proxy: Move handling of headers around to prevent memory usage spikes (#784)

* Move handling of headers around to prevent memory use spikes

While debugging #782, I noticed that using http2 and max_fails=0,
X-Forwarded-For grew infinitely when an upstream request failed after
refreshing the test page. This change ensures that headers are only
set once per request rather than appending in a time-terminated loop.

* Refactor some code into its own function

middleware/proxy/proxy.go

@@ -3,8 +3,10 @@ package proxy
 
 import (
 	"errors"
+	"net"
 	"net/http"
 	"net/url"
+	"strings"
 	"sync/atomic"
 	"time"
 
@@ -75,10 +77,15 @@ var tryDuration = 60 * time.Second
 // ServeHTTP satisfies the middleware.Handler interface.
 func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 	for _, upstream := range p.Upstreams {
-		if middleware.Path(r.URL.Path).Matches(upstream.From()) && upstream.AllowedPath(r.URL.Path) {
+		if !middleware.Path(r.URL.Path).Matches(upstream.From()) ||
+			!upstream.AllowedPath(r.URL.Path) {
+			continue
+		}
+
 		var replacer middleware.Replacer
 		start := time.Now()
-			requestHost := r.Host
+
+		outreq := createUpstreamRequest(r)
 
 		// Since Select() should give us "up" hosts, keep retrying
 		// hosts until timeout (or until we get a nil host).
@@ -87,42 +94,43 @@ func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 			if host == nil {
 				return http.StatusBadGateway, errUnreachable
 			}
-				proxy := host.ReverseProxy
-				r.Host = host.Name
 			if rr, ok := w.(*middleware.ResponseRecorder); ok && rr.Replacer != nil {
 				rr.Replacer.Set("upstream", host.Name)
 			}
 
-				if baseURL, err := url.Parse(host.Name); err == nil {
-					r.Host = baseURL.Host
-					if proxy == nil {
-						proxy = NewSingleHostReverseProxy(baseURL, host.WithoutPathPrefix)
-					}
-				} else if proxy == nil {
-					return http.StatusInternalServerError, err
-				}
-				var extraHeaders http.Header
+			outreq.Host = host.Name
 			if host.ExtraHeaders != nil {
-					extraHeaders = make(http.Header)
+				extraHeaders := make(http.Header)
 				if replacer == nil {
 					rHost := r.Host
-						r.Host = requestHost
 					replacer = middleware.NewReplacer(r, nil, "")
-						r.Host = rHost
+					outreq.Host = rHost
 				}
 				for header, values := range host.ExtraHeaders {
 					for _, value := range values {
-							extraHeaders.Add(header,
-								replacer.Replace(value))
+						extraHeaders.Add(header, replacer.Replace(value))
 						if header == "Host" {
-								r.Host = replacer.Replace(value)
+							outreq.Host = replacer.Replace(value)
 						}
 					}
 				}
+				for k, v := range extraHeaders {
+					outreq.Header[k] = v
+				}
+			}
+
+			proxy := host.ReverseProxy
+			if baseURL, err := url.Parse(host.Name); err == nil {
+				r.Host = baseURL.Host
+				if proxy == nil {
+					proxy = NewSingleHostReverseProxy(baseURL, host.WithoutPathPrefix)
+				}
+			} else if proxy == nil {
+				return http.StatusInternalServerError, err
 			}
 
 			atomic.AddInt64(&host.Conns, 1)
-				backendErr := proxy.ServeHTTP(w, r, extraHeaders)
+			backendErr := proxy.ServeHTTP(w, outreq)
 			atomic.AddInt64(&host.Conns, -1)
 			if backendErr == nil {
 				return 0, nil
@@ -139,7 +147,38 @@ func (p Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 		}
 		return http.StatusBadGateway, errUnreachable
 	}
-	}
 
 	return p.Next.ServeHTTP(w, r)
 }
+
+// createUpstremRequest shallow-copies r into a new request
+// that can be sent upstream.
+func createUpstreamRequest(r *http.Request) *http.Request {
+	outreq := new(http.Request)
+	*outreq = *r // includes shallow copies of maps, but okay
+
+	// Remove hop-by-hop headers to the backend.  Especially
+	// important is "Connection" because we want a persistent
+	// connection, regardless of what the client sent to us.  This
+	// is modifying the same underlying map from r (shallow
+	// copied above) so we only copy it if necessary.
+	for _, h := range hopHeaders {
+		if outreq.Header.Get(h) != "" {
+			outreq.Header = make(http.Header)
+			copyHeader(outreq.Header, r.Header)
+			outreq.Header.Del(h)
+		}
+	}
+
+	if clientIP, _, err := net.SplitHostPort(r.RemoteAddr); err == nil {
+		// If we aren't the first proxy, retain prior
+		// X-Forwarded-For information as a comma+space
+		// separated list and fold multiple headers into one.
+		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
+			clientIP = strings.Join(prior, ", ") + ", " + clientIP
+		}
+		outreq.Header.Set("X-Forwarded-For", clientIP)
+	}
+
+	return outreq
+}

middleware/proxy/reverseproxy.go

@@ -154,54 +154,18 @@ var InsecureTransport http.RoundTripper = &http.Transport{
 	TLSClientConfig:     &tls.Config{InsecureSkipVerify: true},
 }
 
-func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request, extraHeaders http.Header) error {
+func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, outreq *http.Request) error {
 	transport := p.Transport
 	if transport == nil {
 		transport = http.DefaultTransport
 	}
 
-	outreq := new(http.Request)
-	*outreq = *req // includes shallow copies of maps, but okay
-
 	p.Director(outreq)
 	outreq.Proto = "HTTP/1.1"
 	outreq.ProtoMajor = 1
 	outreq.ProtoMinor = 1
 	outreq.Close = false
 
-	// Remove hop-by-hop headers to the backend.  Especially
-	// important is "Connection" because we want a persistent
-	// connection, regardless of what the client sent to us.  This
-	// is modifying the same underlying map from req (shallow
-	// copied above) so we only copy it if necessary.
-	copiedHeaders := false
-	for _, h := range hopHeaders {
-		if outreq.Header.Get(h) != "" {
-			if !copiedHeaders {
-				outreq.Header = make(http.Header)
-				copyHeader(outreq.Header, req.Header)
-				copiedHeaders = true
-			}
-			outreq.Header.Del(h)
-		}
-	}
-
-	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
-		// If we aren't the first proxy retain prior
-		// X-Forwarded-For information as a comma+space
-		// separated list and fold multiple headers into one.
-		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
-			clientIP = strings.Join(prior, ", ") + ", " + clientIP
-		}
-		outreq.Header.Set("X-Forwarded-For", clientIP)
-	}
-
-	if extraHeaders != nil {
-		for k, v := range extraHeaders {
-			outreq.Header[k] = v
-		}
-	}
-
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
 		return err
@@ -237,9 +201,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request, extr
 		for _, h := range hopHeaders {
 			res.Header.Del(h)
 		}
-
 		copyHeader(rw.Header(), res.Header)
-
 		rw.WriteHeader(res.StatusCode)
 		p.copyResponse(rw, res.Body)
 	}
@@ -260,7 +222,6 @@ func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
 			dst = mlw
 		}
 	}
-
 	io.Copy(dst, src)
 }