[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

internal/web/assetscache.go

@@ -0,0 +1,138 @@
+/*
+   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 (
+	// nolint:gosec
+	"crypto/sha1"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"net/http"
+	"path"
+	"strings"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/sirupsen/logrus"
+)
+
+type eTagCacheEntry struct {
+	eTag             string
+	fileLastModified time.Time
+}
+
+// generateEtag generates a strong (byte-for-byte) etag using
+// the entirety of the provided reader.
+func generateEtag(r io.Reader) (string, error) {
+	// nolint:gosec
+	hash := sha1.New()
+
+	if _, err := io.Copy(hash, r); err != nil {
+		return "", err
+	}
+
+	b := make([]byte, 0, sha1.Size)
+	b = hash.Sum(b)
+
+	return `"` + hex.EncodeToString(b) + `"`, nil
+}
+
+// getAssetFileInfo tries to fetch the ETag for the given filePath from the module's
+// assetsETagCache. If it can't be found there, it uses the provided http.FileSystem
+// to generate a new ETag to go in the cache, which it then returns.
+func (m *Module) getAssetETag(filePath string, fs http.FileSystem) (string, error) {
+	file, err := fs.Open(filePath)
+	if err != nil {
+		return "", fmt.Errorf("error opening %s: %s", filePath, err)
+	}
+	defer file.Close()
+
+	fileInfo, err := file.Stat()
+	if err != nil {
+		return "", fmt.Errorf("error statting %s: %s", filePath, err)
+	}
+
+	fileLastModified := fileInfo.ModTime()
+
+	if cachedETag, ok := m.assetsETagCache.Get(filePath); ok && !fileLastModified.After(cachedETag.fileLastModified) {
+		// only return our cached etag if the file wasn't
+		// modified since last time, otherwise generate a
+		// new one; eat fresh!
+		return cachedETag.eTag, nil
+	}
+
+	eTag, err := generateEtag(file)
+	if err != nil {
+		return "", fmt.Errorf("error generating etag: %s", err)
+	}
+
+	// put new entry in cache before we return
+	m.assetsETagCache.Set(filePath, eTagCacheEntry{
+		eTag:             eTag,
+		fileLastModified: fileLastModified,
+	})
+
+	return eTag, nil
+}
+
+// cacheControlMiddleware implements Cache-Control header setting, and checks for
+// files inside the given http.FileSystem.
+//
+// The middleware checks if the file has been modified using If-None-Match etag,
+// if present. If the file hasn't been modified, the middleware returns 304.
+//
+// See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match
+// and: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
+func (m *Module) cacheControlMiddleware(fs http.FileSystem) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		// no-cache prevents clients using default caching or heuristic caching,
+		// and also ensures that clients will validate their cached version against
+		// the version stored on the server to keep up to date.
+		c.Header("Cache-Control", "no-cache")
+
+		ifNoneMatch := c.Request.Header.Get("If-None-Match")
+
+		// derive the path of the requested asset inside the provided filesystem
+		upath := c.Request.URL.Path
+		if !strings.HasPrefix(upath, "/") {
+			upath = "/" + upath
+		}
+		assetFilePath := strings.TrimPrefix(path.Clean(upath), assetsPath)
+
+		// either fetch etag from ttlcache or generate it
+		eTag, err := m.getAssetETag(assetFilePath, fs)
+		if err != nil {
+			logrus.Errorf("error getting ETag for %s: %s", assetFilePath, err)
+			return
+		}
+
+		// Regardless of what happens further down, set the etag header
+		// so that the client has the up-to-date version.
+		c.Header("Etag", eTag)
+
+		// If client already has latest version of the asset, 304 + bail.
+		if ifNoneMatch == eTag {
+			c.AbortWithStatus(http.StatusNotModified)
+			return
+		}
+
+		// else let the rest of the request be processed normally
+	}
+}