HTTP cache rework and enable caching for storage assets (#13569)

This enabled HTTP time-based cache for storage assets, primarily
avatars. I have not observed If-Modified-Since from browsers during
tests but I guess it's good to support regardless.

It introduces a new generic httpcache module that can handle both
time-based and etag-based caching.

Additionally, manifest.json and robots.txt are now also cachable.

modules/httpcache/httpcache.go

@@ -0,0 +1,59 @@
+// Copyright 2020 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package httpcache
+
+import (
+	"encoding/base64"
+	"fmt"
+	"net/http"
+	"os"
+	"strconv"
+	"time"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// GetCacheControl returns a suitable "Cache-Control" header value
+func GetCacheControl() string {
+	if setting.RunMode == "dev" {
+		return "no-store"
+	}
+	return "private, max-age=" + strconv.FormatInt(int64(setting.StaticCacheTime.Seconds()), 10)
+}
+
+// generateETag generates an ETag based on size, filename and file modification time
+func generateETag(fi os.FileInfo) string {
+	etag := fmt.Sprint(fi.Size()) + fi.Name() + fi.ModTime().UTC().Format(http.TimeFormat)
+	return base64.StdEncoding.EncodeToString([]byte(etag))
+}
+
+// HandleTimeCache handles time-based caching for a HTTP request
+func HandleTimeCache(req *http.Request, w http.ResponseWriter, fi os.FileInfo) (handled bool) {
+	ifModifiedSince := req.Header.Get("If-Modified-Since")
+	if ifModifiedSince != "" {
+		t, err := time.Parse(http.TimeFormat, ifModifiedSince)
+		if err == nil && fi.ModTime().Unix() <= t.Unix() {
+			w.WriteHeader(http.StatusNotModified)
+			return true
+		}
+	}
+
+	w.Header().Set("Cache-Control", GetCacheControl())
+	w.Header().Set("Last-Modified", fi.ModTime().Format(http.TimeFormat))
+	return false
+}
+
+// HandleEtagCache handles ETag-based caching for a HTTP request
+func HandleEtagCache(req *http.Request, w http.ResponseWriter, fi os.FileInfo) (handled bool) {
+	etag := generateETag(fi)
+	if req.Header.Get("If-None-Match") == etag {
+		w.WriteHeader(http.StatusNotModified)
+		return true
+	}
+
+	w.Header().Set("Cache-Control", GetCacheControl())
+	w.Header().Set("ETag", etag)
+	return false
+}