reverseproxy: Add more debug logs (#5793)

* reverseproxy: Add more debug logs

This makes debug logging very noisy when reverse proxying, but I guess
that's the point.

This has shown to be useful in troubleshooting infrastructure issues.

* Update modules/caddyhttp/reverseproxy/streaming.go

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* Update modules/caddyhttp/reverseproxy/streaming.go

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

* Add opt-in `trace_logs` option

* Rename to VerboseLogs

---------

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

1 files changed, 19 insertions, 2 deletions

diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index 64a7d7a..08be40d 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -191,6 +191,13 @@ type Handler struct {
 	// - `{http.reverse_proxy.header.*}` The headers from the response
 	HandleResponse []caddyhttp.ResponseHandler `json:"handle_response,omitempty"`
 
+	// If set, the proxy will write very detailed logs about its
+	// inner workings. Enable this only when debugging, as it
+	// will produce a lot of output.
+	//
+	// EXPERIMENTAL: This feature is subject to change or removal.
+	VerboseLogs bool `json:"verbose_logs,omitempty"`
+
 	Transport        http.RoundTripper `json:"-"`
 	CB               CircuitBreaker    `json:"-"`
 	DynamicUpstreams UpstreamSource    `json:"-"`
@@ -943,9 +950,15 @@ func (h *Handler) finalizeResponse(
 	}
 
 	rw.WriteHeader(res.StatusCode)
+	if h.VerboseLogs {
+		logger.Debug("wrote header")
+	}
 
-	err := h.copyResponse(rw, res.Body, h.flushInterval(req, res))
-	res.Body.Close() // close now, instead of defer, to populate res.Trailer
+	err := h.copyResponse(rw, res.Body, h.flushInterval(req, res), logger)
+	errClose := res.Body.Close() // close now, instead of defer, to populate res.Trailer
+	if h.VerboseLogs || errClose != nil {
+		logger.Debug("closed response body from upstream", zap.Error(errClose))
+	}
 	if err != nil {
 		// we're streaming the response and we've already written headers, so
 		// there's nothing an error handler can do to recover at this point;
@@ -979,6 +992,10 @@ func (h *Handler) finalizeResponse(
 		}
 	}
 
+	if h.VerboseLogs {
+		logger.Debug("response finalized")
+	}
+
 	return nil
 }