)
return
}
if r.Method == http.MethodPost {
p.addProductHandler(w, r)
return
}
if r.Method == http.MethodPut {
id := findID(w, r)
p.updateProductHandler(id, w, r)
return
}
// catch all
// if no method is satisfied return an error
w.WriteHeader(http.StatusMethodNotAllowed)
}
// getProducts returns the products from the data store
func (p *ProductHandler) getProductHandler(w http.ResponseWriter, r *http.Request) {
p.l.Println("Handle GET Products")
// fetch the products from the datastore
productList := data.GetProducts()
// serialize the list to JSON
err := productList.WriteJSON(w)
if err != nil {
http.Error(w, "Unable to marshal json", http.StatusInternalServerError)
return
}
}
func (p *ProductHandler) addProductHandler(w http.ResponseWriter, r *http.Request) {
p.l.Println("Handle POST products")
// Read the item from the incoming request
productItem := &data.Product{}
err := productItem.ReadJSON(r.Body)
if err != nil {
http.Error(w, "Unable to unmarshal JSON", http.StatusBadRequest)
return
}
p.l.Printf("Product item: %#v\n", productItem)
data.AddProductItem(productItem)
}
func (p *ProductHandler) updateProductHandler(id int, w http.ResponseWriter, r *http.Request) {
// whatever
return
}
func findID(w http.ResponseWriter, r *http.Request) int {
// expect the id in the URI
dfa := regexp.MustCompile(`/([0-9]+)`)
matches := dfa.FindAllStringSubmatch(r.URL.Path, -1) // returns [][]string
if len(matches) != 1 {
http.Error(w, "Invalid URI", http.StatusBadRequest)
return
}
if len(matches[0]) != 2 {
http.Error(w, "Invlaid URI", http.StatusBadRequest)
return
}
idString := matches[0][1]
id, err := strconv.Atoi(idString)
if err != nil {
http.Error(w, "Invlaid URI", http.StatusBadRequest)
return
}
return id
}
On error, we use http.Error() within handlers that handle POST & GET and then return
But for PUT request, call stack stack is ServeHTTP -> findID() and then ServeHTTP() -> updateProductHandler(). Error handling is required in findID() but cannot return immediately, because findID() returns int.
What is the design pattern to perform error handling for a call stack? Error wrapping using github.com/pkg/errors....

Answer 1

func (p *ProductHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
// handle the request for a list of products
if r.Method == http.MethodGet {
p.getProductHandler(w, r)
return
}
if r.Method == http.MethodPost {
p.addProductHandler(w, r)
return
}
if r.Method == http.MethodPut {
id, err := findID(r.URL.Path)
if err == nil {
p.updateProductHandler(id, w, r)
} else {
http.Error(w, err.Error(), http.StatusBadRequest)
}
return
}
// catch all
// if no method is satisfied return an error
w.WriteHeader(http.StatusMethodNotAllowed)
w.Header().Add("Allow", "GET, POST, PUT")
}
func findID(path string) (int, error) {
// expect the id in the URI
dfa := regexp.MustCompile(`/([0-9]+)`)
matches := dfa.FindAllStringSubmatch(path, -1) // returns [][]string
if len(matches) != 1 {
return 0, fmt.Errorf("Invalid URI %s", path)
}
if len(matches[0]) != 2 {
return 0, fmt.Errorf("Invalid URI %s", path)
}
idString := matches[0][1]
id, err := strconv.Atoi(idString)
if err != nil {
return 0, fmt.Errorf("Invalid URI %s", path)
}
return id, nil
}