@@ -40,6 +40,12 @@ func (fs FileServer) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, err
 	return fs.serveFile(w, r, r.URL.Path)
 }
 
+// calculateEtag produces a strong etag by default. Prefix the result with "W/" to convert this into a weak one.
+// see https://tools.ietf.org/html/rfc7232#section-2.3
+func calculateEtag(d os.FileInfo) string {
+	return fmt.Sprintf(`"%x-%x"`, d.ModTime().Unix(), d.Size())
+}
+
 // serveFile writes the specified file to the HTTP response.
 // name is '/'-separated, not filepath.Separator.
 func (fs FileServer) serveFile(w http.ResponseWriter, r *http.Request, name string) (int, error) {
package staticfiles

import (
	"fmt"
	"math/rand"
	"net/http"
	"os"
	"path"
	"path/filepath"
	"runtime"
	"strconv"
	"strings"
)

// FileServer implements a production-ready file server
// and is the 'default' handler for all requests to Caddy.
// It simply loads and serves the URI requested. FileServer
// is adapted from the one in net/http by the Go authors.
// Significant modifications have been made.
//
// Original license:
//
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
type FileServer struct {
	// Jailed disk access
	Root http.FileSystem

	// List of files to treat as "Not Found"
	Hide []string
}

// ServeHTTP serves static files for r according to fs's configuration.
func (fs FileServer) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
	// r.URL.Path has already been cleaned by Caddy.
	if r.URL.Path == "" {
		r.URL.Path = "/"
	}
	return fs.serveFile(w, r, r.URL.Path)
}

// serveFile writes the specified file to the HTTP response.
// name is '/'-separated, not filepath.Separator.
func (fs FileServer) serveFile(w http.ResponseWriter, r *http.Request, name string) (int, error) {

	location := name

	// Prevent absolute path access on Windows.
	// TODO remove when stdlib http.Dir fixes this.
	if runtime.GOOS == "windows" {
		if filepath.IsAbs(name[1:]) {
			return http.StatusNotFound, nil
		}
	}

	f, err := fs.Root.Open(name)
	if err != nil {
		if os.IsNotExist(err) {
			return http.StatusNotFound, nil
		} else if os.IsPermission(err) {
			return http.StatusForbidden, err
		}
		// Likely the server is under load and ran out of file descriptors
		backoff := int(3 + rand.Int31()%3) // 3–5 seconds to prevent a stampede
		w.Header().Set("Retry-After", strconv.Itoa(backoff))
		return http.StatusServiceUnavailable, err
	}
	defer f.Close()

	d, err := f.Stat()
	if err != nil {
		if os.IsNotExist(err) {
			return http.StatusNotFound, nil
		} else if os.IsPermission(err) {
			return http.StatusForbidden, err
		}
		// Return a different status code than above so as to distinguish these cases
		return http.StatusInternalServerError, err
	}

	// redirect to canonical path
	url := r.URL.Path
	if d.IsDir() {
		// Ensure / at end of directory url
		if !strings.HasSuffix(url, "/") {
			Redirect(w, r, path.Base(url)+"/", http.StatusMovedPermanently)
			return http.StatusMovedPermanently, nil
		}
	} else {
		// Ensure no / at end of file url
		if strings.HasSuffix(url, "/") {
			Redirect(w, r, "../"+path.Base(url), http.StatusMovedPermanently)
			return http.StatusMovedPermanently, nil
		}
	}

	// use contents of an index file, if present, for directory
	if d.IsDir() {
		for _, indexPage := range IndexPages {
			index := strings.TrimSuffix(name, "/") + "/" + indexPage
			ff, err := fs.Root.Open(index)
			if err != nil {
				continue
			}

			// this defer does not leak fds because previous iterations
			// of the loop must have had an err, so nothing to close
			defer ff.Close()

			dd, err := ff.Stat()
			if err != nil {
				ff.Close()
				continue
			}

			// Close previous file - release fd immediately
			f.Close()

			d = dd
			f = ff
			location = index
			break
		}
	}

	// Still a directory? (we didn't find an index file)
	// Return 404 to hide the fact that the folder exists
	if d.IsDir() {
		return http.StatusNotFound, nil
	}

	if fs.IsHidden(d) {
		return http.StatusNotFound, nil
	}

	filename := d.Name()

	for _, encoding := range staticEncodingPriority {
		if !strings.Contains(r.Header.Get("Accept-Encoding"), encoding) {
			continue
		}

		encodedFile, err := fs.Root.Open(location + staticEncoding[encoding])
		if err != nil {
			continue
		}

		encodedFileInfo, err := encodedFile.Stat()
		if err != nil {
			encodedFile.Close()
			continue
		}

		// Close previous file - release fd
		f.Close()

		// Stat is needed for generating valid ETag
		d = encodedFileInfo

		// Encoded file will be served
		f = encodedFile

		w.Header().Add("Vary", "Accept-Encoding")
		w.Header().Set("Content-Encoding", encoding)

		defer f.Close()
		break

	}

	// Experimental ETag header
	e := fmt.Sprintf(`W/"%x-%x"`, d.ModTime().Unix(), d.Size())
	w.Header().Set("ETag", e)

	// Note: Errors generated by ServeContent are written immediately
	// to the response. This usually only happens if seeking fails (rare).
	http.ServeContent(w, r, filename, d.ModTime(), f)

	return http.StatusOK, nil
}

// IsHidden checks if file with FileInfo d is on hide list.
func (fs FileServer) IsHidden(d os.FileInfo) bool {
	// If the file is supposed to be hidden, return a 404
	for _, hiddenPath := range fs.Hide {
		// Check if the served file is exactly the hidden file.
		if hFile, err := fs.Root.Open(hiddenPath); err == nil {
			fs, _ := hFile.Stat()
			hFile.Close()
			if os.SameFile(d, fs) {
				return true
			}
		}
	}
	return false
}

// Redirect sends an HTTP redirect to the client but will preserve
// the query string for the new path. Based on http.localRedirect
// from the Go standard library.
func Redirect(w http.ResponseWriter, r *http.Request, newPath string, statusCode int) {
	if q := r.URL.RawQuery; q != "" {
		newPath += "?" + q
	}
	http.Redirect(w, r, newPath, statusCode)
}

// IndexPages is a list of pages that may be understood as
// the "index" files to directories.
var IndexPages = []string{
	"index.html",
	"index.htm",
	"index.txt",
	"default.html",
	"default.htm",
	"default.txt",
}

// staticEncoding is a map of content-encoding to a file extension.
// If client accepts given encoding (via Accept-Encoding header) and compressed file with given extensions exists
// it will be served to the client instead of original one.
var staticEncoding = map[string]string{
	"gzip": ".gz",
	"br":   ".br",
}

// staticEncodingPriority is a list of preferred static encodings (most efficient compression to least one).
var staticEncodingPriority = []string{
	"br",
	"gzip",
}

This is a strong ETag, but we don't use the actual bytes of the file to produce it. Will that be a problem? (FWIW, I like not reading over the whole file for efficiency purposes.)