1 files changed, 95 insertions, 0 deletions

diff --git a/httputil/buster.go b/httputil/buster.go
new file mode 100644
index 0000000..beab151
--- /dev/null
+++ b/httputil/buster.go
@@ -0,0 +1,95 @@
+// Copyright 2013 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 or at
+// https://developers.google.com/open-source/licenses/bsd.
+
+package httputil
+
+import (
+        "io"
+        "io/ioutil"
+        "net/http"
+        "net/url"
+        "strings"
+        "sync"
+)
+
+type busterWriter struct {
+        headerMap http.Header
+        status    int
+        io.Writer
+}
+
+func (bw *busterWriter) Header() http.Header {
+        return bw.headerMap
+}
+
+func (bw *busterWriter) WriteHeader(status int) {
+        bw.status = status
+}
+
+// CacheBusters maintains a cache of cache busting tokens for static resources served by Handler.
+type CacheBusters struct {
+        Handler http.Handler
+
+        mu     sync.Mutex
+        tokens map[string]string
+}
+
+func sanitizeTokenRune(r rune) rune {
+        if r <= ' ' || r >= 127 {
+                return -1
+        }
+        // Convert percent encoding reserved characters to '-'.
+        if strings.ContainsRune("!#$&'()*+,/:;=?@[]", r) {
+                return '-'
+        }
+        return r
+}
+
+// Get returns the cache busting token for path. If the token is not already
+// cached, Get issues a HEAD request on handler and uses the response ETag and
+// Last-Modified headers to compute a token.
+func (cb *CacheBusters) Get(path string) string {
+        cb.mu.Lock()
+        if cb.tokens == nil {
+                cb.tokens = make(map[string]string)
+        }
+        token, ok := cb.tokens[path]
+        cb.mu.Unlock()
+        if ok {
+                return token
+        }
+
+        w := busterWriter{
+                Writer:    ioutil.Discard,
+                headerMap: make(http.Header),
+        }
+        r := &http.Request{URL: &url.URL{Path: path}, Method: "HEAD"}
+        cb.Handler.ServeHTTP(&w, r)
+
+        if w.status == 200 {
+                token = w.headerMap.Get("Etag")
+                if token == "" {
+                        token = w.headerMap.Get("Last-Modified")
+                }
+                token = strings.Trim(token, `" `)
+                token = strings.Map(sanitizeTokenRune, token)
+        }
+
+        cb.mu.Lock()
+        cb.tokens[path] = token
+        cb.mu.Unlock()
+
+        return token
+}
+
+// AppendQueryParam appends the token as a query parameter to path.
+func (cb *CacheBusters) AppendQueryParam(path string, name string) string {
+        token := cb.Get(path)
+        if token == "" {
+                return path
+        }
+        return path + "?" + name + "=" + token
+}