From e1c362ac2fab2a29dc7360b2087651bad666b11d Mon Sep 17 00:00:00 2001
From: Jan-Lukas Else <mail@jlelse.de>
Date: Tue, 22 Sep 2020 16:42:36 +0200
Subject: [PATCH] Use mutexes to prevent cache stampede

---
 cache.go | 17 +++++++++++++++++
 main.go  |  1 +
 2 files changed, 18 insertions(+)

diff --git a/cache.go b/cache.go
index 6f4db27..5127517 100644
--- a/cache.go
+++ b/cache.go
@@ -6,9 +6,16 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"sync"
 	"time"
 )
 
+var cacheMutexes map[string]*sync.Mutex
+
+func initCache() {
+	cacheMutexes = map[string]*sync.Mutex{}
+}
+
 func cacheMiddleware(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		requestUrl, _ := url.ParseRequestURI(r.RequestURI)
@@ -16,8 +23,16 @@ func cacheMiddleware(next http.Handler) http.Handler {
 		if appConfig.Cache.Enable &&
 			// check bypass query
 			!(requestUrl != nil && requestUrl.Query().Get("cache") == "0") {
+			// Check cache mutex
+			if cacheMutexes[path] == nil {
+				cacheMutexes[path] = &sync.Mutex{}
+			}
+			// Lock mutex - prevents multiple new renderings
+			cacheMutexes[path].Lock()
+			// Get cache
 			cacheTime, header, body := getCache(r.Context(), path)
 			if cacheTime == 0 {
+				// No cache available
 				recorder := httptest.NewRecorder()
 				next.ServeHTTP(recorder, r)
 				// copy values from recorder
@@ -35,8 +50,10 @@ func cacheMiddleware(next http.Handler) http.Handler {
 				if code == http.StatusOK {
 					saveCache(path, now, recorder.Header(), recorder.Body.Bytes())
 				}
+				cacheMutexes[path].Unlock()
 				return
 			}
+			cacheMutexes[path].Unlock()
 			cacheTimeString := time.Unix(cacheTime, 0).Format(time.RFC1123)
 			expiresTimeString := time.Unix(cacheTime+appConfig.Cache.Expiration, 0).Format(time.RFC1123)
 			// check conditional request
diff --git a/main.go b/main.go
index 51e3599..7015d3f 100644
--- a/main.go
+++ b/main.go
@@ -25,6 +25,7 @@ func main() {
 	log.Println("Initialize server components...")
 	initMinify()
 	initMarkdown()
+	initCache()
 	err = initTemplateAssets() // Needs minify
 	if err != nil {
 		log.Fatal(err)