Do not read response body for HEAD requests

Co-authored-by: Romain <rtribotte@users.noreply.github.com>

pkg/proxy/fast/connpool.go

@@ -20,6 +20,7 @@ import (
 // rwWithUpgrade contains a ResponseWriter and an upgradeHandler,
 // used to upgrade the connection (e.g. Websockets).
 type rwWithUpgrade struct {
+	ReqMethod string
 	RW        http.ResponseWriter
 	Upgrade   upgradeHandler
 }
@@ -211,6 +212,10 @@ func (c *conn) handleResponse(r rwWithUpgrade) error {
 
 	r.RW.WriteHeader(res.StatusCode())
 
+	if noResponseBodyExpected(r.ReqMethod) {
+		return nil
+	}
+
 	if res.Header.ContentLength() == 0 {
 		return nil
 	}
@@ -444,8 +449,8 @@ func (c *connPool) askForNewConn(errCh chan<- error) {
 	c.releaseConn(newConn)
 }
 
-// isBodyAllowedForStatus reports whether a given response status code
+// isBodyAllowedForStatus reports whether a given response status code permits a body.
-// permits a body. See RFC 7230, section 3.3.
+// See RFC 7230, section 3.3.
 // From https://github.com/golang/go/blame/master/src/net/http/transfer.go#L459
 func isBodyAllowedForStatus(status int) bool {
 	switch {
@@ -458,3 +463,9 @@ func isBodyAllowedForStatus(status int) bool {
 	}
 	return true
 }
+
+// noResponseBodyExpected reports whether a given request method permits a body.
+// From https://github.com/golang/go/blame/master/src/net/http/transfer.go#L250
+func noResponseBodyExpected(requestMethod string) bool {
+	return requestMethod == "HEAD"
+}

pkg/proxy/fast/proxy.go

@@ -284,6 +284,7 @@ func (p *ReverseProxy) roundTrip(rw http.ResponseWriter, req *http.Request, outR
 
 	// Sending the responseWriter unlocks the connection readLoop, to handle the response.
 	co.RWCh <- rwWithUpgrade{
+		ReqMethod: req.Method,
 		RW:        rw,
 		Upgrade:   upgradeResponseHandler(req.Context(), reqUpType),
 	}

pkg/proxy/fast/proxy_test.go

@@ -278,6 +278,34 @@ func TestPreservePath(t *testing.T) {
 	assert.Equal(t, http.StatusOK, res.Code)
 }
 
+func TestHeadRequest(t *testing.T) {
+	var callCount int
+	server := httptest.NewServer(http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		callCount++
+
+		assert.Equal(t, http.MethodHead, req.Method)
+
+		rw.Header().Set("Content-Length", "42")
+	}))
+	t.Cleanup(server.Close)
+
+	builder := NewProxyBuilder(&transportManagerMock{}, static.FastProxyConfig{})
+
+	serverURL, err := url.JoinPath(server.URL)
+	require.NoError(t, err)
+
+	proxyHandler, err := builder.Build("", testhelpers.MustParseURL(serverURL), true, true)
+	require.NoError(t, err)
+
+	req := httptest.NewRequest(http.MethodHead, "/", http.NoBody)
+	res := httptest.NewRecorder()
+
+	proxyHandler.ServeHTTP(res, req)
+
+	assert.Equal(t, 1, callCount)
+	assert.Equal(t, http.StatusOK, res.Code)
+}
+
 func newCertificate(t *testing.T, domain string) *tls.Certificate {
 	t.Helper()