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.

3 files changed, 67 insertions, 23 deletions

diff --git a/modules/httpcache/httpcache.go b/modules/httpcache/httpcache.go
new file mode 100644
index 0000000000..c4134f8e17
--- /dev/null
+++ b/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
+}
diff --git a/modules/public/public.go b/modules/public/public.go
index 3a2fa4c57c..fc933637d8 100644
--- a/modules/public/public.go
+++ b/modules/public/public.go
@@ -5,15 +5,13 @@
 package public
 
 import (
-	"encoding/base64"
-	"fmt"
 	"log"
 	"net/http"
 	"path"
 	"path/filepath"
 	"strings"
-	"time"
 
+	"code.gitea.io/gitea/modules/httpcache"
 	"code.gitea.io/gitea/modules/setting"
 )
 
@@ -22,11 +20,8 @@ type Options struct {
 	Directory   string
 	IndexFile   string
 	SkipLogging bool
-	// if set to true, will enable caching. Expires header will also be set to
-	// expire after the defined time.
-	ExpiresAfter time.Duration
-	FileSystem   http.FileSystem
-	Prefix       string
+	FileSystem  http.FileSystem
+	Prefix      string
 }
 
 // KnownPublicEntries list all direct children in the `public` directory
@@ -158,23 +153,10 @@ func (opts *Options) handle(w http.ResponseWriter, req *http.Request, opt *Optio
 		log.Println("[Static] Serving " + file)
 	}
 
-	// Add an Expires header to the static content
-	if opt.ExpiresAfter > 0 {
-		w.Header().Set("Expires", time.Now().Add(opt.ExpiresAfter).UTC().Format(http.TimeFormat))
-		tag := GenerateETag(fmt.Sprint(fi.Size()), fi.Name(), fi.ModTime().UTC().Format(http.TimeFormat))
-		w.Header().Set("ETag", tag)
-		if req.Header.Get("If-None-Match") == tag {
-			w.WriteHeader(304)
-			return true
-		}
+	if httpcache.HandleEtagCache(req, w, fi) {
+		return true
 	}
 
 	http.ServeContent(w, req, file, fi.ModTime(), f)
 	return true
 }
-
-// GenerateETag generates an ETag based on size, filename and file modification time
-func GenerateETag(fileSize, fileName, modTime string) string {
-	etag := fileSize + fileName + modTime
-	return base64.StdEncoding.EncodeToString([]byte(etag))
-}
diff --git a/modules/setting/setting.go b/modules/setting/setting.go
index 7ae8bb352d..708dc28233 100644
--- a/modules/setting/setting.go
+++ b/modules/setting/setting.go
@@ -67,6 +67,7 @@ var (
 	// AppVer settings
 	AppVer         string
 	AppBuiltWith   string
+	AppStartTime   time.Time
 	AppName        string
 	AppURL         string
 	AppSubURL      string
@@ -362,6 +363,7 @@ var (
 	PIDFile       = "/run/gitea.pid"
 	WritePIDFile  bool
 	ProdMode      bool
+	RunMode       string
 	RunUser       string
 	IsWindows     bool
 	HasRobotsTxt  bool
@@ -837,6 +839,7 @@ func NewContext() {
 	}
 
 	RunUser = Cfg.Section("").Key("RUN_USER").MustString(user.CurrentUsername())
+	RunMode = Cfg.Section("").Key("RUN_MODE").MustString("dev")
 	// Does not check run user when the install lock is off.
 	if InstallLock {
 		currentUser, match := IsRunUserMatchCurrentUser(RunUser)