[feature] Implement `cache-control` and etags for static assets (#711)

* start working on etag stuff

* add + use cache middleware

* generate etags on the fly

* remove unused field

* clean up filepath

* add license headers to cache files

* add attachgroup function to router interface

* move cache into web module

* rename a couple things

* remove attachStaticFS function from router

* rename + tidy up a few things

* mount assets filesystem

* create assetsFileInfoCache

* update comment

* simplify hash

* fix string fmt

* skip last mod chk, prefer strong etags w/long cache

* move base handler to its own file
this matches the modules in the api folder

* generate new etag if file was modified

* wrap strong etag in quotation marks as per spec

* clarify logic in avatar search

* make hashing a little niftier

1 files changed, 60 insertions, 0 deletions

diff --git a/internal/web/assets.go b/internal/web/assets.go
new file mode 100644
index 000000000..f67f38363
--- /dev/null
+++ b/internal/web/assets.go
@@ -0,0 +1,60 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package web
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+)
+
+type fileSystem struct {
+	fs http.FileSystem
+}
+
+// FileSystem server that only accepts directory listings when an index.html is available
+// from https://gist.github.com/hauxe/f2ea1901216177ccf9550a1b8bd59178
+func (fs fileSystem) Open(path string) (http.File, error) {
+	f, err := fs.fs.Open(path)
+	if err != nil {
+		return nil, err
+	}
+
+	s, _ := f.Stat()
+	if s.IsDir() {
+		index := strings.TrimSuffix(path, "/") + "/index.html"
+		if _, err := fs.fs.Open(index); err != nil {
+			return nil, err
+		}
+	}
+
+	return f, nil
+}
+
+func (m *Module) mountAssetsFilesystem(group *gin.RouterGroup) {
+	fs := fileSystem{http.Dir(m.webAssetsAbsFilePath)}
+
+	// use the cache middleware on all handlers in this group
+	group.Use(m.cacheControlMiddleware(fs))
+
+	// serve static file system in the root of this group,
+	// will end up being something like "/assets/"
+	group.StaticFS("/", fs)
+}