feat: templatized proxy error page

internal/config/proxy_server.go

@@ -7,22 +7,26 @@ import (
 )
 
 type ProxyServerConfig struct {
-	HTTP      HTTPConfig      `yaml:"http"`
-	Metrics   MetricsConfig   `yaml:"metrics"`
-	Transport TransportConfig `yaml:"transport"`
-	Dial      DialConfig      `yaml:"dial"`
-	Sentry    SentryConfig    `yaml:"sentry"`
-	Cache     CacheConfig     `yaml:"cache"`
+	Debug     InterpolatedBool `yaml:"debug"`
+	HTTP      HTTPConfig       `yaml:"http"`
+	Metrics   MetricsConfig    `yaml:"metrics"`
+	Transport TransportConfig  `yaml:"transport"`
+	Dial      DialConfig       `yaml:"dial"`
+	Sentry    SentryConfig     `yaml:"sentry"`
+	Cache     CacheConfig      `yaml:"cache"`
+	Templates TemplatesConfig  `yaml:"templates"`
 }
 
 func NewDefaultProxyServerConfig() ProxyServerConfig {
 	return ProxyServerConfig{
+		Debug:     false,
 		HTTP:      NewHTTPConfig("0.0.0.0", 8080),
 		Metrics:   NewDefaultMetricsConfig(),
 		Transport: NewDefaultTransportConfig(),
 		Dial:      NewDefaultDialConfig(),
 		Sentry:    NewDefaultSentryConfig(),
 		Cache:     NewDefaultCacheConfig(),
+		Templates: NewDefaultTemplatesConfig(),
 	}
 }
 
@@ -115,3 +119,13 @@ func NewDefaultCacheConfig() CacheConfig {
 		TTL: *NewInterpolatedDuration(time.Second * 30),
 	}
 }
+
+type TemplatesConfig struct {
+	Dir InterpolatedString `yaml:"dir"`
+}
+
+func NewDefaultTemplatesConfig() TemplatesConfig {
+	return TemplatesConfig{
+		Dir: "./templates",
+	}
+}

internal/proxy/server.go

@@ -3,11 +3,14 @@ package proxy
 import (
 	"context"
 	"fmt"
+	"html/template"
 	"log"
 	"net"
 	"net/http"
 	"net/http/httputil"
 	"net/url"
+	"path/filepath"
+	"strconv"
 	"time"
 
 	"forge.cadoles.com/Cadoles/go-proxy"
@@ -17,6 +20,8 @@ import (
 	"forge.cadoles.com/cadoles/bouncer/internal/config"
 	"forge.cadoles.com/cadoles/bouncer/internal/proxy/director"
 	"forge.cadoles.com/cadoles/bouncer/internal/store"
+
+	"github.com/Masterminds/sprig/v3"
 	"github.com/getsentry/sentry-go"
 	sentryhttp "github.com/getsentry/sentry-go/http"
 	"github.com/go-chi/chi/v5"
@@ -191,7 +196,63 @@ func (s *Server) errorHandler(w http.ResponseWriter, r *http.Request, err error)
 	logger.Error(r.Context(), "proxy error", logger.E(err))
 	sentry.CaptureException(err)
 
-	http.Error(w, http.StatusText(http.StatusBadGateway), http.StatusBadGateway)
+	s.renderErrorPage(w, r, err, http.StatusBadGateway, http.StatusText(http.StatusBadGateway))
+}
+
+func (s *Server) renderErrorPage(w http.ResponseWriter, r *http.Request, err error, statusCode int, status string) {
+	templateData := struct {
+		StatusCode int
+		Status     string
+		Err        error
+		Debug      bool
+	}{
+		Debug:      bool(s.serverConfig.Debug),
+		StatusCode: statusCode,
+		Status:     status,
+		Err:        err,
+	}
+
+	w.WriteHeader(statusCode)
+	s.renderPage(w, r, "error", strconv.FormatInt(int64(statusCode), 10), templateData)
+}
+
+func (s *Server) renderPage(w http.ResponseWriter, r *http.Request, page string, block string, templateData any) {
+	ctx := r.Context()
+
+	templatesConf := s.serverConfig.Templates
+
+	pattern := filepath.Join(string(templatesConf.Dir), page+".gohtml")
+
+	logger.Info(ctx, "loading proxy templates", logger.F("pattern", pattern))
+
+	tmpl, err := template.New("").Funcs(sprig.FuncMap()).ParseGlob(pattern)
+	if err != nil {
+		logger.Error(ctx, "could not load proxy templates", logger.E(errors.WithStack(err)))
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+
+		return
+	}
+
+	w.Header().Add("Cache-Control", "no-cache")
+
+	blockTmpl := tmpl.Lookup(block)
+	if blockTmpl == nil {
+		blockTmpl = tmpl.Lookup("default")
+	}
+
+	if blockTmpl == nil {
+		logger.Error(ctx, "could not find template block nor default one", logger.F("block", block))
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+
+		return
+	}
+
+	if err := blockTmpl.Execute(w, templateData); err != nil {
+		logger.Error(ctx, "could not render proxy page", logger.E(errors.WithStack(err)))
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+
+		return
+	}
 }
 
 func NewServer(funcs ...OptionFunc) *Server {