Add AroundRoundTrip option, unexport CachedResponse

httpcache.go

@@ -45,9 +45,9 @@ func cacheKey(req *http.Request) string {
 	}
 }
 
-// CachedResponse returns the cached http.Response for req if present, and nil
+// cachedResponse returns the cached http.Response for req if present, and nil
 // otherwise.
-func CachedResponse(c Cache, req *http.Request) (resp *http.Response, err error) {
+func cachedResponse(c Cache, req *http.Request) (resp *http.Response, err error) {
 	cachedVal, ok := c.Get(cacheKey(req))
 	if !ok {
 		return
@@ -98,9 +98,18 @@ type Transport struct {
 	// The RoundTripper interface actually used to make requests
 	// If nil, http.DefaultTransport is used
 	Transport http.RoundTripper
-	Cache     Cache
+
+	// The Cache interface used to store and retrieve responses.
+	Cache Cache
+
 	// If true, responses returned from the cache will be given an extra header, X-From-Cache
 	MarkCachedResponses bool
+
+	// AroundRoundTrip is an optional func.
+	// If set, the Transport will call AroundRoundTrip at the start of RoundTrip
+	// and defer the returned func until the end of RoundTrip.
+	// Typically used to implement a lock that is held for the duration of the RoundTrip.
+	AroundRoundTrip func(key string) func()
 }
 
 // NewTransport returns a new Transport with the
@@ -136,10 +145,13 @@ func varyMatches(cachedResp *http.Response, req *http.Request) bool {
 // will be returned.
 func (t *Transport) RoundTrip(req *http.Request) (resp *http.Response, err error) {
 	cacheKey := cacheKey(req)
+	if n := t.AroundRoundTrip; n != nil {
+		defer n(cacheKey)()
+	}
 	cacheable := (req.Method == "GET" || req.Method == "HEAD") && req.Header.Get("range") == ""
 	var cachedResp *http.Response
 	if cacheable {
-		cachedResp, err = CachedResponse(t.Cache, req)
+		cachedResp, err = cachedResponse(t.Cache, req)
 	} else {
 		// Need to invalidate an existing value
 		t.Cache.Delete(cacheKey)