From 7becbd6aadee548e6ed19f913008e65dd339d8b4 Mon Sep 17 00:00:00 2001
From: Jan-Lukas Else <mail@jlelse.de>
Date: Wed, 29 Jul 2020 22:45:26 +0200
Subject: [PATCH] Implement basic db-based cache

---
 cache.go               | 53 ++++++++++++++++++++++++++++++++++++++++++
 config.go              | 16 +++++++++++++
 database_migrations.go |  7 ++++++
 http.go                |  4 ++--
 posts.go               |  9 +++++--
 5 files changed, 85 insertions(+), 4 deletions(-)
 create mode 100644 cache.go

diff --git a/cache.go b/cache.go
new file mode 100644
index 0000000..bf2925e
--- /dev/null
+++ b/cache.go
@@ -0,0 +1,53 @@
+package main
+
+import (
+	"context"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+func CacheMiddleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestUrl, _ := url.ParseRequestURI(r.RequestURI)
+		if appConfig.cache.enable &&
+			// Check bypass query
+			!(requestUrl != nil && requestUrl.Query().Get("cache") == "0") {
+			mime, t, cache := getCache(SlashTrimmedPath(r), r.Context())
+			if cache == nil {
+				next.ServeHTTP(w, r)
+				return
+			} else {
+				expiresTime := time.Unix(t+appConfig.cache.expiration, 0).Format(time.RFC1123)
+				w.Header().Set("Expires", expiresTime)
+				w.Header().Set("Content-Type", mime)
+				_, _ = w.Write(cache)
+			}
+		} else {
+			next.ServeHTTP(w, r)
+		}
+	})
+}
+
+func getCache(path string, context context.Context) (string, int64, []byte) {
+	var mime string
+	var t int64
+	var cache []byte
+	allowedTime := time.Now().Unix() - appConfig.cache.expiration
+	row := appDb.QueryRowContext(context, "select COALESCE(mime, ''), COALESCE(time, 0), value from cache where path=? and time>=?", path, allowedTime)
+	_ = row.Scan(&mime, &t, &cache)
+	return mime, t, cache
+}
+
+func saveCache(path string, mime string, value []byte) {
+	now := time.Now().Unix()
+	startWritingToDb()
+	tx, err := appDb.Begin()
+	if err != nil {
+		return
+	}
+	_, _ = tx.Exec("delete from cache where time<?;", now-appConfig.cache.expiration)
+	_, _ = tx.Exec("insert or replace into cache (path, time, mime, value) values (?, ?, ?, ?);", path, now, mime, value)
+	_ = tx.Commit()
+	finishWritingToDb()
+}
diff --git a/config.go b/config.go
index 8de459f..0b20d29 100644
--- a/config.go
+++ b/config.go
@@ -8,6 +8,7 @@ import (
 type config struct {
 	server *configServer
 	db     *configDb
+	cache  *configCache
 }
 
 type configServer struct {
@@ -19,9 +20,15 @@ type configDb struct {
 	file string
 }
 
+type configCache struct {
+	enable     bool
+	expiration int64
+}
+
 var appConfig = &config{
 	server: &configServer{},
 	db:     &configDb{},
+	cache:  &configCache{},
 }
 
 func initConfig() error {
@@ -45,6 +52,15 @@ func initConfig() error {
 	viper.SetDefault(databaseFile, "data/db.sqlite")
 	appConfig.db.file = viper.GetString(databaseFile)
 	logConfig(databaseFile, appConfig.db.file)
+	// Caching
+	cacheEnable := "cache.enable"
+	viper.SetDefault(cacheEnable, true)
+	appConfig.cache.enable = viper.GetBool(cacheEnable)
+	logConfig(cacheEnable, appConfig.cache.enable)
+	cacheExpiration := "cache.expiration"
+	viper.SetDefault(cacheExpiration, 600)
+	appConfig.cache.expiration = viper.GetInt64(cacheExpiration)
+	logConfig(cacheExpiration, appConfig.cache.expiration)
 	return nil
 }
 
diff --git a/database_migrations.go b/database_migrations.go
index 8e8ba57..e1640ab 100644
--- a/database_migrations.go
+++ b/database_migrations.go
@@ -31,6 +31,13 @@ func migrateDb() error {
 					return err
 				},
 			},
+			&migrator.Migration{
+				Name: "00004",
+				Func: func(tx *sql.Tx) error {
+					_, err := tx.Exec("create table cache (path text not null primary key, time integer, mime text, value blob);")
+					return err
+				},
+			},
 		),
 	)
 	if err != nil {
diff --git a/http.go b/http.go
index 48354ce..ec4e219 100644
--- a/http.go
+++ b/http.go
@@ -63,7 +63,7 @@ func buildHandler() (http.Handler, error) {
 	} else {
 		for _, path := range allPostPaths {
 			if path != "" {
-				r.Get(path, servePost)
+				r.With(CacheMiddleware).Get(path, servePost)
 			}
 		}
 	}
@@ -109,4 +109,4 @@ func SlashTrimmedPath(r *http.Request) string {
 		path = strings.TrimSuffix(path, "/")
 	}
 	return path
-}
\ No newline at end of file
+}
diff --git a/posts.go b/posts.go
index fcc48ec..61aac34 100644
--- a/posts.go
+++ b/posts.go
@@ -18,7 +18,8 @@ type post struct {
 }
 
 func servePost(w http.ResponseWriter, r *http.Request) {
-	post, err := getPost(SlashTrimmedPath(r), r.Context())
+	path := SlashTrimmedPath(r)
+	post, err := getPost(path, r.Context())
 	if err == postNotFound {
 		http.NotFound(w, r)
 		return
@@ -31,7 +32,11 @@ func servePost(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 		return
 	}
-	w.Header().Set("Content-Type", "text/html")
+	mime := "text/html"
+	if appConfig.cache.enable {
+		saveCache(path, mime, htmlContent)
+	}
+	w.Header().Set("Content-Type", mime)
 	_, _ = w.Write(htmlContent)
 }