reverse_proxy: flush HTTP/2 response when ContentLength is unknown (#3561)

* reverse proxy: Support more h2 stream scenarios (#3556)

* reverse proxy: add integration test for better h2 stream (#3556)

* reverse proxy: adjust comments as francislavoie suggests

* link to issue #3556 in the comments

3 files changed, 214 insertions, 0 deletions

diff --git a/caddytest/integration/stream_test.go b/caddytest/integration/stream_test.go
new file mode 100644
index 0000000..c0ab32b
--- /dev/null
+++ b/caddytest/integration/stream_test.go
@@ -0,0 +1,201 @@
+package integration
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/caddyserver/caddy/v2/caddytest"
+	"golang.org/x/net/http2"
+	"golang.org/x/net/http2/h2c"
+)
+
+// (see https://github.com/caddyserver/caddy/issues/3556 for use case)
+func TestH2ToH2CStream(t *testing.T) {
+	tester := caddytest.NewTester(t)
+	tester.InitServer(` 
+  {
+    "apps": {
+      "http": {
+        "http_port": 9080,
+        "https_port": 9443,
+        "servers": {
+          "srv0": {
+            "listen": [
+              ":9443"
+            ],
+            "routes": [
+              {
+                "handle": [
+                  {
+                    "handler": "reverse_proxy",
+                    "transport": {
+                      "protocol": "http",
+                      "compression": false,
+                      "versions": [
+                        "h2c",
+                        "2"
+                      ]
+                    },
+                    "upstreams": [
+                      {
+                        "dial": "localhost:54321"
+                      }
+                    ]
+                  }
+                ],
+                "match": [
+                  {
+                    "path": [
+                      "/tov2ray"
+                    ]
+                  }
+                ]
+              }
+            ],
+            "tls_connection_policies": [
+              {
+                "certificate_selection": {
+                  "any_tag": ["cert0"]
+                },
+                "default_sni": "a.caddy.localhost"
+              }
+            ]
+          }
+        }
+      },
+      "tls": {
+        "certificates": {
+          "load_files": [
+            {
+              "certificate": "/a.caddy.localhost.crt",
+              "key": "/a.caddy.localhost.key",
+              "tags": [
+                "cert0"
+              ]
+            }
+          ]
+        }
+      },
+      "pki": {
+        "certificate_authorities" : {
+          "local" : {
+            "install_trust": false
+          }
+        }
+      }
+    }
+  }
+  `, "json")
+
+	expectedBody := "some data to be echoed"
+	// start the server
+	server := testH2ToH2CStreamServeH2C(t)
+	go server.ListenAndServe()
+	defer func() {
+		ctx, cancel := context.WithTimeout(context.Background(), time.Nanosecond)
+		defer cancel()
+		server.Shutdown(ctx)
+	}()
+
+	r, w := io.Pipe()
+	req := &http.Request{
+		Method: "PUT",
+		Body:   ioutil.NopCloser(r),
+		URL: &url.URL{
+			Scheme: "https",
+			Host:   "127.0.0.1:9443",
+			Path:   "/tov2ray",
+		},
+		Proto:      "HTTP/2",
+		ProtoMajor: 2,
+		ProtoMinor: 0,
+		Header:     make(http.Header),
+	}
+	// Disable any compression method from server.
+	req.Header.Set("Accept-Encoding", "identity")
+
+	resp := tester.AssertResponseCode(req, 200)
+	if 200 != resp.StatusCode {
+		return
+	}
+	go func() {
+		fmt.Fprint(w, expectedBody)
+		w.Close()
+	}()
+
+	defer resp.Body.Close()
+	bytes, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		t.Fatalf("unable to read the response body %s", err)
+	}
+
+	body := string(bytes)
+
+	if !strings.Contains(body, expectedBody) {
+		t.Errorf("requesting \"%s\" expected response body \"%s\" but got \"%s\"", req.RequestURI, expectedBody, body)
+	}
+	return
+}
+
+func testH2ToH2CStreamServeH2C(t *testing.T) *http.Server {
+	h2s := &http2.Server{}
+	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		rstring, err := httputil.DumpRequest(r, false)
+		if err == nil {
+			t.Logf("h2c server received req: %s", rstring)
+		}
+		// We only accept HTTP/2!
+		if r.ProtoMajor != 2 {
+			t.Error("Not a HTTP/2 request, rejected!")
+			w.WriteHeader(http.StatusInternalServerError)
+			return
+		}
+
+		if r.Host != "127.0.0.1:9443" {
+			t.Errorf("r.Host doesn't match, %v!", r.Host)
+			w.WriteHeader(http.StatusNotFound)
+			return
+		}
+
+		if !strings.HasPrefix(r.URL.Path, "/tov2ray") {
+			w.WriteHeader(http.StatusNotFound)
+			return
+		}
+
+		w.Header().Set("Cache-Control", "no-store")
+		w.WriteHeader(200)
+		if f, ok := w.(http.Flusher); ok {
+			f.Flush()
+		}
+
+		buf := make([]byte, 4*1024)
+
+		for {
+			n, err := r.Body.Read(buf)
+			if n > 0 {
+				w.Write(buf[:n])
+			}
+
+			if err != nil {
+				if err == io.EOF {
+					r.Body.Close()
+				}
+				break
+			}
+		}
+	})
+
+	server := &http.Server{
+		Addr:    "127.0.0.1:54321",
+		Handler: h2c.NewHandler(handler, h2s),
+	}
+	return server
+}
diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index bb1453a..0a53db4 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -611,6 +611,14 @@ func (h *Handler) reverseProxy(rw http.ResponseWriter, req *http.Request, di Dia
 
 	rw.WriteHeader(res.StatusCode)
 
+	// some apps need the response headers before starting to stream content with http2,
+	// so it's important to explicitly flush the headers to the client before streaming the data.
+	// (see https://github.com/caddyserver/caddy/issues/3556 for use case)
+	if req.ProtoMajor == 2 && res.ContentLength == -1 {
+		if wf, ok := rw.(http.Flusher); ok {
+			wf.Flush()
+		}
+	}
 	err = h.copyResponse(rw, res.Body, h.flushInterval(req, res))
 	res.Body.Close() // close now, instead of defer, to populate res.Trailer
 	if err != nil {
diff --git a/modules/caddyhttp/reverseproxy/streaming.go b/modules/caddyhttp/reverseproxy/streaming.go
index 8a7c6f7..105ff32 100644
--- a/modules/caddyhttp/reverseproxy/streaming.go
+++ b/modules/caddyhttp/reverseproxy/streaming.go
@@ -96,6 +96,11 @@ func (h Handler) flushInterval(req *http.Request, res *http.Response) time.Durat
 		return -1 // negative means immediately
 	}
 
+	// for h2 and h2c upstream streaming data to client (issue #3556)
+	if req.ProtoMajor == 2 && res.ContentLength == -1 {
+		return -1
+	}
+
 	// TODO: more specific cases? e.g. res.ContentLength == -1? (this TODO is from the std lib)
 	return time.Duration(h.FlushInterval)
 }