From 852afa65de21b3cb4c7bac0ff0b505ea02392913 Mon Sep 17 00:00:00 2001
From: "Gustavo \"Guz\" L de Mello" <contact@guz.one>
Date: Tue, 25 Feb 2025 14:31:02 -0300
Subject: [PATCH] feat(smalltrip,exceptions): exception middleware to provide
 handlers depending of Accept header

---
 exceptions/middleware.go | 165 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 165 insertions(+)

diff --git a/exceptions/middleware.go b/exceptions/middleware.go
index 7d487a6..50a41a3 100644
--- a/exceptions/middleware.go
+++ b/exceptions/middleware.go
@@ -1,12 +1,177 @@
 package exceptions
 
 import (
+	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"html/template"
 	"net/http"
+	"slices"
+	"strings"
+
+	"forge.capytal.company/loreddev/x/groute/middleware"
 )
 
+func Middleware(options ...MiddlewareOption) middleware.Middleware {
+	opts := middlewareOpts{
+		templates:      make(map[int]*template.Template),
+		handlers:       make(map[string]HandlerFunc),
+		defaultHandler: HandlerJSON(HandlerText),
+	}
+
+	for _, option := range options {
+		option(&opts)
+	}
+
+	if _, ok := opts.templates[0]; !ok {
+		opts.templates[0] = defaultTemplate
+	}
+
+	if _, ok := opts.handlers["application/json"]; !ok {
+		opts.handlers["application/json"] = HandlerJSON(HandlerText)
+	}
+	if _, ok := opts.handlers["text/html"]; !ok {
+		opts.handlers["text/html"] = HandlerTemplates(opts.templates, opts.defaultHandler)
+	}
+	if _, ok := opts.handlers["application/xhtml+xml"]; !ok {
+		opts.handlers["application/xhtml+xml"] = HandlerTemplates(opts.templates, opts.defaultHandler)
+	}
+	if _, ok := opts.handlers["application/xml"]; !ok {
+		opts.handlers["application/xml"] = HandlerTemplates(opts.templates, opts.defaultHandler)
+	}
+
+	return NewMiddleware(func(e Exception, w http.ResponseWriter, r *http.Request) {
+		for k, v := range opts.handlers {
+			if strings.Contains(r.Header.Get("Accept"), k) {
+				v(e, w, r)
+				return
+			}
+		}
+		opts.defaultHandler(e, w, r)
+	})
+}
+
+var defaultTemplate = template.Must(template.New("xx-small-trip-default-Exception-template").Parse(`
+Status: {{ .Status }}
+Code: {{ .Code }}
+Message: {{ .Message }}
+Err: {{ .Err }}
+Severity: {{ .Severity }}
+`))
+
+type MiddlewareOption = func(*middlewareOpts)
+
+func MiddlewareTemplate(t *template.Template, statusCode ...int) MiddlewareOption {
+	return func(mo *middlewareOpts) {
+		if len(statusCode) > 0 {
+			mo.templates[statusCode[0]] = t
+		} else {
+			mo.templates[0] = t
+		}
+	}
+}
+
+func MiddlewareHandler(h HandlerFunc, mimeType ...string) MiddlewareOption {
+	return func(mo *middlewareOpts) {
+		if len(mimeType) > 0 {
+			mo.handlers[mimeType[0]] = h
+		} else {
+			mo.defaultHandler = h
+		}
+	}
+}
+
+type middlewareOpts struct {
+	templates      map[int]*template.Template
+	handlers       map[string]HandlerFunc
+	defaultHandler HandlerFunc
+}
+
+func NewMiddleware(handler HandlerFunc) middleware.Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			r = r.WithContext(context.WithValue(r.Context(), handlerFuncCtxKey, handler))
+			next.ServeHTTP(w, r)
+		})
+	}
+}
+
+const handlerFuncCtxKey = "xx-smalltrip-Exception-handler-func"
+
+type HandlerFunc = func(e Exception, w http.ResponseWriter, r *http.Request)
+
+func HandlerTemplates(ts map[int]*template.Template, fallback HandlerFunc) HandlerFunc {
+	return func(e Exception, w http.ResponseWriter, r *http.Request) {
+		if len(ts) == 0 {
+			fallback(e, w, r)
+			return
+		}
+
+		t, ok := ts[e.Status]
+		if ok {
+			HandlerTemplate(t, fallback)(e, w, r)
+			return
+		}
+
+		// Loops over ordered list and gets the last one that is small or equal
+		// the current Status. For example, if the current Exception has Status 404,
+		// and we provide a map like:
+		//
+		// map[int]*template.Template{
+		//   100: Template100,
+		//   200: Template200,
+		//   300: Template300,
+		//   400: Template400,
+		//   500: Template500,
+		// }
+		//
+		// It will be converted to a ordered list of keys: 100, 200, 300, 400, 500.
+		// This loops iterates on all keys until a value bigger than the current Status
+		// is found (in this example, 500), then it uses the previous (in this example 400).
+		//
+		// So the 404 Exception will be rendered using the Template400.
+
+		keys := make([]int, len(ts), len(ts))
+
+		var i int
+		for k := range ts {
+			keys[i] = k
+			i++
+		}
+
+		slices.Sort(keys)
+
+		key := keys[0]
+		for _, k := range keys {
+			if k > e.Status {
+				break
+			}
+			key = k
+		}
+
+		t, ok = ts[key]
+		if ok {
+			HandlerTemplate(t, fallback)(e, w, r)
+			return
+		}
+
+		fallback(e, w, r)
+	}
+}
+
+func HandlerTemplate(t *template.Template, fallback HandlerFunc) HandlerFunc {
+	return func(e Exception, w http.ResponseWriter, r *http.Request) {
+		err := t.Execute(w, e)
+		if err != nil {
+			e.Err = errors.Join(fmt.Errorf("executing Exception template: %s", e.Error()), e.Err)
+
+			fallback(e, w, r)
+			return
+		}
+	}
+}
+
 func HandlerJSON(fallback HandlerFunc) HandlerFunc {
 	return func(e Exception, w http.ResponseWriter, r *http.Request) {
 		j, err := json.Marshal(e)