[MM-30092] api4/image: add support for protocol rel. urls (#16189)

* api4/image: add support for protocol rel. urls

* reflect review comments

* Update api4/image.go

Co-authored-by: Juho Nurminen <juho.nurminen@mattermost.com>

* api4/image: prevent opaque urls to be processed

Co-authored-by: Juho Nurminen <juho.nurminen@mattermost.com>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/image.go

@@ -20,12 +20,27 @@ func getImage(c *Context, w http.ResponseWriter, r *http.Request) {
 	if err != nil {
 		c.Err = model.NewAppError("getImage", "api.image.get.app_error", nil, err.Error(), http.StatusBadRequest)
 		return
+	} else if parsedURL.Opaque != "" {
+		c.Err = model.NewAppError("getImage", "api.image.get.app_error", nil, "", http.StatusBadRequest)
+		return
+	}
+	siteURL, err := url.Parse(*c.App.Config().ServiceSettings.SiteURL)
+	if err != nil {
+		c.Err = model.NewAppError("getImage", "model.config.is_valid.site_url.app_error", nil, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	if parsedURL.Scheme == "" {
+		parsedURL.Scheme = siteURL.Scheme
+	}
+	if parsedURL.Host == "" {
+		parsedURL.Host = siteURL.Host
 	}
 
 	// in case image proxy is enabled and we are fetching a remote image (NOT static or served by plugins), pass request to proxy
-	if *c.App.Config().ImageProxySettings.Enable && parsedURL.IsAbs() {
-		c.App.ImageProxy().GetImage(w, r, actualURL)
+	if *c.App.Config().ImageProxySettings.Enable && parsedURL.Host != siteURL.Host {
+		c.App.ImageProxy().GetImage(w, r, parsedURL.String())
 	} else {
-		http.Redirect(w, r, actualURL, http.StatusFound)
+		http.Redirect(w, r, parsedURL.String(), http.StatusFound)
 	}
 }