Prevents superfluous WriteHeader call in the error middleware

Co-authored-by: LandryBe <lbenguigui@gmail.com>

integration/error_pages_test.go

@@ -2,6 +2,7 @@ package integration
 
 import (
 	"net/http"
+	"net/http/httptest"
 	"os"
 	"time"
 
@@ -29,7 +30,7 @@ func (s *ErrorPagesSuite) TestSimpleConfiguration(c *check.C) {
 	file := s.adaptFile(c, "fixtures/error_pages/simple.toml", struct {
 		Server1 string
 		Server2 string
-	}{s.BackendIP, s.ErrorPageIP})
+	}{"http://" + s.BackendIP + ":80", s.ErrorPageIP})
 	defer os.Remove(file)
 
 	cmd, display := s.traefikCmd(withConfigFile(file))
@@ -67,3 +68,33 @@ func (s *ErrorPagesSuite) TestErrorPage(c *check.C) {
 	err = try.Request(frontendReq, 2*time.Second, try.BodyContains("An error occurred."))
 	c.Assert(err, checker.IsNil)
 }
+
+func (s *ErrorPagesSuite) TestErrorPageFlush(c *check.C) {
+	srv := httptest.NewServer(http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
+		rw.Header().Add("Transfer-Encoding", "chunked")
+		rw.WriteHeader(http.StatusInternalServerError)
+		_, _ = rw.Write([]byte("KO"))
+	}))
+
+	file := s.adaptFile(c, "fixtures/error_pages/simple.toml", struct {
+		Server1 string
+		Server2 string
+	}{srv.URL, s.ErrorPageIP})
+	defer os.Remove(file)
+
+	cmd, display := s.traefikCmd(withConfigFile(file))
+	defer display(c)
+	err := cmd.Start()
+	c.Assert(err, checker.IsNil)
+	defer s.killCmd(cmd)
+
+	frontendReq, err := http.NewRequest(http.MethodGet, "http://127.0.0.1:8080", nil)
+	c.Assert(err, checker.IsNil)
+	frontendReq.Host = "test.local"
+
+	err = try.Request(frontendReq, 2*time.Second,
+		try.BodyContains("An error occurred."),
+		try.HasHeaderValue("Content-Type", "text/html", true),
+	)
+	c.Assert(err, checker.IsNil)
+}