reverse_proxy: fix bidirectional streams with encodings (fix #3606) (#3620)

* reverse_proxy: fix bi-h2stream breaking gzip encode handle(#3606).

* reverse_proxy: check http version of both sides to avoid affecting non-h2 upstream.

* Minor cleanup; apply review suggestions

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

2 files changed, 23 insertions, 5 deletions

diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index 0a53db4..7fdf55a 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -613,8 +613,8 @@ func (h *Handler) reverseProxy(rw http.ResponseWriter, req *http.Request, di Dia
 
 	// 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 {
+	// (see https://github.com/caddyserver/caddy/issues/3556 for use case and nuances)
+	if h.isBidirectionalStream(req, res) {
 		if wf, ok := rw.(http.Flusher); ok {
 			wf.Flush()
 		}
diff --git a/modules/caddyhttp/reverseproxy/streaming.go b/modules/caddyhttp/reverseproxy/streaming.go
index 105ff32..127c0f0 100644
--- a/modules/caddyhttp/reverseproxy/streaming.go
+++ b/modules/caddyhttp/reverseproxy/streaming.go
@@ -96,15 +96,33 @@ 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 {
+	// for h2 and h2c upstream streaming data to client (issues #3556 and #3606)
+	if h.isBidirectionalStream(req, res) {
 		return -1
 	}
 
-	// TODO: more specific cases? e.g. res.ContentLength == -1? (this TODO is from the std lib)
+	// TODO: more specific cases? e.g. res.ContentLength == -1? (this TODO is from the std lib, but
+	// strangely similar to our isBidirectionalStream function that we implemented ourselves)
 	return time.Duration(h.FlushInterval)
 }
 
+// isBidirectionalStream returns whether we should work in bi-directional stream mode.
+//
+// See https://github.com/caddyserver/caddy/pull/3620 for discussion of nuances.
+func (h Handler) isBidirectionalStream(req *http.Request, res *http.Response) bool {
+	// We have to check the encoding here; only flush headers with identity encoding.
+	// Non-identity encoding might combine with "encode" directive, and in that case,
+	// if body size larger than enc.MinLength, upper level encode handle might have
+	// Content-Encoding header to write.
+	// (see https://github.com/caddyserver/caddy/issues/3606 for use case)
+	ae := req.Header.Get("Accept-Encoding")
+
+	return req.ProtoMajor == 2 &&
+		res.ProtoMajor == 2 &&
+		res.ContentLength == -1 &&
+		(ae == "identity" || ae == "")
+}
+
 func (h Handler) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
 	if flushInterval != 0 {
 		if wf, ok := dst.(writeFlusher); ok {