fix(gateway): no duplicate WriteHeader calls (#515)

* fix(gateway): do not act on template errors
* docs: changelog
* feat: write error on template execution error

Closes #487

CHANGELOG.md

@@ -27,7 +27,9 @@ The following emojis are used to highlight certain changes:
 
 ### Fixed
 
-* `boxo/gateway`: a panic (which is recovered) could sporadically be triggered inside a CAR request, if the right [conditions were met](https://github.com/ipfs/boxo/pull/511). 
+* `boxo/gateway`
+  * a panic (which is recovered) could sporadically be triggered inside a CAR request, if the right [conditions were met](https://github.com/ipfs/boxo/pull/511). 
+  * no longer emits `http: superfluous response.WriteHeader` warnings when an error happens.
 
 ### Security
 

gateway/assets/test/main.go

@@ -159,8 +159,7 @@ func runTemplate(w http.ResponseWriter, filename string, data interface{}) {
 	}
 	err = tpl.Execute(w, data)
 	if err != nil {
-		http.Error(w, fmt.Sprintf("failed to execute template: %s", err), http.StatusInternalServerError)
-		return
+		_, _ = w.Write([]byte(fmt.Sprintf("error during body generation: %v", err)))
 	}
 }
 

gateway/errors.go

@@ -165,14 +165,17 @@ func webError(w http.ResponseWriter, r *http.Request, c *Config, err error, defa
 	if acceptsHTML {
 		w.Header().Set("Content-Type", "text/html")
 		w.WriteHeader(code)
-		_ = assets.ErrorTemplate.Execute(w, assets.ErrorTemplateData{
+		err = assets.ErrorTemplate.Execute(w, assets.ErrorTemplateData{
 			GlobalData: assets.GlobalData{
 				Menu: c.Menu,
 			},
 			StatusCode: code,
 			StatusText: http.StatusText(code),
 			Error:      err.Error(),
 		})
+		if err != nil {
+			_, _ = w.Write([]byte(fmt.Sprintf("error during body generation: %v", err)))
+		}
 	} else {
 		http.Error(w, err.Error(), code)
 	}

gateway/handler.go

@@ -912,12 +912,13 @@ func (i *handler) handleSuperfluousNamespace(w http.ResponseWriter, r *http.Requ
 	// - redirects to intendedURL after a short delay
 
 	w.WriteHeader(http.StatusBadRequest)
-	if err := redirectTemplate.Execute(w, redirectTemplateData{
+	err = redirectTemplate.Execute(w, redirectTemplateData{
 		RedirectURL:   intendedURL,
 		SuggestedPath: intendedPath.String(),
 		ErrorMsg:      fmt.Sprintf("invalid path: %q should be %q", r.URL.Path, intendedPath.String()),
-	}); err != nil {
-		i.webError(w, r, fmt.Errorf("failed to redirect when fixing superfluous namespace: %w", err), http.StatusBadRequest)
+	})
+	if err != nil {
+		_, _ = w.Write([]byte(fmt.Sprintf("error during body generation: %v", err)))
 	}
 
 	return true

gateway/handler_codec.go

@@ -195,20 +195,19 @@ func (i *handler) serveCodecHTML(ctx context.Context, w http.ResponseWriter, r *
 	w.Header().Del("Cache-Control")
 
 	cidCodec := mc.Code(resolvedPath.RootCid().Prefix().Codec)
-	if err := assets.DagTemplate.Execute(w, assets.DagTemplateData{
+	err = assets.DagTemplate.Execute(w, assets.DagTemplateData{
 		GlobalData: i.getTemplateGlobalData(r, contentPath),
 		Path:       contentPath.String(),
 		CID:        resolvedPath.RootCid().String(),
 		CodecName:  cidCodec.String(),
 		CodecHex:   fmt.Sprintf("0x%x", uint64(cidCodec)),
 		Node:       parseNode(blockCid, blockData),
-	}); err != nil {
-		err = fmt.Errorf("failed to generate HTML listing for this DAG: try fetching raw block with ?format=raw: %w", err)
-		i.webError(w, r, err, http.StatusInternalServerError)
-		return false
+	})
+	if err != nil {
+		_, _ = w.Write([]byte(fmt.Sprintf("error during body generation: %v", err)))
 	}
 
-	return true
+	return err == nil
 }
 
 // parseNode does a best effort attempt to parse this request's block such that

gateway/handler_unixfs_dir.go

@@ -212,7 +212,7 @@ func (i *handler) serveDirectory(ctx context.Context, w http.ResponseWriter, r *
 	rq.logger.Debugw("request processed", "tplDataDNSLink", globalData.DNSLink, "tplDataSize", size, "tplDataBackLink", backLink, "tplDataHash", hash)
 
 	if err := assets.DirectoryTemplate.Execute(w, tplData); err != nil {
-		i.webError(w, r, err, http.StatusInternalServerError)
+		_, _ = w.Write([]byte(fmt.Sprintf("error during body generation: %v", err)))
 		return false
 	}