agent/http: refactor proxy for simpler error handling

pkg/agent/protocol/http/proxy.go

@@ -74,16 +74,6 @@ func NewProxy(c ProxyConfig, d Disruption) (protocol.Proxy, error) {
 	}, nil
 }
 
-// contains verifies if a list of strings contains the given string
-func contains(list []string, target string) bool {
-	for _, element := range list {
-		if element == target {
-			return true
-		}
-	}
-	return false
-}
-
 // httpClient defines the method for executing HTTP requests. It is used to allow mocking
 // the client in tests
 type httpClient interface {
@@ -97,56 +87,85 @@ type httpHandler struct {
 	client      httpClient
 }
 
-func (h *httpHandler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
-	var statusCode int
-	headers := http.Header{}
-	body := io.NopCloser(strings.NewReader(h.disruption.ErrorBody))
-
-	excluded := contains(h.disruption.Excluded, req.URL.Path)
-
-	if !excluded && h.disruption.ErrorRate > 0 && rand.Float32() <= h.disruption.ErrorRate {
-		// force error code
-		statusCode = int(h.disruption.ErrorCode)
-	} else {
-		req.Host = h.upstreamURL.Host
-		req.URL.Host = h.upstreamURL.Host
-		req.URL.Scheme = h.upstreamURL.Scheme
-		req.RequestURI = ""
-		originServerResponse, srvErr := h.client.Do(req)
-		if srvErr != nil {
-			rw.WriteHeader(http.StatusInternalServerError)
-			_, _ = fmt.Fprint(rw, srvErr)
-			return
+// isExcluded checks whether a request should be proxied through without any kind of modification whatsoever.
+func (h *httpHandler) isExcluded(r *http.Request) bool {
+	for _, excluded := range h.disruption.Excluded {
+		if strings.EqualFold(r.URL.Path, excluded) {
+			return true
 		}
+	}
 
-		headers = originServerResponse.Header
-		statusCode = originServerResponse.StatusCode
-		body = originServerResponse.Body
+	return false
+}
 
-		defer func() {
-			_ = originServerResponse.Body.Close()
-		}()
-	}
+// proxy proxies a request to the upstream URL.
+// Request is performed immediately, but response won't be copied to the client until wait has been consumed.
+func (h *httpHandler) proxy(rw http.ResponseWriter, req *http.Request, wait <-chan time.Time) {
+	upstreamReq := req.Clone(context.Background())
+	upstreamReq.Host = h.upstreamURL.Host
+	upstreamReq.URL.Host = h.upstreamURL.Host
+	upstreamReq.URL.Scheme = h.upstreamURL.Scheme
+	upstreamReq.RequestURI = "" // It is an error to set this field in an HTTP client request.
 
-	if !excluded && h.disruption.AverageDelay > 0 {
-		delay := int64(h.disruption.AverageDelay)
-		if h.disruption.DelayVariation > 0 {
-			variation := int64(h.disruption.DelayVariation)
-			delay = delay + variation - 2*rand.Int63n(variation)
-		}
-		time.Sleep(time.Duration(delay))
+	response, err := h.client.Do(req)
+	<-wait
+
+	if err != nil {
+		rw.WriteHeader(http.StatusBadGateway)
+		_, _ = fmt.Fprint(rw, err)
+		return
 	}
 
-	// return response to the client
-	for key, values := range headers {
+	defer func() {
+		// Fully consume and then close upstream response body.
+		_, _ = io.Copy(io.Discard, response.Body)
+		_ = response.Body.Close()
+	}()
+
+	// Mirror headers.
+	for key, values := range response.Header {
 		for _, value := range values {
 			rw.Header().Add(key, value)
 		}
 	}
-	rw.WriteHeader(statusCode)
+
+	// Mirror status code.
+	rw.WriteHeader(response.StatusCode)
 
 	// ignore errors writing body, nothing to do.
-	_, _ = io.Copy(rw, body)
+	_, _ = io.Copy(rw, response.Body)
+}
+
+// fault consumes wait and then writes to rw the configured error code and body.
+func (h *httpHandler) fault(rw http.ResponseWriter, wait <-chan time.Time) {
+	<-wait
+
+	rw.WriteHeader(int(h.disruption.ErrorCode))
+	_, _ = rw.Write([]byte(h.disruption.ErrorBody))
+}
+
+func (h *httpHandler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	if h.isExcluded(req) {
+		//nolint:contextcheck // Unclear which context the linter requires us to propagate here.
+		h.proxy(rw, req, time.After(0))
+		return
+	}
+
+	delay := h.disruption.AverageDelay
+	if h.disruption.DelayVariation > 0 {
+		variation := int64(h.disruption.DelayVariation)
+		delay += time.Duration(variation - 2*rand.Int63n(variation))
+	}
+
+	responseTimer := time.After(delay)
+
+	if h.disruption.ErrorRate > 0 && rand.Float32() <= h.disruption.ErrorRate {
+		h.fault(rw, responseTimer)
+		return
+	}
+
+	//nolint:contextcheck // Unclear which context the linter requires us to propagate here.
+	h.proxy(rw, req, responseTimer)
 }
 
 // Start starts the execution of the proxy